src/usr.bin/mktemp/mktemp.1 - diff

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Diff for /src/usr.bin/mktemp/mktemp.1 between version 1.33 and 1.34

version 1.33, 2024/03/01 23:37:42

version 1.34, 2024/03/03 15:24:45

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.Os

.Sh NAME

.Nm mktemp

.Nd make temporary filename (unique)

.Nd make temporary file or directory (unique)

.Sh SYNOPSIS

.Nm mktemp

.Op Fl dqtu

Line 29

.Sh DESCRIPTION

The

.Nm mktemp

utility takes the given filename

utility takes the specified file name

.Ar template

and overwrites a portion of it to create a unique filename.

and overwrites a portion of it to create a unique file name.

The

.Ar template

may be any filename with at least six

may be any file name containing at least six

.Ql X Ns s

in the last component of the filename, for example

in the last component of the path, for example

.Pa /tmp/tfile.XXXXXXXXXX

.Pa /tmp/editor.XXXXXXXXXX.txt .

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only the last one will be considered.

If no

.Ar template

is specified, a default of

is specified, a default value of

.Pa tmp.XXXXXXXXXX

is used and the

.Fl t

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.Ql X Ns s

in the

.Ar template

and the number of collisions with pre-existing files.

and the number of collisions with pre-existing entries.

The number of unique filenames

The number of unique names

.Nm

can return depends on the number of

.Ql X Ns s

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.Pp

.Nm

can successfully generate a unique filename, the file (or directory)

successfully generates a unique name, the file (or directory)

is created with file permissions such that it is only readable and writable

is created with permissions such that it is only readable and writable

by its owner (unless the

.Fl u

flag is given) and the filename is printed to standard output.

flag is given) and the name is printed to the standard output.

.Pp

.Nm 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.

the PID as a suffix and use that as a temporary file name.

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

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.

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.It Fl p Ar directory

Use the specified

.Ar directory

as a prefix when generating the temporary filename.

as a prefix when generating the temporary name.

The

.Ar directory

will be overridden by the user's

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fragment illustrates a simple use of

.Nm

where the script should quit if it cannot get a safe

temporary file.

temporary file:

.Bd -literal -offset indent

TMPFILE=`mktemp /tmp/example.XXXXXXXXXX` || exit 1

echo "program output" >> $TMPFILE

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.Pp

The same fragment with support for a user's

.Ev TMPDIR

environment variable can be written as follows.

environment variable can be written as follows:

.Bd -literal -offset indent

TMPFILE=`mktemp -t example.XXXXXXXXXX` || exit 1

echo "program output" >> $TMPFILE

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the temporary file.

In this case the

.Fl t

flag is implied.

flag is implied:

.Bd -literal -offset indent

TMPFILE=`mktemp` || exit 1

echo "program output" >> $TMPFILE

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.Pa /extra/tmp

unless the user's

.Ev TMPDIR

environment variable specifies otherwise.

environment variable specifies otherwise:

.Bd -literal -offset indent

TMPFILE=`mktemp -p /extra/tmp example.XXXXXXXXXX` || exit 1

echo "program output" >> $TMPFILE

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.Pp

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.

the second one fails we need to remove the first before exiting:

.Bd -literal -offset indent

TMP1=`mktemp -t example.1.XXXXXXXXXX` || exit 1

TMP1=`mktemp -t example.XXXXXXXXXX.1` || exit 1

TMP2=`mktemp -t example.2.XXXXXXXXXX`

TMP2=`mktemp -t example.XXXXXXXXXX.2`

if [ $? -ne 0 ]; then

rm -f $TMP1

exit 1

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Or perhaps you don't want to exit if

.Nm

is unable to create the file.

In this case you can protect that part of the script thusly.

In this case you can protect that part of the script thusly:

.Bd -literal -offset indent

TMPFILE=`mktemp -q -t example.XXXXXXXXXX` && {

# Safe to use $TMPFILE in this block