Lock files for single instance in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of using a lock file in a script?

A lock file prevents multiple instances of the same script from running at the same time. It acts like a 'do not enter' sign to avoid conflicts or duplicated work.

intermediate

How do you create a lock file safely in bash to avoid race conditions?

Use the 'mkdir' command to create a directory as a lock. This is atomic, so only one script can create it at a time. If 'mkdir' fails, another instance is running.

beginner

What should a script do if it finds the lock file already exists?

It should exit or wait, because the lock means another instance is running. This avoids running two copies at once.

intermediate

Why is it better to use a directory as a lock file instead of a regular file?

Creating a directory with 'mkdir' is an atomic operation in Unix systems, so it avoids race conditions. Creating a regular file is not atomic and can cause conflicts.

beginner

Show a simple bash snippet that uses a lock directory to allow only one script instance.

if mkdir /tmp/myscript.lock 2>/dev/null; then echo "Lock acquired, running script..." # script commands here rmdir /tmp/myscript.lock else echo "Another instance is running. Exiting." exit 1 fi

What does a lock file prevent in scripting?

A File permission issues

B Script syntax errors

C Slow script execution

D Multiple script instances running simultaneously

Lock files are used to prevent multiple instances of the same script from running at the same time.

Which command is best for creating an atomic lock in bash?

A touch /tmp/lockfile

B echo 'lock' > /tmp/lockfile

C mkdir /tmp/lockdir

D rm /tmp/lockfile

'mkdir' is atomic and safe for creating lock directories, preventing race conditions.

What should a script do if it cannot create the lock directory?

A Continue running normally

B Exit or wait because another instance is running

C Create a new lock file

D Delete other files

If the lock directory exists, it means another instance is running, so the script should exit or wait.

Why is using a regular file for locking less safe than a directory?

A File creation is not atomic and can cause race conditions

B Files cannot be deleted

C Directories use less disk space

D Files are slower to access

Creating a regular file is not atomic, so two scripts might create it simultaneously causing conflicts.

What command removes the lock directory after the script finishes?

A rmdir /tmp/lockdir

B rm /tmp/lockdir

C touch /tmp/lockdir

D mkdir /tmp/lockdir

'rmdir' removes an empty directory, which is used to release the lock.

Explain how to use a lock file to ensure only one instance of a bash script runs at a time.

Think about using mkdir as an atomic operation.

Describe why atomic operations are important when creating lock files in scripts.

Atomic means 'all or nothing' in one step.