18.4 mktemp: Create temporary file or directory
mktemp manages the creation of temporary files and
directories. Synopsis:
mktemp [option]... [template]
Safely create a temporary file or directory based on template,
and print its name. If given, template must include at least
three consecutive ‘X’s in the last component. If omitted, the template
‘tmp.XXXXXXXXXX’ is used, and option --tmpdir is
implied. The final run of ‘X’s in the template will be replaced
by alpha-numeric characters; thus, on a case-sensitive file system,
and with a template including a run of n instances of ‘X’,
there are ‘62**n’ potential file names.
Older scripts used to create temporary files by simply joining the
name of the program with the process id (‘$$’) as a suffix.
However, that naming scheme is easily predictable, and suffers from a
race condition where the attacker can create an appropriately named
symbolic link, such that when the script then opens a handle to what
it thought was an unused file, it is instead modifying an existing
file. Using the same scheme to create a directory is slightly safer,
since the mkdir will fail if the target already exists, but
it is still inferior because it allows for denial of service attacks.
Therefore, modern scripts should use the mktemp command to
guarantee that the generated name will be unpredictable, and that
knowledge of the temporary file name implies that the file was created
by the current script and cannot be modified by other users.
When creating a file, the resulting file has read and write
permissions for the current user, but no permissions for the group or
others; these permissions are reduced if the current umask is more
restrictive.
Here are some examples (although note that if you repeat them, you
will most likely get different file names):
- Create a temporary file in the current directory.
$ mktemp file.XXXX
file.H47c
- Create a temporary file with a known suffix.
$ mktemp --suffix=.txt file-XXXX
file-H08W.txt
$ mktemp file-XXXX-XXXX.txt
file-XXXX-eI9L.txt
- Create a secure fifo relative to the user's choice of TMPDIR,
but falling back to the current directory rather than /tmp.
Note that mktemp does not create fifos, but can create a
secure directory in which the fifo can live. Exit the shell if the
directory or fifo could not be created.
$ dir=$(mktemp -p "${TMPDIR:-.}" -d dir-XXXX) || exit 1
$ fifo=$dir/fifo
$ mkfifo "$fifo" || { rmdir "$dir"; exit 1; }
- Create and use a temporary file if possible, but ignore failure. The
file will reside in the directory named by TMPDIR, if specified,
or else in /tmp.
$ file=$(mktemp -q) && {
> # Safe to use $file only within this block. Use quotes,
> # since $TMPDIR, and thus $file, may contain whitespace.
> echo ... > "$file"
> rm "$file"
> }
- Act as a semi-random character generator (it is not fully random,
since it is impacted by the contents of the current directory). To
avoid security holes, do not use the resulting names to create a file.
$ mktemp -u XXX
Gb9
$ mktemp -u XXX
nzC
The program accepts the following options. Also see Common options.
- ‘-d’
- ‘--directory’
- Create a directory rather than a file. The directory will have read,
write, and search permissions for the current user, but no permissions
for the group or others; these permissions are reduced if the current
umask is more restrictive.
- ‘-q’
- ‘--quiet’
- Suppress diagnostics about failure to create a file or directory. The
exit status will still reflect whether a file was created.
- ‘-u’
- ‘--dry-run’
- Generate a temporary name that does not name an existing file, without
changing the file system contents. Using the output of this command
to create a new file is inherently unsafe, as there is a window of
time between generating the name and using it where another process
can create an object by the same name.
- ‘-p dir’
- ‘--tmpdir[=dir]’
- Treat template relative to the directory dir. If
dir is not specified (only possible with the long option
--tmpdir) or is the empty string, use the value of
TMPDIR if available, otherwise use ‘/tmp’. If this is
specified, template must not be absolute. However,
template can still contain slashes, although intermediate
directories must already exist.
- ‘--suffix=suffix’
- Append suffix to the template. suffix must not
contain slash. If --suffix is specified, template must
end in ‘X’; if it is not specified, then an appropriate
--suffix is inferred by finding the last ‘X’ in
template. This option exists for use with the default
template and for the creation of a suffix that starts with
‘X’.
- ‘-t’
- Treat template as a single file relative to the value of
TMPDIR if available, or to the directory specified by
-p, otherwise to ‘/tmp’. template must not
contain slashes. This option is deprecated; the use of -p
without -t offers better defaults (by favoring the command
line over TMPDIR) and more flexibility (by allowing intermediate
directories).
Exit status:
0 if the file was created,
1 otherwise.