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- make temporary filename
mktemp [-dtqu] [--directory] [--quiet] [--dry-run] [-p directory] [--suffix=suff] [--tmpdir[=dir]] [template]
The mktemp utility makes a temporary filename. To do this, mktemp takes the specified filename template and overwrites a portion of it to create a unique filename. See OPERANDS.
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 allows shell scripts to safely use temporary files. Traditionally, many shell scripts take the name of the program with the PID as a suffix and used 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 guarantees that a temporary file is not subverted, it still allows a simple denial of service attack. Use mktemp instead.
The following options are supported:
Make a directory instead of a file.
Use the specified directory as a prefix when generating the temporary filename. The directory is overridden by the user's TMPDIR environment variable if it is set. This option implies the -t flag.
Fail silently if an error occurs. This is useful if a script does not want error output to go to standard error.
Append suff to the template. suff must not contain a slash (/). This option is implied when the template ends in characters other than replaceable X characters, and does not remove the requirement to include such X characters.
'template' is relative to dir. If dir is not specified, the user's environment variable TMPDIR is used if set, else /tmp is used. This option does not support a template with an absolute name and unlike with the use of -t, a template may contain slashes.
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.
Operate in unsafe mode. The temp file is unlinked before mktemp exits. This is slightly better than mktemp(3C), but still introduces a race condition. Use of this option is discouraged.
The following operands are supported:
template can be any filename with three to six Xs included in it, for example /tmp/tfile.XXXXXX.
If template is not specified, a default of tmp.XXXXXX is used and the -t flag is implied.
If template has multiple series of Xs, the final series is used for the replacement text, unless --suffix specifies otherwise.
If template has characters after the final set of Xs, then the --suffix option is implied to be all such characters, unless explicitly specified.
Example 1 Using mktemp
The following example illustrates a simple use of mktemp in a sh(1) script. In this example, the script quits if it cannot get a safe temporary file.
TMPFILE=`mktemp /tmp/example.XXXXXX` if [ -z "$TMPFILE" ]; then exit 1; fi echo "program output" >> $TMPFILE
Example 2 Using mktemp to Support TMPDIR
The following example uses mktemp to support for a user's TMPDIR environment variable:
TMPFILE=`mktemp -t example.XXXXXX` if [ -z "$TMPFILE" ]; then exit 1; fi echo "program output" >> $TMPFILE
Example 3 Using mktemp Without Specifying the Name of the Temporary File
The following example uses mktemp without specifying the name of the temporary file. In this case the -t flag is implied.
TMPFILE=`mktemp` if [ -z "$TMPFILE" ]; then exit 1; fi echo "program output" >> $TMPFILE
Example 4 Using mktemp with a Default Temporary Directory Other than /tmp
The following example creates the temporary file in /extra/tmp unless the user's TMPDIR environment variable specifies otherwise:
TMPFILE=`mktemp -p /extra/tmp example.XXXXX` if [ -z "$TMPFILE" ]; then exit 1; fi echo "program output" >> $TMPFILE
Example 5 Using mktemp to Remove a File
The following example attempts to create two temporary files. If creation of the second temporary file fails, mktemp removes the first file before exiting:
TMP1=`mktemp -t example.1.XXXXXX` if [ -z "$TMP1" ]; then exit 1; fi TMP2=`mktemp -t example.2.XXXXXX` if [ -z "$TMP2" ]; then rm -f $TMP1 exit 1 fi
Example 6 Using mktemp
The following example does not exit if mktemp is unable to create the file. That part of the script has been protected.
TMPFILE=`mktemp -q -t example.XXXXXX` if [ ! -z "$TMPFILE" ] then # Safe to use $TMPFILE in this block echo data > $TMPFILE ... rm -f $TMPFILE fi
Example 7 Using mktemp with Suffix Option
The following command illustrates the use of the suffix option. The effect of this command is to create the temporary file ex.q5N.SUFF.
# mktemp --suffix=.SUFF ex.XXXXXX ex.q5Ngid.SUFF
Example 8 Using Suffix and Tmpdir Options
The following command illustrates the use of the suffix and tmpdir options.
# mktemp --tmpdir=$HOME --suffix=.bar foo.XXXXXX /root/foo.7ZaO_N.bar
Example 9 Using Directory and Suffix Options
The following command uses both the directory and suffix options.
# mktemp --directory --suffix=.bar foo.XXXXXX foo.GSaO3d.bar # ls -l drwx------ 2 root staff 512 Mar 19 2012 foo.GSaO.bar
Example 10 Supporting a Template with Non-Trailing Xs
The following command shows the use of the directory option with non-trailing X characters. In this command, the -–suffix=suff option is implied, where bar is used as the suffix.
# mktemp XXfooXXXXXXbar XXfooaFY0N6bar
Example 11 Using the Quiet and Tmpdir Options
The following command illustrates the use of the quiet and tmpdir options.
# mktemp --quiet --tmpdir=/tmp foo [No diagnostic message is returned]
Example 12 Using mktemp with Multiple Options
The following command combines the use of the dry-run, tmpdir, and suffix options.
# mktemp --dry-run --tmpdir=$HOME --suffix=SUFF /root/tmp.qdaGcOSUFF # ls -l /root/tmp.qdaGcOSUFF /root/tmp.qdaGcOSUFF: No such file or directory
See environ(5) for descriptions of the following environment variables that affect the execution of mktemp with the -t option: TMPDIR.
Name a directory used for creating temporary files to override system default; used by mktemp.
The following exit values are returned:
An error occurred.
See attributes(5) for descriptions of the following attributes: