usr.bin/mktemp/mktemp.1

.\"	$OpenBSD: mktemp.1,v 1.30 2019/01/25 00:19:26 millert Exp $
.\"
.\" Copyright (c) 1996, 2000, 2001, 2003, 2010, 2013
.\"	Todd C. Miller <millert@openbsd.org>
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.Dd $Mdocdate: January 25 2019 $
.Dt MKTEMP 1
.Os
.Sh NAME
.Nm mktemp
.Nd make temporary filename (unique)
.Sh SYNOPSIS
.Nm mktemp
.Op Fl dqtu
.Op Fl p Ar directory
.Op Ar template
.Sh DESCRIPTION
The
.Nm mktemp
utility takes the given filename
.Ar template
and overwrites a portion of it to create a unique filename.
The
.Ar template
may be any filename with at least six
.Ql X Ns s
appended
to it, for example
.Pa /tmp/tfile.XXXXXXXXXX .
If no
.Ar template
is specified a default of
.Pa tmp.XXXXXXXXXX
is used and the
.Fl t
flag is implied (see below).
.Pp
The trailing
.Ql X Ns s
are replaced with a unique digit and letter combination.
The name chosen depends both on the number of
.Ql X Ns s
in the
.Ar template
and the number of collisions with pre-existing files.
The number of unique filenames
.Nm
can return depends on the number of
.Ql X Ns s
provided; ten
.Ql X Ns s
will
result in
.Nm
testing roughly 26 ** 10 combinations.
.Pp
If
.Nm
can successfully generate a unique filename, the file (or directory)
is created with file permissions such that it is only readable and writable
by its owner (unless the
.Fl u
flag is given) and the filename is printed to standard output.
.Pp
.Nm mktemp
is provided to allow shell scripts to safely use temporary files.
Traditionally, many shell scripts take the name of the program with
the PID as a suffix and use that as a temporary filename.
This kind of naming scheme is predictable and the race condition it creates
is easy for an attacker to win.
A safer, though still inferior approach
is to make a temporary directory using the same naming scheme.
While this does allow one to guarantee that a temporary file will not be
subverted, it still allows a simple denial of service attack.
For these reasons it is suggested that
.Nm
be used instead.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl d
Make a directory instead of a file.
.It Fl p Ar directory
Use the specified
.Ar directory
as a prefix when generating the temporary filename.
The
.Ar directory
will be overridden by the user's
.Ev TMPDIR
environment variable if it is set.
This option implies the
.Fl t
flag (see below).
.It Fl q
Fail silently if an error occurs.
This is useful if
a script does not want error output to go to standard error.
.It Fl t
Generate a path rooted in a temporary directory.
This directory is chosen as follows:
.Bl -bullet
.It
If the user's
.Ev TMPDIR
environment variable is set, the directory contained therein is used.
.It
Otherwise, if the
.Fl p
flag was given the specified directory is used.
.It
If none of the above apply,
.Pa /tmp
is used.
.El
.Pp
In this mode, the
.Ar template
(if specified) should be a directory component (as opposed to a full path)
and thus should not contain any forward slashes.
.It Fl u
Operate in
.Dq unsafe
mode.
The temp file will be unlinked before
.Nm
exits.
This is slightly better than
.Xr mktemp 3
but still introduces a race condition.
Use of this option is not encouraged.
.El
.Sh ENVIRONMENT
.Bl -tag -width TMPDIR
.It Ev TMPDIR
directory in which to place the temporary file when in
.Fl t
mode
.El
.Sh EXIT STATUS
.Ex -std mktemp
.Sh EXAMPLES
The following
.Xr sh 1
fragment illustrates a simple use of
.Nm
where the script should quit if it cannot get a safe
temporary file.
.Bd -literal -offset indent
TMPFILE=`mktemp /tmp/example.XXXXXXXXXX` || exit 1
echo "program output" >> $TMPFILE
.Ed
.Pp
The same fragment with support for a user's
.Ev TMPDIR
environment variable can be written as follows.
.Bd -literal -offset indent
TMPFILE=`mktemp -t example.XXXXXXXXXX` || exit 1
echo "program output" >> $TMPFILE
.Ed
.Pp
This can be further simplified if we don't care about the actual name of
the temporary file.
In this case the
.Fl t
flag is implied.
.Bd -literal -offset indent
TMPFILE=`mktemp` || exit 1
echo "program output" >> $TMPFILE
.Ed
.Pp
In some cases, it may be desirable to use a default temporary directory
other than
.Pa /tmp .
In this example the temporary file will be created in
.Pa /extra/tmp
unless the user's
.Ev TMPDIR
environment variable specifies otherwise.
.Bd -literal -offset indent
TMPFILE=`mktemp -p /extra/tmp example.XXXXXXXXXX` || exit 1
echo "program output" >> $TMPFILE
.Ed
.Pp
In other cases, we want the script to catch the error.
For instance, if we attempt to create two temporary files and
the second one fails we need to remove the first before exiting.
.Bd -literal -offset indent
TMP1=`mktemp -t example.1.XXXXXXXXXX` || exit 1
TMP2=`mktemp -t example.2.XXXXXXXXXX`
if [ $? -ne 0 ]; then
	rm -f $TMP1
	exit 1
fi
.Ed
.Pp
Or perhaps you don't want to exit if
.Nm
is unable to create the file.
In this case you can protect that part of the script thusly.
.Bd -literal -offset indent
TMPFILE=`mktemp -q -t example.XXXXXXXXXX` && {
	# Safe to use $TMPFILE in this block
	echo data > $TMPFILE
	...
	rm -f $TMPFILE
}
.Ed
.Sh DIAGNOSTICS
One of the following error messages may be displayed if
.Nm
does not succeed and the
.Fl q
option was not specified:
.Bl -tag -width indent
.It Li "insufficient number of Xs in template"
The specified
.Ar template
contained fewer than six
.Ql X Ns s
at the end.
.It Li "template must not contain directory separators in -t mode"
The
.Ar template
contained one or more directory components and the
.Fl t
option was specified.
.It Li "cannot make temp dir"
.Nm
was unable to create the temporary directory for any of the reasons
specified by
.Xr mkdtemp 3 .
.It Li "cannot make temp file"
.Nm
was unable to create the temporary file for any of the reasons
specified by
.Xr mkstemp 3 .
.It Li "cannot allocate memory"
.Nm
was unable to allocate memory for any of the reasons specified by
.Xr malloc 3 .
.El
.Sh SEE ALSO
.Xr mktemp 3
.Sh HISTORY
The
.Nm
utility first appeared in
.Ox 2.1 .