MKTEMP(1) | General Commands Manual | MKTEMP(1) |
mktemp
— make
temporary filename (unique)
mktemp |
[-dqtu ] [-p
directory] [template] |
The mktemp
utility takes the given
filename template and overwrites a portion of it to
create a unique filename. The template may be any
filename with at least six ‘Xs
’
appended to it, for example /tmp/tfile.XXXXXXXXXX.
If no template is specified a default of
tmp.XXXXXXXXXX is used and the
-t
flag is implied (see below).
The trailing ‘Xs
’ are
replaced with a unique digit and letter combination. The name chosen depends
both on the number of ‘Xs
’ in the
template and the number of collisions with
pre-existing files. The number of unique filenames
mktemp
can return depends on the number of
‘Xs
’ provided; ten
‘Xs
’ will result in
mktemp
testing roughly 26 ** 10 combinations.
If mktemp
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
-u
flag is given) and the filename is printed to
standard output.
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 mktemp
be used
instead.
The options are as follows:
-d
-p
directoryTMPDIR
environment variable if it is set. This option implies the
-t
flag (see below).-q
-t
TMPDIR
environment variable is
set, the directory contained therein is used.-p
flag was given the
specified directory is used.In this mode, the template (if specified) should be a directory component (as opposed to a full path) and thus should not contain any forward slashes.
-u
mktemp
exits. This is slightly better than
mktemp(3) but still
introduces a race condition. Use of this option is not encouraged.The mktemp
utility exits with a value of 0
on success or 1 on failure.
TMPDIR
-t
modeThe 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/example.XXXXXXXXXX` || exit 1 echo "program output" >> $TMPFILE
The same fragment with support for a user's
TMPDIR
environment variable can be written as
follows.
TMPFILE=`mktemp -t example.XXXXXXXXXX` || exit 1 echo "program output" >> $TMPFILE
This can be further simplified if we don't care about the actual
name of the temporary file. In this case the -t
flag
is implied.
TMPFILE=`mktemp` || exit 1 echo "program output" >> $TMPFILE
In some cases, it may be desirable to use a default temporary
directory other than /tmp. In this example the
temporary file will be created in /extra/tmp unless
the user's TMPDIR
environment variable specifies
otherwise.
TMPFILE=`mktemp -p /extra/tmp example.XXXXXXXXXX` || exit 1 echo "program output" >> $TMPFILE
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.
TMP1=`mktemp -t example.1.XXXXXXXXXX` || exit 1 TMP2=`mktemp -t example.2.XXXXXXXXXX` if [ $? -ne 0 ]; then rm -f $TMP1 exit 1 fi
Or perhaps you don't want to exit if
mktemp
is unable to create the file. In this case
you can protect that part of the script thusly.
TMPFILE=`mktemp -q -t example.XXXXXXXXXX` && { # Safe to use $TMPFILE in this block echo data > $TMPFILE ... rm -f $TMPFILE }
One of the following error messages may be displayed if
mktemp
does not succeed and the
-q
option was not specified:
insufficient
number of Xs in template
Xs
’ at the end.template
must not contain directory separators in -t mode
-t
option was specified.cannot
make temp dir
mktemp
was unable to create the temporary
directory for any of the reasons specified by
mkdtemp(3).cannot
make temp file
mktemp
was unable to create the temporary file for
any of the reasons specified by
mkstemp(3).cannot
allocate memory
mktemp
was unable to allocate memory for any of
the reasons specified by
malloc(3).The mktemp
utility first appeared in
OpenBSD 2.1.
August 7, 2013 | OpenBSD-6.1 |