This week in Bash Scripting Sunday, we’re diving into a small topic with a big impact: handling temporary files safely and securely.

Temporary files are common in scripting — for holding intermediate data, logs, or scratch work. But if you’re using something like:

tmpfile="/tmp/myscript.tmp"

…you might be exposing yourself to race conditions, file collisions, or even security issues.

Let’s fix that. 🛠️

🧪 The Problem with Hardcoded Temp Files

Hardcoding paths like /tmp/foo.txt can lead to problems:

  • If two users/scripts run at once, they’ll overwrite each other’s data
  • It can leak sensitive information
  • It’s predictable, so it might be exploited by a malicious user

Instead, use tools designed for safe, atomic temporary file creation.

✅ The Solution: mktemp

mktemp is your friend for securely creating temporary files or directories.

tmpfile=$(mktemp)
echo "Working in $tmpfile"

This guarantees a unique file path like:

/tmp/tmp.zsY9Xc2S0b

Want a custom prefix?

tmpfile=$(mktemp /tmp/myscript.XXXXXX)

Use XXXXXX to mark where randomness should go.

🧽 Cleaning Up: trap to the Rescue

A good script is a tidy script. Use trap to clean up your temp file automatically, even if your script exits early.

tmpfile=$(mktemp)
trap 'rm -f "$tmpfile"' EXIT

Now, when the script exits (normally or due to an error), the temp file is gone.

🧪 Example: Sorting a Large File Without Modifying the Original

#!/usr/bin/env bash

input_file="$1"
[ -f "$input_file" ] || { echo "File not found"; exit 1; }

sorted_tmp=$(mktemp)
trap 'rm -f "$sorted_tmp"' EXIT

sort "$input_file" > "$sorted_tmp"

# Do something with the sorted data
head "$sorted_tmp"

This ensures the original file remains untouched, and cleanup happens automatically.

📁 Temporary Directories Too!

tmpdir=$(mktemp -d)
trap 'rm -rf "$tmpdir"' EXIT

Now you’ve got a whole isolated workspace that cleans itself up.

🚫 Common Pitfalls

  • ❌ Don’t use predictable filenames in /tmp
  • ❌ Don’t forget to clean up — use trap
  • ✅ Always quote variable expansions: "$tmpfile"

🧵 Summary

  • Use mktemp to safely create temp files/dirs
  • Set traps to clean up automatically
  • Avoid fixed names and race conditions

Next week, we’ll explore logging techniques — from simple echo lines to syslog integration.

Happy scripting! 🐚