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November 21, 2013 21:39
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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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