MKTEMP(1)                  OpenBSD Reference Manual                  MKTEMP(1)

NAME
     mktemp - make temporary file name (unique)

SYNOPSIS
     mktemp [-d] [-q] [-u] template

DESCRIPTION
     The mktemp utility takes the given file name template and overwrites a
     portion of it to create a unique file name.  The template may be any file
     name with some number of `Xs' appended to it, for example
     /tmp/temp.XXXXXXXXXX.

     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 file names 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 file name, the file (or di-
     rectory) 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 file name.  This kind of nam-
     ing 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 sug-
     gested that mktemp be used instead.

     The options are as follows:

     -d      Make a directory instead of a file.

     -q      Fail silently if an error occurs.  This is useful if a script
             does not want error output to go to standard error.

     -u      Operate in ``unsafe'' mode.  The temp file will be unlinked be-
             fore mktemp exits.  This is slightly better than mktemp(3) but
             still introduces a race condition.  Use of this option is not en-
             couraged.

     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.

           CMD=`basename $0`
           TMPFILE=`mktemp /tmp/$CMD.XXXXXXXXXX` || exit 1
           echo "program output" >> $TMPFILE

     In this case, we want the script to catch the error ourselves.

           CMD=`basename $0`
           TMPFILE=`mktemp -q /tmp/$CMD.XXXXXXXXXX`
           if [ $? -ne 0 ]; then
                   echo "$CMD: Can't create temp file, exiting..."
                   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 the part of the script thusly.

           CMD=`basename $0`
           TMPFILE=`mktemp /tmp/$CMD.XXXXXXXXXX` && {
                   # Safe to use $TMPFILE in this block
                   echo data > $TMPFILE
                   ...
                   rm -f $TMPFILE
           }

SEE ALSO
     mkdtemp(3), mkstemp(3), mktemp(3)

HISTORY
     The mktemp utility appeared in OpenBSD 2.1.

OpenBSD 2.9                    November 20, 1996                             2
