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 some number of
`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 combination
of the current process number and random letters.
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:
-V
Print the version and exit.
-d
Make a directory instead of a file.
-p directory
Use the specified
directory
as a prefix when generating the temporary filename.
The
directory
will be overridden by the user's
TMPDIR
environment variable if it is set.
This option implies the
-t
flag (see below).
-q
Fail silently if an error occurs.
This is useful if
a script does not want error output to go to standard error.
-t
Generate a path rooted in a temporary directory.
This directory is chosen as follows:
*
If the user's
TMPDIR
environment variable is set, the directory contained therein is used.
*
Otherwise, if the
-p
flag was given the specified directory is used.
*
If none of the above apply,
/tmp
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
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.
The
mktemp
utility
exits with a value of 0 on success or 1 on 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.
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.
In some 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 -t example.XXXXXXXXXX` && {
# Safe to use $TMPFILE in this block
echo data > $TMPFILE
...
rm -f $TMPFILE
}
ENVIRONMENT
TMPDIR
directory in which to place the temporary file when in
-t
mode
