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\" $FreeBSD: src/usr.bin/mktemp/mktemp.1,v 1.5 1999/08/28 01:04:13 peter Exp $
\"
Dd July 25, 2021
Dt MKTEMP 1
Os
Sh NAME
Nm mktemp
Nd make temporary file name (unique)
Sh SYNOPSIS
Nm mktemp
Op Fl dqu
Op Fl p Ar tmpdir
Bro
Fl t Ar prefix
No |
Ar template ...
Brc
Sh DESCRIPTION
The
Nm
utility
is provided to allow shell scripts to safely use temporary files.
It creates temporary files or directories using unique names,
and prints the names.
Pp
The name of each temporary file or directory is derived from a
template that includes several trailing
Ql X
characters, such as
Pa /tmp/prefix.XXXX .
The trailing
Ql X
characters in the template are replaced with a unique letter and number
combination.
Any
Ql X
characters other than at the end of the template are taken literally.
The number of unique file names
Nm
can return depends on the number of trailing
Ql X Ns s
in the template; six
Ql X Ns s
will result in
Nm
testing roughly 62 ** 6 (56800235584) combinations.
Pp
The templates used to create the unique names are derived from the
Fl t Ar prefix
option, or the
Ar template
arguments, possibly modified by other options.
Any number of temporary files or directories may be created
in a single invocation using multiple
Ar template
arguments.
It is possible to specify both a
Fl t Ar prefix
option and one or more
Ar template
arguments,
but this is not usually done.
Pp
If neither a
Fl t Ar prefix
option, nor any
Ar template
arguments are specified, then the default is equivalent to
Fl t Li mktemp .
Pp
If
Nm
can successfully generate a unique file name, the file
is created with mode 0600 (unless the
Fl u
flag is given) and the filename is printed to standard output.
Sh OPTIONS
The available options are as follows:
Bl -tag -width indent
It Fl d
Make a directory instead of a file.
It Fl p Ar tmpdir
Specifies a directory in which temporary files should be created.
If this option is specified, then it applies to all temporary files,
including those created as a result of a
Fl t Ar prefix
option, and those created as a result of a
Ar template
argument.
Pp
If the
Fl p Ar tmpdir
option is not specified, then
temporary files created as a result of a
Fl t Ar prefix
option will use a default temporary directory
(as described under the
Fl t
option),
but temporary files created as a result of a
Ar template
argument will not use a default temporary directory
(so they will be created relative to the current working directory, if the
Ar template
does not begin with
Ql \&/ ) .
It Fl t Ar prefix
Generate a template using an appropriate directory name, followed by the
supplied
Ar prefix ,
followed by
Ql \&.XXXXXXXX .
Any
Ql X
characters in the supplied
Ar prefix
are taken literally, but the trailing
Ql X
characters in the appended
Ql \&.XXXXXXXX
are replaced by unique values.
Pp
The directory name used for the template generated by the
Fl t Ar prefix
option is taken from the
Fl p Ar tmpdir
option, or from the
Ev TMPDIR
environment variable, or
Pa /tmp
as a default.
Pp
If one or more
Ar template
arguments are used in addition to the
Fl t Ar prefix
option, then the
Ar prefix
does not apply to the
Ar template
arguments.
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 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 NOTES
Nm
takes care to create the files or directories in a way that is
safe from race conditions (provided the
Fl u
option is not used).
Pp
Traditionally, without
Nm ,
many shell scripts created temporary files
using the name of the program with
the pid as a suffix.
This kind of naming scheme is predictable and creates a race condition that
allows an attacker to subvert the program by
creating a different file, directory, or symbolic link
under the same name.
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 recommended that
Nm
be used instead of simpler schemes.
Pp
Care should be taken to ensure that it is appropriate to use an
environment variable potentially supplied by the user.
Sh EXIT STATUS
The
Nm
utility exits with a value of 0 on success, and 1 on any failure.
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/${0##*/}.XXXXXX` || exit 1
echo "program output" >> $TMPFILE
Ed
Pp
To allow the use of $TMPDIR:
Bd -literal -offset indent
TMPFILE=`mktemp -t ${0##*/}` || exit 1
echo "program output" >> $TMPFILE
Ed
Pp
In this case, we want the script to catch the error itself.
Bd -literal -offset indent
TMPFILE=`mktemp -q /tmp/${0##*/}.XXXXXX`
if [ $? -ne 0 ]; then
       echo "$0: Can't create temp file, exiting..."
       exit 1
fi
Ed
Sh SEE ALSO
Xr mkdtemp 3 ,
Xr mkstemp 3 ,
Xr mktemp 3 ,
Xr environ 7
Sh HISTORY
The
Nm
utility appeared in
Nx 1.5 .
It was imported from
Fx ,
and the idea and the manual page were taken from
Ox .

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