Linux mktemp Command: Create Secure Temporary Files and Directories

This guide explains how to create unpredictable temporary files and directories with Linux mktemp, use templates and TMPDIR, clean up with trap, and avoid unsafe dry runs. It focuses on practical Linux usage, predictable automation, and the cases where a seemingly small path or option mistake can change the result.

The examples use GNU Coreutils behavior. Check mktemp --help, man mktemp, and your distribution documentation when portability or a version-specific option matters.

What Is the mktemp Command?

mktemp atomically creates a temporary file or directory with an unpredictable unique name and prints that path. This avoids the predictable-name race conditions common in scripts that combine a program name with a process ID.

Basic Syntax

mktemp [OPTION]... [TEMPLATE]

Quote paths that contain spaces. When supported, place -- before operands that begin with a hyphen so they are not interpreted as options.

Basic mktemp Usage

Create a temporary file

With no template, mktemp uses the system temporary-directory policy, creates the file, and prints its path. Stop if creation fails.

tmp_file=$(mktemp) || exit 1
printf '%s\n' "$tmp_file"

Choose a filename pattern

A template ends with a run of X characters that mktemp replaces. A suffix can be inferred or supplied explicitly.

mktemp report-XXXXXXXX.csv
mktemp --suffix=.log app-XXXXXXXX

Create a temporary directory

Use -d for a private temporary workspace. A stricter umask may further restrict its permissions.

tmp_dir=$(mktemp -d) || exit 1
printf '%s\n' "$tmp_dir"

Checking the Actual Creation Path

mktemp returns the path of the securely created temporary file to standard output. Do not guess the name to create it or rely only on the -u result.

tmpfile=$(mktemp)
printf '%s\n' "$tmpfile"
test -f "$tmpfile"
echo $?

The output is one line with the temporary path and 0. The string of the temporary path differs with each execution. Once the task is complete, make sure to verify that it is the file you actually created and clean up the variable path enclosed in quotes.

Key Options

Long options often make reviewed scripts easier to understand. When a script must run on non-GNU systems, verify every non-POSIX option on the target platform.

Practical Examples

Clean up a temporary file with trap

Register cleanup immediately after successful creation. Quote every path variable and use -- where supported.

tmp_file=$(mktemp) || exit 1
trap 'rm -f -- "$tmp_file"' EXIT HUP INT TERM

printf '%s\n' 'temporary data' > "$tmp_file"

Use a temporary work directory

When several intermediate files are needed, create one private directory and use fixed names only inside that directory.

work_dir=$(mktemp -d) || exit 1
trap 'rm -rf -- "$work_dir"' EXIT HUP INT TERM

input=$work_dir/input
output=$work_dir/output

Select the parent directory

Use -p when temporary data must reside on a particular filesystem. The parent directory must already exist and have suitable permissions.

mkdir -p "$HOME/.cache/myapp"
cache_file=$(mktemp -p "$HOME/.cache/myapp" cache-XXXXXXXX) || exit 1

mktemp and Related Commands

Important Cautions

Do not create a file later from mktemp -u output

Another process can claim the name between selection and use. Let mktemp create the object itself.

Quote cleanup variables

An empty or split path can make a cleanup command target the wrong place. Verify creation and quote the variable every time.

Know where sensitive temporary data is stored

The temporary directory may be disk-backed, swapped, snapshotted, or backed up. Apply a separate data-handling policy for secrets.

FAQ

Why is mktemp safer than adding a process ID to a name?

It chooses an unpredictable name and creates the object atomically, reducing name-squatting and symlink races.

How do I create a temporary directory?

Use mktemp -d and store its printed path in a quoted variable.

Are temporary files removed automatically?

Usually no. Register cleanup with trap in a shell script.

When is TMPDIR used?

It can select the parent used by the default template or --tmpdir. Check the installed version for exact behavior.

Related Resources

• Essential Linux Commands: A Practical Reference by Task
• GNU Coreutils mktemp documentation
• GNU Coreutils manual