mktemp



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 ** 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 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 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 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 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 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


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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