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MKTEMP(1) MKTEMP(1)
NAME mktemp - make temporary filename (unique) SYNOPSIS mktemp [-V] | [-dqtu] [-p directory] [template] DESCRIPTION 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 pro cess 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 b2.txt bin.txt binout.txt binout2.txt binout2.txt~ docs html loopcmd loopcmd~ makehtml makehtml (copy) makehtml~ readpasswd showdoc.php showdoc.php~ space2newline space2newline~ testphp.php testphp.php~ 10 combinations. If mktemp can successfully generate a unique filename, the file (or directory) is created with file permissions such that it is only read able 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 tem porary filename. The directory will be overridden by the users 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 users TMPDIR environment variable is set, the directory contained therein is used. Otherwise, if the -p flag was given the specified direc tory 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. TMPFILE=mktemp /tmp/example.XXXXXXXXXX || exit 1 echo "program output" >> $TMPFILE The same fragment with support for a users 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 dont 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 users TMPDIR environment variable specifies oth erwise. TMPFILE=mktemp -p /extra/tmp example.XXXXXXXXXX || exit 1 echo "program output" >> $TMPFILE 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 dont 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 SEE ALSO mkdtemp(3), mkstemp(3), mktemp(3) HISTORY The mktemp utility appeared in OpenBSD 2.1. 30 September 2001 MKTEMP(1) |
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