NAME
mktemp —
make temporary file name
(unique)
SYNOPSIS
mktemp |
[-dqu]
[-p
tmpdir] {-t
prefix | template ...} |
DESCRIPTION
The
mktemp 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.
The name of each temporary file or directory is derived from a template that
includes several trailing ‘
X
’ characters,
such as
/tmp/prefix.XXXX. The trailing
‘
X
’ characters in the template are
replaced by unique values derived from the current process number and
additional letters or numbers. Any ‘
X
’
characters other than at the end of the template are taken literally. The
number of unique file names
mktemp can return depends on the
number of trailing ‘
Xs
’ in the template;
six ‘
Xs
’ will result in
mktemp testing roughly 26 ** 6 combinations.
The templates used to create the unique names are derived from the
-t prefix option, or the
template arguments, possibly modified by other options.
Any number of temporary files or directories may be created in a single
invocation using multiple
template arguments. It is
possible to specify both a
-t prefix
option and one or more
template arguments, but this is
not usually done.
If neither a
-t prefix option, nor any
template arguments are specified, then the default is
equivalent to
-t mktemp
.
If
mktemp can successfully generate a unique file name, the
file is created with mode 0600 (unless the
-u flag is given)
and the filename is printed to standard output.
OPTIONS
The available options are as follows:
-
-
- -d
- Make a directory instead of a file.
-
-
- -p
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 -t
prefix option, and those created as a result of a
template argument.
If the -p tmpdir option is not
specified, then temporary files created as a result of a
-t prefix option will use a
default temporary directory (as described under the -t
option), but temporary files created as a result of a
template argument will not use a default temporary
directory (so they will be created relative to the current working
directory, if the template does not begin with
‘
/
’).
-
-
- -t
prefix
- Generate a template using an appropriate directory name,
followed by the supplied prefix, followed by
‘
.XXXXXXXX
’. Any
‘X
’ characters in the supplied
prefix are taken literally, but the trailing
‘X
’ characters in the appended
‘.XXXXXXXX
’ are replaced by unique
values.
The directory name used for the template generated by the
-t prefix option is taken from the
-p tmpdir option, or from the
TMPDIR
environment variable, or
/tmp as a default.
If one or more template arguments are used in addition
to the -t prefix option, then the
prefix does not apply to the
template arguments.
-
-
- -q
- Fail silently if an error occurs. This is useful if a
script does not want error output to go to standard error.
-
-
- -u
- Operate in “unsafe” mode. The temp file will be
unlinked before mktemp exits. This is slightly better
than mktemp(3) but still
introduces a race condition. Use of this option is not encouraged.
NOTES
mktemp takes care to create the files or directories in a way
that is safe from race conditions (provided the
-u option is
not used).
Traditionally, without
mktemp, 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
mktemp be used instead of simpler schemes.
Care should be taken to ensure that it is appropriate to use an environment
variable potentially supplied by the user.
EXIT STATUS
The
mktemp utility exits with a value of 0 on success, and 1
on any failure.
EXAMPLES
The following
sh(1) fragment
illustrates a simple use of
mktemp where the script should
quit if it cannot get a safe temporary file.
TMPFILE=`mktemp /tmp/${0##*/}.XXXXXX` || exit 1
echo "program output" >> $TMPFILE
To allow the use of $TMPDIR:
TMPFILE=`mktemp -t ${0##*/}` || exit 1
echo "program output" >> $TMPFILE
In this case, we want the script to catch the error itself.
TMPFILE=`mktemp -q /tmp/${0##*/}.XXXXXX`
if [ $? -ne 0 ]; then
echo "$0: Can't create temp file, exiting..."
exit 1
fi
SEE ALSO
mkdtemp(3),
mkstemp(3),
mktemp(3),
environ(7)
HISTORY
The
mktemp utility appeared in
NetBSD
1.5. It was imported from
FreeBSD, and the idea
and the manual page were taken from
OpenBSD.