1.\" Copyright (c) 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" This code is derived from software contributed to Berkeley by 5.\" Kenneth Almquist. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. All advertising materials mentioning features or use of this software 16.\" must display the following acknowledgement: 17.\" This product includes software developed by the University of 18.\" California, Berkeley and its contributors. 19.\" 4. Neither the name of the University nor the names of its contributors 20.\" may be used to endorse or promote products derived from this software 21.\" without specific prior written permission. 22.\" 23.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 24.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 25.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 26.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 27.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 28.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 29.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 30.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 31.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 32.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 33.\" SUCH DAMAGE. 34.\" 35.\" from: @(#)sh.1 8.6 (Berkeley) 5/4/95 36.\" $FreeBSD: src/bin/sh/sh.1,v 1.39.2.25 2003/02/13 19:28:08 fanf Exp $ 37.\" 38.Dd May 5, 1995 39.Dt SH 1 40.Os 41.Sh NAME 42.Nm sh 43.Nd command interpreter (shell) 44.Sh SYNOPSIS 45.Nm 46.Op Fl /+abCEefIimnPpsTuVvx 47.Op Fl /+o Ar longname 48.Op Fl c Ar string 49.Op Ar arg ... 50.Sh DESCRIPTION 51The 52.Nm 53utility is the standard command interpreter for the system. 54The current version of 55.Nm 56is in the process of being changed to 57conform with the 58.St -p1003.2 59specification for the shell. This version has many features which make 60it appear 61similar in some respects to the Korn shell, but it is not a Korn 62shell clone like 63.Xr pdksh 1 . 64Only features 65designated by POSIX, plus a few Berkeley extensions, are being 66incorporated into this shell. 67This man page is not intended to be a tutorial nor a complete 68specification of the shell. 69.Ss Overview 70The shell is a command that reads lines from 71either a file or the terminal, interprets them, and 72generally executes other commands. 73It is the program that is started when a user logs into the system, 74although a user can select a different shell with the 75.Xr chsh 1 76command. 77The shell 78implements a language that has flow control constructs, 79a macro facility that provides a variety of features in 80addition to data storage, along with built-in history and line 81editing capabilities. It incorporates many features to 82aid interactive use and has the advantage that the interpretative 83language is common to both interactive and non-interactive 84use (shell scripts). That is, commands can be typed directly 85to the running shell or can be put into a file, 86which can be executed directly by the shell. 87.Ss Invocation 88.\" 89.\" XXX This next sentence is incredibly confusing. 90.\" 91If no arguments are present and if the standard input of the shell 92is connected to a terminal 93(or if the 94.Fl i 95option is set), 96the shell is considered an interactive shell. An interactive shell 97generally prompts before each command and handles programming 98and command errors differently (as described below). 99When first starting, the shell inspects argument 0, and 100if it begins with a dash 101.Pq Li - , 102the shell is also considered a login shell. 103This is normally done automatically by the system 104when the user first logs in. A login shell first reads commands 105from the files 106.Pa /etc/profile 107and then 108.Pa .profile 109if they exist. If the environment variable 110.Ev ENV 111is set on entry to a shell, or is set in the 112.Pa .profile 113of a login shell, the shell then reads commands from the file named in 114.Ev ENV . 115Therefore, a user should place commands that are to be executed only 116at login time in the 117.Pa .profile 118file, and commands that are executed for every shell inside the 119.Ev ENV 120file. 121The user can set the 122.Ev ENV 123variable to some file by placing the following line in the file 124.Pa .profile 125in the home directory, 126substituting for 127.Pa .shinit 128the filename desired: 129.Pp 130.Dl ENV=$HOME/.shinit; export ENV 131.Pp 132The first non-option argument specified on the command line 133will be treated as the 134name of a file from which to read commands (a shell script), and 135the remaining arguments are set as the positional parameters 136of the shell ($1, $2, etc). Otherwise, the shell reads commands 137from its standard input. 138.Pp 139Unlike older versions of 140.Nm 141the 142.Ev ENV 143script is only sourced on invocation of interactive shells. This 144closes a well-known, and sometimes easily exploitable security 145hole related to poorly thought out 146.Ev ENV 147scripts. 148.Ss Argument List Processing 149All of the single letter options to 150.Nm 151have a corresponding long name, 152with the exception of 153.Fl c 154and 155.Fl /+o . 156These long names are provided next to the single letter options 157in the descriptions below. 158The long name for an option may be specified as an argument to the 159.Fl /+o 160option of 161.Nm . 162Once the shell is running, 163the long name for an option may be specified as an argument to the 164.Fl /+o 165option of the 166.Ic set 167built-in command 168(described later in the section called 169.Sx Built-in Commands ) . 170Introducing an option with a dash 171.Pq Li - 172enables the option, 173while using a plus 174.Pq Li + 175disables the option. 176A 177.Dq Li -- 178or plain 179.Dq Li - 180will stop option processing and will force the remaining 181words on the command line to be treated as arguments. 182The 183.Fl /+o 184and 185.Fl c 186options do not have long names. 187They take arguments and are described after the single letter options. 188.Bl -tag -width indent 189.It Fl a Li allexport 190Flag variables for export when assignments are made to them. 191.It Fl b Li notify 192Enable asynchronous notification of background job 193completion. 194(UNIMPLEMENTED) 195.It Fl C Li noclobber 196Do not overwrite existing files with 197.Dq Li > . 198.It Fl E Li emacs 199Enable the built-in 200.Xr emacs 1 201command line editor (disables the 202.Fl V 203option if it has been set). 204.It Fl e Li errexit 205Exit immediately if any untested command fails in non-interactive mode. 206The exit status of a command is considered to be 207explicitly tested if the command is used to control 208an if, elif, while, or until; or if the command is the left 209hand operand of an 210.Dq Li && 211or 212.Dq Li || 213operator. 214.It Fl f Li noglob 215Disable pathname expansion. 216.It Fl I Li ignoreeof 217Ignore 218.Dv EOF Ns ' Ns s 219from input when in interactive mode. 220.It Fl i Li interactive 221Force the shell to behave interactively. 222.It Fl m Li monitor 223Turn on job control (set automatically when interactive). 224.It Fl n Li noexec 225If not interactive, read commands but do not 226execute them. This is useful for checking the 227syntax of shell scripts. 228.It Fl P Li physical 229Change the default for the 230.Ic cd 231and 232.Ic pwd 233commands from 234.Fl L 235(logical directory layout) 236to 237.Fl P 238(physical directory layout). 239.It Fl p Li privileged 240Turn on privileged mode. This mode is enabled on startup 241if either the effective user or group id is not equal to the 242real user or group id. Turning this mode off sets the 243effective user and group ids to the real user and group ids. 244When this mode is enabled for interactive shells, the file 245.Pa /etc/suid_profile 246is sourced instead of 247.Pa ~/.profile 248after 249.Pa /etc/profile 250is sourced, and the contents of the 251.Ev ENV 252variable are ignored. 253.It Fl s Li stdin 254Read commands from standard input (set automatically 255if no file arguments are present). This option has 256no effect when set after the shell has already started 257running (i.e. when set with the 258.Ic set 259command). 260.It Fl T Li trapsasync 261When waiting for a child, execute traps immediately. 262If this option is not set, 263traps are executed after the child exits, 264as specified in 265.St -p1003.2 266This nonstandard option is useful for putting guarding shells around 267children that block signals. The surrounding shell may kill the child 268or it may just return control to the tty and leave the child alone, 269like this: 270.Bd -literal -offset indent 271sh -T -c "trap 'exit 1' 2 ; some-blocking-program" 272.Ed 273.Pp 274.It Fl u Li nounset 275Write a message to standard error when attempting 276to expand a variable that is not set, and if the 277shell is not interactive, exit immediately. 278.It Fl V Li vi 279Enable the built-in 280.Xr vi 1 281command line editor (disables 282.Fl E 283if it has been set). 284.It Fl v Li verbose 285The shell writes its input to standard error 286as it is read. Useful for debugging. 287.It Fl x Li xtrace 288Write each command 289(preceded by 290.Dq Li +\ ) 291to standard error before it is executed. 292Useful for debugging. 293.El 294.Pp 295The 296.Fl c 297option may be used to pass its string argument to the shell 298to be interpreted as input. 299Keep in mind that this option only accepts a single string as its 300argument, hence multi-word strings must be quoted. 301.Pp 302The 303.Fl /+o 304option takes as its only argument the long name of an option 305to be enabled or disabled. 306For example, the following two invocations of 307.Nm 308both enable the built-in 309.Xr emacs 1 310command line editor: 311.Bd -literal -offset indent 312set -E 313set -o emacs 314.Ed 315.Pp 316If used without an argument, the 317.Fl o 318option displays the current option settings in a human-readable format. 319If 320.Cm +o 321is used without an argument, the current option settings are output 322in a format suitable for re-input into the shell. 323.Ss Lexical Structure 324The shell reads input in terms of lines from a file and breaks 325it up into words at whitespace (blanks and tabs), and at 326certain sequences of 327characters called 328.Dq operators , 329which are special to the shell. 330There are two types of operators: control operators and 331redirection operators (their meaning is discussed later). 332The following is a list of valid operators: 333.Bl -tag -width indent 334.It Control operators: 335.Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact 336.It Li & Ta Li && Ta Li ( Ta Li ) Ta Li \en 337.It Li ;; Ta Li ; Ta Li | Ta Li || 338.El 339.It Redirection operators: 340.Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact 341.It Li < Ta Li > Ta Li << Ta Li >> Ta Li <> 342.It Li <& Ta Li >& Ta Li <<- Ta Li >| 343.El 344.El 345.Ss Quoting 346Quoting is used to remove the special meaning of certain characters 347or words to the shell, such as operators, whitespace, or 348keywords. There are three types of quoting: matched single quotes, 349matched double quotes, and backslash. 350.Bl -tag -width indent 351.It Single Quotes 352Enclosing characters in single quotes preserves the literal 353meaning of all the characters (except single quotes, making 354it impossible to put single-quotes in a single-quoted string). 355.It Double Quotes 356Enclosing characters within double quotes preserves the literal 357meaning of all characters except dollarsign 358.Pq Li $ , 359backquote 360.Pq Li ` , 361and backslash 362.Pq Li \e . 363The backslash inside double quotes is historically weird. 364It remains literal unless it precedes the following characters, 365which it serves to quote: 366.Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact 367.It Li $ Ta Li ` Ta Li \&" Ta Li \e\ Ta Li \en 368.El 369.It Backslash 370A backslash preserves the literal meaning of the following 371character, with the exception of the newline character 372.Pq Li \en . 373A backslash preceding a newline is treated as a line continuation. 374.El 375.Ss Reserved Words 376Reserved words are words that have special meaning to the 377shell and are recognized at the beginning of a line and 378after a control operator. The following are reserved words: 379.Bl -column "doneXX" "elifXX" "elseXX" "untilXX" "whileX" -offset center 380.It Li \&! Ta { Ta } Ta Ic case Ta Ic do 381.It Ic done Ta Ic elif Ta Ic else Ta Ic esac Ta Ic fi 382.It Ic for Ta Ic if Ta Ic then Ta Ic until Ta Ic while 383.El 384.Ss Aliases 385An alias is a name and corresponding value set using the 386.Ic alias 387built-in command. Whenever a reserved word may occur (see above), 388and after checking for reserved words, the shell 389checks the word to see if it matches an alias. 390If it does, it replaces it in the input stream with its value. 391For example, if there is an alias called 392.Dq Li lf 393with the value 394.Dq Li ls -F , 395then the input 396.Bd -literal -offset indent 397lf foobar 398.Ed 399.Pp 400would become 401.Bd -literal -offset indent 402ls -F foobar 403.Ed 404.Pp 405Aliases provide a convenient way for naive users to 406create shorthands for commands without having to learn how 407to create functions with arguments. They can also be 408used to create lexically obscure code. This use is discouraged. 409.Ss Commands 410The shell interprets the words it reads according to a 411language, the specification of which is outside the scope 412of this man page (refer to the BNF in the 413.St -p1003.2 414document). Essentially though, a line is read and if 415the first word of the line (or after a control operator) 416is not a reserved word, then the shell has recognized a 417simple command. Otherwise, a complex command or some 418other special construct may have been recognized. 419.Ss Simple Commands 420If a simple command has been recognized, the shell performs 421the following actions: 422.Bl -enum 423.It 424Leading words of the form 425.Dq Li name=value 426are stripped off and assigned to the environment of 427the simple command. Redirection operators and 428their arguments (as described below) are stripped 429off and saved for processing. 430.It 431The remaining words are expanded as described in 432the section called 433.Sx Word Expansions , 434and the first remaining word is considered the command 435name and the command is located. The remaining 436words are considered the arguments of the command. 437If no command name resulted, then the 438.Dq Li name=value 439variable assignments recognized in 1) affect the 440current shell. 441.It 442Redirections are performed as described in 443the next section. 444.El 445.Ss Redirections 446Redirections are used to change where a command reads its input 447or sends its output. In general, redirections open, close, or 448duplicate an existing reference to a file. The overall format 449used for redirection is: 450.Pp 451.Dl [n] redir-op file 452.Pp 453The 454.Ql redir-op 455is one of the redirection operators mentioned 456previously. The following gives some examples of how these 457operators can be used. 458Note that stdin and stdout are commonly used abbreviations 459for standard input and standard output respectively. 460.Bl -tag -width "1234567890XX" -offset indent 461.It Li [n]> file 462redirect stdout (or file descriptor n) to file 463.It Li [n]>| file 464same as above, but override the 465.Fl C 466option 467.It Li [n]>> file 468append stdout (or file descriptor n) to file 469.It Li [n]< file 470redirect stdin (or file descriptor n) from file 471.It Li [n]<> file 472redirect stdin (or file descriptor n) to and from file 473.It Li [n1]<&n2 474duplicate stdin (or file descriptor n1) from file descriptor n2 475.It Li [n]<&- 476close stdin (or file descriptor n) 477.It Li [n1]>&n2 478duplicate stdout (or file descriptor n1) to file descriptor n2 479.It Li [n]>&- 480close stdout (or file descriptor n) 481.El 482.Pp 483The following redirection is often called a 484.Dq here-document . 485.Bd -literal -offset indent 486[n]<< delimiter 487 here-doc-text 488 ... 489delimiter 490.Ed 491.Pp 492All the text on successive lines up to the delimiter is 493saved away and made available to the command on standard 494input, or file descriptor n if it is specified. If the delimiter 495as specified on the initial line is quoted, then the here-doc-text 496is treated literally, otherwise the text is subjected to 497parameter expansion, command substitution, and arithmetic 498expansion (as described in the section on 499.Sx Word Expansions ) . 500If the operator is 501.Dq Li <<- 502instead of 503.Dq Li << , 504then leading tabs 505in the here-doc-text are stripped. 506.Ss Search and Execution 507There are three types of commands: shell functions, 508built-in commands, and normal programs. 509The command is searched for (by name) in that order. 510The three types of commands are all executed in a different way. 511.Pp 512When a shell function is executed, all of the shell positional 513parameters (except $0, which remains unchanged) are 514set to the arguments of the shell function. 515The variables which are explicitly placed in the environment of 516the command (by placing assignments to them before the 517function name) are made local to the function and are set 518to the values given. 519Then the command given in the function definition is executed. 520The positional parameters are restored to their original values 521when the command completes. 522This all occurs within the current shell. 523.Pp 524Shell built-in commands are executed internally to the shell, without 525spawning a new process. 526.Pp 527Otherwise, if the command name does not match a function 528or built-in command, the command is searched for as a normal 529program in the file system (as described in the next section). 530When a normal program is executed, the shell runs the program, 531passing the arguments and the environment to the program. 532If the program is not a normal executable file 533(i.e. if it does not begin with the 534.Qq magic number 535whose 536.Tn ASCII 537representation is 538.Qq #! , 539resulting in an 540.Er ENOEXEC 541return value from 542.Xr execve 2 ) 543the shell will interpret the program in a subshell. 544The child shell will reinitialize itself in this case, 545so that the effect will be 546as if a new shell had been invoked to handle the ad-hoc shell script, 547except that the location of hashed commands located in 548the parent shell will be remembered by the child. 549.Pp 550Note that previous versions of this document 551and the source code itself misleadingly and sporadically 552refer to a shell script without a magic number 553as a 554.Qq shell procedure . 555.Ss Path Search 556When locating a command, the shell first looks to see if 557it has a shell function by that name. Then it looks for a 558built-in command by that name. If a built-in command is not found, 559one of two things happen: 560.Bl -enum 561.It 562Command names containing a slash are simply executed without 563performing any searches. 564.It 565The shell searches each entry in 566.Ev PATH 567in turn for the command. The value of the 568.Ev PATH 569variable should be a series of 570entries separated by colons. Each entry consists of a 571directory name. 572The current directory 573may be indicated implicitly by an empty directory name, 574or explicitly by a single period. 575.El 576.Ss Command Exit Status 577Each command has an exit status that can influence the behavior 578of other shell commands. The paradigm is that a command exits 579with zero for normal or success, and non-zero for failure, 580error, or a false indication. The man page for each command 581should indicate the various exit codes and what they mean. 582Additionally, the built-in commands return exit codes, as does 583an executed shell function. 584.Pp 585If a command is terminated by a signal, its exit status is 128 plus 586the signal number. Signal numbers are defined in the header file 587.Aq Pa sys/signal.h . 588.Ss Complex Commands 589Complex commands are combinations of simple commands 590with control operators or reserved words, together creating a larger complex 591command. More generally, a command is one of the following: 592.Bl -item -offset indent 593.It 594simple command 595.It 596pipeline 597.It 598list or compound-list 599.It 600compound command 601.It 602function definition 603.El 604.Pp 605Unless otherwise stated, the exit status of a command is 606that of the last simple command executed by the command. 607.Ss Pipelines 608A pipeline is a sequence of one or more commands separated 609by the control operator |. The standard output of all but 610the last command is connected to the standard input 611of the next command. The standard output of the last 612command is inherited from the shell, as usual. 613.Pp 614The format for a pipeline is: 615.Pp 616.Dl [!] command1 [ | command2 ...] 617.Pp 618The standard output of command1 is connected to the standard 619input of command2. The standard input, standard output, or 620both of a command is considered to be assigned by the 621pipeline before any redirection specified by redirection 622operators that are part of the command. 623.Pp 624If the pipeline is not in the background (discussed later), 625the shell waits for all commands to complete. 626.Pp 627If the reserved word ! does not precede the pipeline, the 628exit status is the exit status of the last command specified 629in the pipeline. Otherwise, the exit status is the logical 630NOT of the exit status of the last command. That is, if 631the last command returns zero, the exit status is 1; if 632the last command returns greater than zero, the exit status 633is zero. 634.Pp 635Because pipeline assignment of standard input or standard 636output or both takes place before redirection, it can be 637modified by redirection. For example: 638.Pp 639.Dl $ command1 2>&1 | command2 640.Pp 641sends both the standard output and standard error of 642.Ql command1 643to the standard input of 644.Ql command2 . 645.Pp 646A 647.Dq Li \&; 648or newline terminator causes the preceding 649AND-OR-list 650(described below in the section called 651.Sx Short-Circuit List Operators ) 652to be executed sequentially; 653an 654.Dq Li & 655causes asynchronous execution of the preceding AND-OR-list. 656.Pp 657Note that unlike some other shells, 658.Nm 659executes each process in the pipeline as a child of the 660.Nm 661process. 662Shell built-in commands are the exception to this rule. 663They are executed in the current shell, although they do not affect its 664environment when used in pipelines. 665.Ss Background Commands (&) 666If a command is terminated by the control operator ampersand 667.Pq Li & , 668the shell executes the command asynchronously; 669the shell does not wait for the command to finish 670before executing the next command. 671.Pp 672The format for running a command in background is: 673.Bd -literal -offset indent 674command1 & [command2 & ...] 675.Ed 676.Pp 677If the shell is not interactive, the standard input of an 678asynchronous command is set to /dev/null. 679.Ss Lists (Generally Speaking) 680A list is a sequence of zero or more commands separated by 681newlines, semicolons, or ampersands, 682and optionally terminated by one of these three characters. 683The commands in a 684list are executed in the order they are written. 685If command is followed by an ampersand, the shell starts the 686command and immediately proceeds onto the next command; 687otherwise it waits for the command to terminate before 688proceeding to the next one. 689.Ss Short-Circuit List Operators 690.Dq Li && 691and 692.Dq Li || 693are AND-OR list operators. 694.Dq Li && 695executes the first command, and then executes the second command 696if the exit status of the first command is zero. 697.Dq Li || 698is similar, but executes the second command if the exit 699status of the first command is nonzero. 700.Dq Li && 701and 702.Dq Li || 703both have the same priority. 704.Ss Flow-Control Constructs (if, while, for, case) 705The syntax of the 706.Ic if 707command is: 708.\" 709.\" XXX Use .Dl to work around broken handling of .Ic inside .Bd and .Ed . 710.\" 711.Dl Ic if Ar list 712.Dl Ic then Ar list 713.Dl [ Ic elif Ar list 714.Dl Ic then Ar list ] ... 715.Dl [ Ic else Ar list ] 716.Dl Ic fi 717.Pp 718The syntax of the 719.Ic while 720command is: 721.Dl Ic while Ar list 722.Dl Ic do Ar list 723.Dl Ic done 724.Pp 725The two lists are executed repeatedly while the exit status of the 726first list is zero. 727The 728.Ic until 729command is similar, but has the word 730.Ic until 731in place of 732.Ic while , 733which causes it to 734repeat until the exit status of the first list is zero. 735.Pp 736The syntax of the 737.Ic for 738command is: 739.Dl Ic for Ar variable Ic in Ar word ... 740.Dl Ic do Ar list 741.Dl Ic done 742.Pp 743The words are expanded, and then the list is executed 744repeatedly with the variable set to each word in turn. 745The 746.Ic do 747and 748.Ic done 749commands may be replaced with 750.Dq Li { 751and 752.Dq Li } . 753.Pp 754The syntax of the 755.Ic break 756and 757.Ic continue 758commands is: 759.Dl Ic break Op Ar num 760.Dl Ic continue Op Ar num 761.Pp 762The 763.Ic break 764command terminates the 765.Ar num 766innermost 767.Ic for 768or 769.Ic while 770loops. 771The 772.Ic continue 773command continues with the next iteration of the innermost loop. 774These are implemented as built-in commands. 775.Pp 776The syntax of the 777.Ic case 778command is 779.Dl Ic case Ar word Ic in 780.Dl pattern) list ;; 781.Dl ... 782.Dl Ic esac 783.Pp 784The pattern can actually be one or more patterns 785(see 786.Sx Shell Patterns 787described later), 788separated by 789.Dq Li \&| 790characters. 791.Ss Grouping Commands Together 792Commands may be grouped by writing either 793.Bd -literal -offset indent 794(list) 795.Ed 796.Pp 797or 798.Bd -literal -offset indent 799{ list; } 800.Ed 801.Pp 802The first form executes the commands in a subshell. 803Note that built-in commands thus executed do not affect the current shell. 804The second form does not fork another shell, 805so it is slightly more efficient. 806Grouping commands together this way allows the user to 807redirect their output as though they were one program: 808.Bd -literal -offset indent 809{ echo -n "hello"; echo " world"; } > greeting 810.Ed 811.Ss Functions 812The syntax of a function definition is 813.Bd -literal -offset indent 814name ( ) command 815.Ed 816.Pp 817A function definition is an executable statement; when 818executed it installs a function named name and returns an 819exit status of zero. The command is normally a list 820enclosed between 821.Dq Li { 822and 823.Dq Li } . 824.Pp 825Variables may be declared to be local to a function by 826using the 827.Ic local 828command. 829This should appear as the first statement of a function, 830and the syntax is: 831.Bd -ragged -offset indent 832.Ic local 833.Op Ar variable ... 834.Op Ar - 835.Ed 836.Pp 837The 838.Ic local 839command is implemented as a built-in command. 840.Pp 841When a variable is made local, it inherits the initial 842value and exported and readonly flags from the variable 843with the same name in the surrounding scope, if there is 844one. Otherwise, the variable is initially unset. The shell 845uses dynamic scoping, so that if the variable 846.Em x 847is made local to function 848.Em f , 849which then calls function 850.Em g , 851references to the variable 852.Em x 853made inside 854.Em g 855will refer to the variable 856.Em x 857declared inside 858.Em f , 859not to the global variable named 860.Em x . 861.Pp 862The only special parameter than can be made local is 863.Dq Li - . 864Making 865.Dq Li - 866local causes any shell options that are 867changed via the set command inside the function to be 868restored to their original values when the function 869returns. 870.Pp 871The syntax of the 872.Ic return 873command is 874.Bd -ragged -offset indent 875.Ic return 876.Op Ar exitstatus 877.Ed 878.Pp 879It terminates the currently executing function. 880The 881.Ic return 882command is implemented as a built-in command. 883.Ss Variables and Parameters 884The shell maintains a set of parameters. A parameter 885denoted by a name is called a variable. When starting up, 886the shell turns all the environment variables into shell 887variables. New variables can be set using the form 888.Bd -literal -offset indent 889name=value 890.Ed 891.Pp 892Variables set by the user must have a name consisting solely 893of alphabetics, numerics, and underscores. 894The first letter of a variable name must not be numeric. 895A parameter can also be denoted by a number 896or a special character as explained below. 897.Ss Positional Parameters 898A positional parameter is a parameter denoted by a number greater than zero. 899The shell sets these initially to the values of its command line 900arguments that follow the name of the shell script. The 901.Ic set 902built-in command can also be used to set or reset them. 903.Ss Special Parameters 904A special parameter is a parameter denoted by one of the following 905special characters. The value of the parameter is listed 906next to its character. 907.Bl -hang 908.It Li * 909Expands to the positional parameters, starting from one. When 910the expansion occurs within a double-quoted string 911it expands to a single field with the value of each parameter 912separated by the first character of the 913.Ev IFS 914variable, 915or by a 916.Aq space 917if 918.Ev IFS 919is unset. 920.It Li @ 921Expands to the positional parameters, starting from one. When 922the expansion occurs within double-quotes, each positional 923parameter expands as a separate argument. 924If there are no positional parameters, the 925expansion of 926.Li @ 927generates zero arguments, even when 928.Li @ 929is double-quoted. What this basically means, for example, is 930if $1 is 931.Dq abc 932and $2 is 933.Dq def ghi , 934then 935.Qq Li $@ 936expands to 937the two arguments: 938.Bd -literal -offset indent 939"abc" "def ghi" 940.Ed 941.It Li # 942Expands to the number of positional parameters. 943.It Li \&? 944Expands to the exit status of the most recent pipeline. 945.It Li - 946(hyphen) Expands to the current option flags (the single-letter 947option names concatenated into a string) as specified on 948invocation, by the set built-in command, or implicitly 949by the shell. 950.It Li $ 951Expands to the process ID of the invoked shell. A subshell 952retains the same value of $ as its parent. 953.It Li \&! 954Expands to the process ID of the most recent background 955command executed from the current shell. For a 956pipeline, the process ID is that of the last command in the 957pipeline. 958.It Li 0 959(zero) Expands to the name of the shell or shell script. 960.El 961.Ss Word Expansions 962This clause describes the various expansions that are 963performed on words. Not all expansions are performed on 964every word, as explained later. 965.Pp 966Tilde expansions, parameter expansions, command substitutions, 967arithmetic expansions, and quote removals that occur within 968a single word expand to a single field. It is only field 969splitting or pathname expansion that can create multiple 970fields from a single word. 971The single exception to this rule is 972the expansion of the special parameter 973.Li @ 974within double-quotes, 975as was described above. 976.Pp 977The order of word expansion is: 978.Bl -enum 979.It 980Tilde Expansion, Parameter Expansion, Command Substitution, 981Arithmetic Expansion (these all occur at the same time). 982.It 983Field Splitting is performed on fields generated by step (1) 984unless the 985.Ev IFS 986variable is null. 987.It 988Pathname Expansion (unless the 989.Fl f 990option is in effect). 991.It 992Quote Removal. 993.El 994.Pp 995The 996.Dq Li $ 997character is used to introduce parameter expansion, command 998substitution, or arithmetic evaluation. 999.Ss Tilde Expansion (substituting a user's home directory) 1000A word beginning with an unquoted tilde character 1001.Pq Li ~ 1002is 1003subjected to tilde expansion. 1004All the characters up to a slash 1005.Pq Li / 1006or the end of the word are treated as a username 1007and are replaced with the user's home directory. If the 1008username is missing (as in ~/foobar), the tilde is replaced 1009with the value of the HOME variable (the current user's 1010home directory). 1011.Ss Parameter Expansion 1012The format for parameter expansion is as follows: 1013.Bd -literal -offset indent 1014${expression} 1015.Ed 1016.Pp 1017where expression consists of all characters until the matching 1018.Dq Li } . 1019Any 1020.Dq Li } 1021escaped by a backslash or within a quoted string, and characters in 1022embedded arithmetic expansions, command substitutions, and variable 1023expansions, are not examined in determining the matching 1024.Dq Li } . 1025.Pp 1026The simplest form for parameter expansion is: 1027.Bd -literal -offset indent 1028${parameter} 1029.Ed 1030.Pp 1031The value, if any, of parameter is substituted. 1032.Pp 1033The parameter name or symbol can be enclosed in braces, which are 1034optional except for positional parameters with more than one digit or 1035when parameter is followed by a character that could be interpreted as 1036part of the name. 1037If a parameter expansion occurs inside double-quotes: 1038.Bl -enum 1039.It 1040Pathname expansion is not performed on the results of the 1041expansion. 1042.It 1043Field splitting is not performed on the results of the 1044expansion, with the exception of the special parameter 1045.Li @ . 1046.El 1047.Pp 1048In addition, a parameter expansion can be modified by using one of the 1049following formats. 1050.Bl -tag -width indent 1051.It Li ${parameter:-word} 1052Use Default Values. If parameter is unset or 1053null, the expansion of word is 1054substituted; otherwise, the value of 1055parameter is substituted. 1056.It Li ${parameter:=word} 1057Assign Default Values. If parameter is unset 1058or null, the expansion of word is 1059assigned to parameter. In all cases, the 1060final value of parameter is 1061substituted. Only variables, not positional 1062parameters or special parameters, can be 1063assigned in this way. 1064.It Li ${parameter:?[word]} 1065Indicate Error if Null or Unset. If 1066parameter is unset or null, the expansion of 1067word (or a message indicating it is unset if 1068word is omitted) is written to standard 1069error and the shell exits with a nonzero 1070exit status. 1071Otherwise, the value of 1072parameter is substituted. An 1073interactive shell need not exit. 1074.It Li ${parameter:+word} 1075Use Alternate Value. If parameter is unset 1076or null, null is substituted; 1077otherwise, the expansion of word is 1078substituted. 1079.El 1080.Pp 1081In the parameter expansions shown previously, use of the colon in the 1082format results in a test for a parameter that is unset or null; omission 1083of the colon results in a test for a parameter that is only unset. 1084.Bl -tag -width indent 1085.It Li ${#parameter} 1086String Length. The length in characters of 1087the value of parameter. 1088.El 1089.Pp 1090The following four varieties of parameter expansion provide for substring 1091processing. 1092In each case, pattern matching notation 1093(see 1094.Sx Shell Patterns ) , 1095rather than regular expression notation, 1096is used to evaluate the patterns. 1097If parameter is one of the special parameters 1098.Li * 1099or 1100.Li @ , 1101the result of the expansion is unspecified. 1102Enclosing the full parameter expansion string in double-quotes does not 1103cause the following four varieties of pattern characters to be quoted, 1104whereas quoting characters within the braces has this effect. 1105.Bl -tag -width indent 1106.It Li ${parameter%word} 1107Remove Smallest Suffix Pattern. The word 1108is expanded to produce a pattern. The 1109parameter expansion then results in 1110parameter, with the smallest portion of the 1111suffix matched by the pattern deleted. 1112.It Li ${parameter%%word} 1113Remove Largest Suffix Pattern. The word 1114is expanded to produce a pattern. The 1115parameter expansion then results in 1116parameter, with the largest portion of the 1117suffix matched by the pattern deleted. 1118.It Li ${parameter#word} 1119Remove Smallest Prefix Pattern. The word 1120is expanded to produce a pattern. The 1121parameter expansion then results in 1122parameter, with the smallest portion of the 1123prefix matched by the pattern deleted. 1124.It Li ${parameter##word} 1125Remove Largest Prefix Pattern. The word 1126is expanded to produce a pattern. The 1127parameter expansion then results in 1128parameter, with the largest portion of the 1129prefix matched by the pattern deleted. 1130.El 1131.Ss Command Substitution 1132Command substitution allows the output of a command to be substituted in 1133place of the command name itself. Command substitution occurs when 1134the command is enclosed as follows: 1135.Bd -literal -offset indent 1136$(command) 1137.Ed 1138.Pp 1139or the backquoted version: 1140.Bd -literal -offset indent 1141`command` 1142.Ed 1143.Pp 1144The shell expands the command substitution by executing command in a 1145subshell environment and replacing the command substitution 1146with the standard output of the command, 1147removing sequences of one or more newlines at the end of the substitution. 1148Embedded newlines before the end of the output are not removed; 1149however, during field splitting, they may be translated into spaces 1150depending on the value of 1151.Ev IFS 1152and the quoting that is in effect. 1153.Ss Arithmetic Expansion 1154Arithmetic expansion provides a mechanism for evaluating an arithmetic 1155expression and substituting its value. 1156The format for arithmetic expansion is as follows: 1157.Bd -literal -offset indent 1158$((expression)) 1159.Ed 1160.Pp 1161The expression is treated as if it were in double-quotes, except 1162that a double-quote inside the expression is not treated specially. The 1163shell expands all tokens in the expression for parameter expansion, 1164command substitution, and quote removal. 1165.Pp 1166Next, the shell treats this as an arithmetic expression and 1167substitutes the value of the expression. 1168.Ss White Space Splitting (Field Splitting) 1169After parameter expansion, command substitution, and 1170arithmetic expansion the shell scans the results of 1171expansions and substitutions that did not occur in double-quotes for 1172field splitting and multiple fields can result. 1173.Pp 1174The shell treats each character of the 1175.Ev IFS 1176as a delimiter and uses 1177the delimiters to split the results of parameter expansion and command 1178substitution into fields. 1179.Ss Pathname Expansion (File Name Generation) 1180Unless the 1181.Fl f 1182option is set, 1183file name generation is performed 1184after word splitting is complete. Each word is 1185viewed as a series of patterns, separated by slashes. The 1186process of expansion replaces the word with the names of 1187all existing files whose names can be formed by replacing 1188each pattern with a string that matches the specified pattern. 1189There are two restrictions on this: first, a pattern cannot match 1190a string containing a slash, and second, 1191a pattern cannot match a string starting with a period 1192unless the first character of the pattern is a period. 1193The next section describes the patterns used for both 1194Pathname Expansion and the 1195.Ic case 1196command. 1197.Ss Shell Patterns 1198A pattern consists of normal characters, which match themselves, 1199and meta-characters. 1200The meta-characters are 1201.Dq Li \&! , 1202.Dq Li * , 1203.Dq Li \&? , 1204and 1205.Dq Li [ . 1206These characters lose their special meanings if they are quoted. 1207When command or variable substitution is performed and the dollar sign 1208or back quotes are not double-quoted, the value of the 1209variable or the output of the command is scanned for these 1210characters and they are turned into meta-characters. 1211.Pp 1212An asterisk 1213.Pq Li * 1214matches any string of characters. 1215A question mark 1216.Pq Li \&? 1217matches any single character. 1218A left bracket 1219.Pq Li [ 1220introduces a character class. 1221The end of the character class is indicated by a 1222.Dq Li \&] ; 1223if the 1224.Dq Li \&] 1225is missing then the 1226.Dq Li [ 1227matches a 1228.Dq Li [ 1229rather than introducing a character class. 1230A character class matches any of the characters between the square brackets. 1231A range of characters may be specified using a minus sign. 1232The character class may be complemented by making an exclamation point 1233.Pq Li !\& 1234the first character of the character class. 1235.Pp 1236To include a 1237.Dq Li \&] 1238in a character class, make it the first character listed 1239(after the 1240.Dq Li \&! , 1241if any). 1242To include a 1243.Dq Li - , 1244make it the first or last character listed. 1245.Ss Built-in Commands 1246This section lists the commands which 1247are built-in because they need to perform some operation 1248that cannot be performed by a separate process. In addition to 1249these, built-in versions of the 1250.Xr printf 1 1251and 1252.Xr test 1 1253commands are provided for efficiency. 1254.Bl -tag -width indent 1255.It Ic \&: 1256A null command that returns a 0 (true) exit value. 1257.It Ic \&. Ar file 1258The commands in the specified file are read and executed by the shell. 1259If 1260.Ar file 1261contains any 1262.Dq / 1263characters, it is used as is. Otherwise, the shell searches the 1264.Ev PATH 1265for the file. If it is not found in the 1266.Ev PATH , 1267it is sought in the current working directory. 1268.It Ic alias Op Ar name ... 1269.It Ic alias Op Ar name Ns = Ns Ar string ... 1270If 1271.Ar name Ns = Ns Ar string 1272is specified, the shell defines the alias 1273.Ar name 1274with value 1275.Ar string . 1276If just 1277.Ar name 1278is specified, the value of the alias 1279.Ar name 1280is printed. 1281With no arguments, the 1282.Ic alias 1283built-in command prints the names and values of all defined aliases 1284(see 1285.Ic unalias ) . 1286Alias values are written with appropriate quoting so that they are 1287suitable for re-input to the shell. 1288.It Ic bg Op Ar job ... 1289Continue the specified jobs 1290(or the current job if no jobs are given) 1291in the background. 1292.It Ic builtin Ar cmd Op Ar arg ... 1293Execute the specified built-in command, 1294.Ar cmd . 1295This is useful when the user wishes to override a shell function 1296with the same name as a built-in command. 1297.It Ic bind Oo Fl aeklrsv Oc Oo Ar key Oo Ar command Oc Oc 1298List or alter key bindings for the line editor. 1299This command is documented in 1300.Xr editrc 5 . 1301.It Ic cd Oo Fl LP Oc Op Ar directory 1302Switch to the specified 1303.Ar directory , 1304or to the directory specified in the 1305.Ev HOME 1306environment variable if no 1307.Ar directory 1308is specified. 1309If 1310.Ar directory 1311does not begin with 1312.Pa / , \&. , 1313or 1314.Pa .. , 1315then the directories listed in the 1316.Ev CDPATH 1317variable will be 1318searched for the specified 1319.Ar directory . 1320If 1321.Ev CDPATH 1322is unset, the current directory is searched. 1323The format of 1324.Ar CDPATH 1325is the same as that of 1326.Ev PATH . 1327In an interactive shell, 1328the 1329.Ic cd 1330command will print out the name of the directory 1331that it actually switched to 1332if this is different from the name that the user gave. 1333These may be different either because the 1334.Ev CDPATH 1335mechanism was used or because a symbolic link was crossed. 1336.Pp 1337If the 1338.Fl P 1339option is specified, 1340.Pa .. 1341is handled physically and symbolic links are resolved before 1342.Pa .. 1343components are processed. 1344If the 1345.Fl L 1346option is specified, 1347.Pa .. 1348is handled logically. 1349This is the default. 1350.It Ic chdir 1351A synonym for the 1352.Ic cd 1353built-in command. 1354.It Ic command Oo Fl p Oc Op Ar utility Op Ar argument ... 1355Execute the specified 1356.Ar utility 1357as a simple command (see the 1358.Sx Simple Commands 1359section). 1360.Pp 1361If the 1362.Fl p 1363option is specified, the command search is performed using a 1364default value of 1365.Ev PATH 1366that is guaranteed to find all of the standard utilities. 1367.It Ic echo Oo Fl e | n Oc Op Ar string 1368Print 1369.Ar string 1370to the standard output with a newline appended. 1371.Bl -tag -width indent 1372.It Fl n 1373Suppress the output of the trailing newline. 1374.It Fl e 1375Process C-style backslash escape sequences. 1376.Ic echo 1377understands the following character escapes: 1378.Bl -tag -width indent 1379.It \ea 1380Alert (ring the terminal bell) 1381.It \eb 1382Backspace 1383.It \ec 1384Suppress the trailing newline (this has the side-effect of truncating the 1385line if it is not the last character) 1386.It \ee 1387The ESC character (ASCII 0x1b) 1388.It \ef 1389Formfeed 1390.It \en 1391Newline 1392.It \er 1393Carriage return 1394.It \et 1395Horizontal tab 1396.It \ev 1397Vertical tab 1398.It \e\e 1399Literal backslash 1400.It \e0nnn 1401(Zero) The character whose octal value is nnn 1402.El 1403.Pp 1404If 1405.Ar string 1406is not enclosed in quotes then the backslash itself must be escaped 1407with a backslash to protect it from the shell. For example 1408.Bd -literal -offset indent 1409$ echo -e "a\evb" 1410a 1411 b 1412$ echo -e a\e\evb 1413a 1414 b 1415$ echo -e "a\e\eb" 1416a\eb 1417$ echo -e a\e\e\e\eb 1418a\eb 1419.Ed 1420.El 1421.Pp 1422Only one of the 1423.Fl e 1424and 1425.Fl n 1426options may be specified. 1427.It Ic eval Ar string ... 1428Concatenate all the arguments with spaces. 1429Then re-parse and execute the command. 1430.It Ic exec Op Ar command Op arg ... 1431Unless 1432.Ar command 1433is omitted, 1434the shell process is replaced with the specified program 1435(which must be a real program, not a shell built-in command or function). 1436Any redirections on the 1437.Ic exec 1438command are marked as permanent, 1439so that they are not undone when the 1440.Ic exec 1441command finishes. 1442.It Ic exit Op Ar exitstatus 1443Terminate the shell process. 1444If 1445.Ar exitstatus 1446is given 1447it is used as the exit status of the shell; 1448otherwise the exit status of the preceding command is used. 1449.It Ic export Oo Fl p Oc Op Ar name ... 1450The specified names are exported so that they will 1451appear in the environment of subsequent commands. 1452The only way to un-export a variable is to 1453.Ic unset 1454it. 1455The shell allows the value of a variable to be set 1456at the same time as it is exported by writing 1457.Bd -literal -offset indent 1458export name=value 1459.Ed 1460.Pp 1461With no arguments the export command lists the names 1462of all exported variables. 1463If the 1464.Fl p 1465option is specified, the exported variables are printed as 1466.Dq Ic export Ar name Ns = Ns Ar value 1467lines, suitable for re-input to the shell. 1468.It Ic fc Oo Fl e Ar editor Oc Op Ar first Op Ar last 1469.It Ic fc Fl l Oo Fl nr Oc Op Ar first Op Ar last 1470.It Ic fc Fl s Oo Ar old Ns = Ns Ar new Oc Op Ar first 1471The 1472.Ic fc 1473built-in command lists, or edits and re-executes, 1474commands previously entered to an interactive shell. 1475.Bl -tag -width indent 1476.It Fl e Ar editor 1477Use the editor named by 1478.Ar editor 1479to edit the commands. 1480The editor string is a command name, 1481subject to search via the 1482.Ev PATH 1483variable. 1484The value in the 1485.Ev FCEDIT 1486variable is used as a default when 1487.Fl e 1488is not specified. 1489If 1490.Ev FCEDIT 1491is null or unset, the value of the 1492.Ev EDITOR 1493variable is used. 1494If 1495.Ev EDITOR 1496is null or unset, 1497.Xr ed 1 1498is used as the editor. 1499.It Fl l No (ell) 1500List the commands rather than invoking 1501an editor on them. The commands are written in the 1502sequence indicated by the first and last operands, as 1503affected by 1504.Fl r , 1505with each command preceded by the command number. 1506.It Fl n 1507Suppress command numbers when listing with 1508.Fl l . 1509.It Fl r 1510Reverse the order of the commands listed 1511(with 1512.Fl l ) 1513or edited 1514(with neither 1515.Fl l 1516nor 1517.Fl s ) . 1518.It Fl s 1519Re-execute the command without invoking an editor. 1520.It Ar first 1521.It Ar last 1522Select the commands to list or edit. 1523The number of previous commands that can be accessed 1524are determined by the value of the 1525.Ev HISTSIZE 1526variable. 1527The value of 1528.Ar first 1529or 1530.Ar last 1531or both are one of the following: 1532.Bl -tag -width indent 1533.It Ar [+]num 1534A positive number representing a command number; 1535command numbers can be displayed with the 1536.Fl l 1537option. 1538.It Ar -num 1539A negative decimal number representing the 1540command that was executed 1541.Ar num 1542of 1543commands previously. 1544For example, -1 is the immediately previous command. 1545.It Ar string 1546A string indicating the most recently entered command 1547that begins with that string. 1548If the 1549.Ar old=new 1550operand is not also specified with 1551.Fl s , 1552the string form of the first operand cannot contain an embedded equal sign. 1553.El 1554.El 1555.Pp 1556The following environment variables affect the execution of 1557.Ic fc : 1558.Bl -tag -width indent 1559.It Ev FCEDIT 1560Name of the editor to use. 1561.It Ev HISTSIZE 1562The number of previous commands that are accessible. 1563.El 1564.It Ic fg Op Ar job 1565Move the specified 1566.Ar job 1567or the current job to the foreground. 1568.It Ic getopts Ar optstring Ar var 1569The POSIX 1570.Ic getopts 1571command. 1572The 1573.Ic getopts 1574command deprecates the older 1575.Xr getopt 1 1576command. 1577The first argument should be a series of letters, each possibly 1578followed by a colon which indicates that the option takes an argument. 1579The specified variable is set to the parsed option. The index of 1580the next argument is placed into the shell variable 1581.Ev OPTIND . 1582If an option takes an argument, it is placed into the shell variable 1583.Ev OPTARG . 1584If an invalid option is encountered, 1585.Ev var 1586is set to 1587.Dq Li \&? . 1588It returns a false value (1) when it encounters the end of the options. 1589.It Ic hash Oo Fl rv Oc Op Ar command ... 1590The shell maintains a hash table which remembers the locations of commands. 1591With no arguments whatsoever, the 1592.Ic hash 1593command prints out the contents of this table. 1594Entries which have not been looked at since the last 1595.Ic cd 1596command are marked with an asterisk; 1597it is possible for these entries to be invalid. 1598.Pp 1599With arguments, the 1600.Ic hash 1601command removes each specified 1602.Ar command 1603from the hash table (unless they are functions) and then locates it. 1604With the 1605.Fl v 1606option, 1607.Ic hash 1608prints the locations of the commands as it finds them. 1609The 1610.Fl r 1611option causes the 1612.Ic hash 1613command to delete all the entries in the hash table except for functions. 1614.It Ic jobid Op Ar job 1615Print the process id's of the processes in the specified 1616.Ar job . 1617If the 1618.Ar job 1619argument is omitted, use the current job. 1620.It Ic jobs Oo Fl ls Oc Op Ar job ... 1621Print information about the specified jobs, or all jobs if no 1622.Ar job 1623argument is given. 1624The information printed includes job ID, status and command name. 1625.Pp 1626If the 1627.Fl l 1628option is specified, the PID of each job is also printed. 1629If the 1630.Fl s 1631option is specified, only the PIDs of the jobs are printed, one per line. 1632.It Ic pwd Op Fl LP 1633Print the path of the current directory. The built-in command may 1634differ from the program of the same name because the 1635built-in command remembers what the current directory 1636is rather than recomputing it each time. This makes 1637it faster. However, if the current directory is 1638renamed, 1639the built-in version of 1640.Xr pwd 1 1641will continue to print the old name for the directory. 1642.Pp 1643If the 1644.Fl P 1645option is specified, symbolic links are resolved. 1646If the 1647.Fl L 1648option is specified, the shell's notion of the current directory 1649is printed (symbolic links are not resolved). 1650This is the default. 1651.It Ic read Oo Fl p Ar prompt Oc Oo Fl t Ar timeout Oc Oo Fl er Oc Ar variable ... 1652The 1653.Ar prompt 1654is printed if the 1655.Fl p 1656option is specified 1657and the standard input is a terminal. Then a line is 1658read from the standard input. The trailing newline 1659is deleted from the line and the line is split as 1660described in the section on 1661.Sx White Space Splitting (Field Splitting) 1662above, and 1663the pieces are assigned to the variables in order. 1664If there are more pieces than variables, the remaining 1665pieces (along with the characters in 1666.Ev IFS 1667that separated them) 1668are assigned to the last variable. 1669If there are more variables than pieces, the remaining 1670variables are assigned the null string. 1671.Pp 1672Backslashes are treated specially, unless the 1673.Fl r 1674option is 1675specified. If a backslash is followed by 1676a newline, the backslash and the newline will be 1677deleted. If a backslash is followed by any other 1678character, the backslash will be deleted and the following 1679character will be treated as though it were not in 1680.Ev IFS , 1681even if it is. 1682.Pp 1683If the 1684.Fl t 1685option is specified and the 1686.Ar timeout 1687elapses before any input is supplied, 1688the 1689.Ic read 1690command will return without assigning any values. 1691The 1692.Ar timeout 1693value may optionally be followed by one of 1694.Dq s , 1695.Dq m 1696or 1697.Dq h 1698to explicitly specify seconds, minutes or hours. 1699If none is supplied, 1700.Dq s 1701is assumed. 1702.Pp 1703The 1704.Fl e 1705option exists only for backward compatibility with older scripts. 1706.It Ic readonly Oo Fl p Oc Op Ar name ... 1707Each specified 1708.Ar name 1709is marked as read only, 1710so that it cannot be subsequently modified or unset. 1711The shell allows the value of a variable to be set 1712at the same time as it is marked read only 1713by using the following form: 1714.Bd -literal -offset indent 1715readonly name=value 1716.Ed 1717.Pp 1718With no arguments the 1719.Ic readonly 1720command lists the names of all read only variables. 1721If the 1722.Fl p 1723option is specified, the read-only variables are printed as 1724.Dq Ic readonly Ar name Ns = Ns Ar value 1725lines, suitable for re-input to the shell. 1726.It Ic set Oo Fl /+abCEefIimnpTuVvx Oc Oo Fl /+o Ar longname Oc Oo 1727.Fl c Ar string Oc Op Fl - Ar arg ... 1728The 1729.Ic set 1730command performs three different functions: 1731.Bl -item 1732.It 1733With no arguments, it lists the values of all shell variables. 1734.It 1735If options are given, 1736either in short form or using the long 1737.Dq Fl /+o Ar longname 1738form, 1739it sets or clears the specified options as described in the section called 1740.Sx Argument List Processing . 1741.It 1742If the 1743.Dq Fl - 1744option is specified, 1745.Ic set 1746will replace the shell's positional parameters with the subsequent 1747arguments. 1748If no arguments follow the 1749.Dq Fl - 1750option, 1751all the positional parameters will be cleared, 1752which is equivalent to executing the command 1753.Dq Li shift $# . 1754The 1755.Dq Fl - 1756flag may be omitted when specifying arguments to be used 1757as positional replacement parameters. 1758This is not recommended, 1759because the first argument may begin with a dash 1760.Pq Li - 1761or a plus 1762.Pq Li + , 1763which the 1764.Ic set 1765command will interpret as a request to enable or disable options. 1766.El 1767.It Ic setvar Ar variable Ar value 1768Assigns the specified 1769.Ar value 1770to the specified 1771.Ar variable . 1772.Ic Setvar 1773is intended to be used in functions that 1774assign values to variables whose names are passed as parameters. 1775In general it is better to write 1776.Bd -literal -offset indent 1777variable=value 1778.Ed 1779rather than using 1780.Ic setvar . 1781.It Ic shift Op Ar n 1782Shift the positional parameters 1783.Ar n 1784times, or once if 1785.Ar n 1786is not specified. 1787A shift sets the value of $1 to the value of $2, 1788the value of $2 to the value of $3, and so on, 1789decreasing the value of $# by one. 1790If there are zero positional parameters, shifting does not do anything. 1791.It Ic trap Oo Ar action Oc Ar signal ... 1792Cause the shell to parse and execute 1793.Ar action 1794when any specified 1795.Ar signal 1796is received. 1797The signals are specified by name or number. 1798In addition, the pseudo-signal 1799.Cm EXIT 1800may be used to specify an action that is performed when the shell terminates. 1801The 1802.Ar action 1803may be null or omitted; 1804the former causes the specified signal to be ignored 1805and the latter causes the default action to be taken. 1806When the shell forks off a subshell, 1807it resets trapped (but not ignored) signals to the default action. 1808The 1809.Ic trap 1810command has no effect on signals that were ignored on entry to the shell. 1811.It Ic type Op Ar name ... 1812Interpret each 1813.Ar name 1814as a command and print the resolution of the command search. 1815Possible resolutions are: 1816shell keyword, alias, shell built-in command, command, tracked alias 1817and not found. 1818For aliases the alias expansion is printed; 1819for commands and tracked aliases 1820the complete pathname of the command is printed. 1821.It Ic ulimit Oo Fl HSabcdflmnstuv Oc Op Ar limit 1822Set or display resource limits (see 1823.Xr getrlimit 2 ) . 1824If 1825.Ar limit 1826is specified, the named resource will be set; 1827otherwise the current resource value will be displayed. 1828.Pp 1829If 1830.Fl H 1831is specified, the hard limits will be set or displayed. 1832While everybody is allowed to reduce a hard limit, 1833only the superuser can increase it. 1834The 1835.Fl S 1836option 1837specifies the soft limits instead. When displaying limits, 1838only one of 1839.Fl S 1840or 1841.Fl H 1842can be given. 1843The default is to display the soft limits, 1844and to set both the hard and the soft limits. 1845.Pp 1846Option 1847.Fl a 1848causes the 1849.Ic ulimit 1850command to display all resources. 1851The parameter 1852.Ar limit 1853is not acceptable in this mode. 1854.Pp 1855The remaining options specify which resource value is to be 1856displayed or modified. 1857They are mutually exclusive. 1858.Bl -tag -width indent 1859.It Fl b Ar sbsize 1860The maximum size of socket buffer usage, in bytes. 1861.It Fl c Ar coredumpsize 1862The maximal size of core dump files, in 512-byte blocks. 1863.It Fl d Ar datasize 1864The maximal size of the data segment of a process, in kilobytes. 1865.It Fl f Ar filesize 1866The maximal size of a file, in 512-byte blocks. 1867.It Fl l Ar lockedmem 1868The maximal size of memory that can be locked by a process, in 1869kilobytes. 1870.It Fl m Ar memoryuse 1871The maximal resident set size of a process, in kilobytes. 1872.It Fl n Ar nofiles 1873The maximal number of descriptors that could be opened by a process. 1874.It Fl s Ar stacksize 1875The maximal size of the stack segment, in kilobytes. 1876.It Fl t Ar time 1877The maximal amount of CPU time to be used by each process, in seconds. 1878.It Fl u Ar userproc 1879The maximal number of simultaneous processes for this user ID. 1880.It Fl v Ar virtualmem 1881The maximal virtual size of a process, in kilobytes. 1882.El 1883.It Ic umask Op Ar mask 1884Set the file creation mask (see 1885.Xr umask 2 ) 1886to the octal value specified by 1887.Ar mask . 1888If the argument is omitted, the current mask value is printed. 1889.It Ic unalias Oo Fl a Oc Op Ar name 1890If 1891.Ar name 1892is specified, the shell removes that alias. 1893If 1894.Fl a 1895is specified, all aliases are removed. 1896.It Ic unset Oo Fl fv Oc Ar name ... 1897The specified variables or functions are unset and unexported. 1898If the 1899.Fl v 1900option is specified or no options are given, the 1901.Ar name 1902arguments are treated as variable names. 1903If the 1904.Fl f 1905option is specified, the 1906.Ar name 1907arguments are treated as function names. 1908.It Ic wait Op Ar job 1909Wait for the specified 1910.Ar job 1911to complete and return the exit status of the last process in the 1912.Ar job . 1913If the argument is omitted, wait for all jobs to complete 1914and return an exit status of zero. 1915.El 1916.Ss Commandline Editing 1917When 1918.Nm 1919is being used interactively from a terminal, the current command 1920and the command history 1921(see 1922.Ic fc 1923in 1924.Sx Built-in Commands ) 1925can be edited using vi-mode command line editing. 1926This mode uses commands similar 1927to a subset of those described in the vi man page. 1928The command 1929.Dq Li set -o vi 1930(or 1931.Dq Li set -V ) 1932enables vi-mode editing and places 1933.Nm 1934into vi insert mode. With vi-mode enabled, 1935.Nm 1936can be switched between insert mode and command mode by typing 1937.Aq ESC . 1938Hitting 1939.Aq return 1940while in command mode will pass the line to the shell. 1941.Pp 1942Similarly, the 1943.Dq Li set -o emacs 1944(or 1945.Dq Li set -E ) 1946command can be used to enable a subset of 1947emacs-style command line editing features. 1948.Sh SEE ALSO 1949.Xr builtin 1 , 1950.Xr echo 1 , 1951.Xr expr 1 , 1952.Xr printf 1 , 1953.Xr pwd 1 , 1954.Xr test 1 1955.Sh HISTORY 1956A 1957.Nm 1958command appeared in 1959.At v1 . 1960