mktemp



MKTEMP(1)                  OpenBSD Reference Manual                  MKTEMP(1)


NAME

     mktemp - make temporary filename (unique)


SYNOPSIS

     mktemp [-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 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      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:

             o   If the user's TMPDIR environment variable is set, the
                 directory contained therein is used.

             o   Otherwise, if the -p flag was given the specified directory
                 is used.

             o   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.


ENVIRONMENT

     TMPDIR  directory in which to place the temporary file when in -t mode


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 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
           }


DIAGNOSTICS

     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
             The specified template contained fewer than six `Xs' at the end.

     template must not contain directory separators in -t mode
             The template contained one or more directory components and the
             -t option was specified.

     cannot make temp dir
             mktemp was unable to create the temporary directory for any of
             the reasons specified by mkdir(2).

     cannot make temp file
             mktemp was unable to create the temporary file for any of the
             reasons specified by open(2).

     cannot allocate memory
             mktemp was unable to allocate memory for any of the reasons
             specified by malloc(3).


SEE ALSO

     mktemp(3)


HISTORY

     The mktemp utility first appeared in OpenBSD 2.1.

OpenBSD 5.4                     March 12, 2013                     OpenBSD 5.4

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