Lock files for single instance in Bash Scripting - Step-by-Step Execution

Concept Flow - Lock files for single instance

Start Script

↓

Check if lock file exists

↓

Exit: Another instance

↓

Run main script

↓

Remove lock file

↓

End Script

The script first checks if a lock file exists. If yes, it exits to avoid multiple instances. If no, it creates the lock file, runs the main task, then removes the lock file.

Execution Sample

Bash Scripting

#!/bin/bash
LOCKFILE=/tmp/myscript.lock
if [ -e "$LOCKFILE" ]; then
  echo "Already running"
  exit 1
fi
trap "rm -f $LOCKFILE" EXIT
touch "$LOCKFILE"
# main work here
sleep 2
echo "Script done"

This script uses a lock file to ensure only one instance runs. It exits if the lock file exists, otherwise creates it, runs work, then cleans up.

Execution Table

Step	Check Lock File	Lock File Exists?	Action	Output
1	Check if /tmp/myscript.lock exists	No	Create lock file
2	Run main work (sleep 2)	N/A	Sleep 2 seconds
3	Print message	N/A	echo 'Script done'	Script done
4	Remove lock file on exit	N/A	rm -f /tmp/myscript.lock
5	Script ends	N/A	Exit normally

💡 Script ends after removing lock file to allow future runs

Variable Tracker

Variable	Start	After Step 1	After Step 4	Final
LOCKFILE	/tmp/myscript.lock	/tmp/myscript.lock (created)	None (removed)	None (removed)

Key Moments - 3 Insights

Why does the script exit immediately if the lock file exists?

Because the lock file signals another instance is running. The script checks this at Step 1 in the execution_table and exits to avoid conflicts.

What ensures the lock file is removed even if the script is interrupted?

The trap command sets a cleanup action on EXIT (Step 4). This means the lock file is removed no matter how the script ends.

Why do we create the lock file after checking it doesn't exist?

Creating the lock file after confirming it doesn't exist prevents race conditions. This is shown in Step 1 where the file is created only if absent.

Visual Quiz - 3 Questions

Look at the execution_table, what happens at Step 1 if the lock file exists?

A The script creates a new lock file

B The script exits immediately

C The script runs main work

D The script removes the lock file

💡 Hint

Refer to Step 1 in execution_table where the lock file existence check leads to exit if true

According to variable_tracker, what is the state of LOCKFILE after Step 4?

A LOCKFILE is renamed

B LOCKFILE is created and exists

C LOCKFILE is removed and does not exist

D LOCKFILE is empty but exists

💡 Hint

Check variable_tracker row for LOCKFILE after Step 4 showing removal

If the trap command was missing, what would likely happen?

A Lock file might remain after script ends

B Lock file would never be created

C Script would run twice simultaneously

D Script would not check for lock file

💡 Hint

trap ensures cleanup at exit; without it, lock file removal at Step 4 might not happen

Concept Snapshot

Lock files prevent multiple script instances.
Check if lock file exists; if yes, exit.
If no, create lock file and run work.
Use trap to remove lock file on exit.
This avoids conflicts and ensures single instance.

Full Transcript

This bash script uses a lock file to allow only one instance to run at a time. It starts by checking if the lock file exists. If it does, the script prints a message and exits immediately to avoid running twice. If the lock file does not exist, the script creates it and sets a trap to remove it when the script ends. Then it runs the main work, here simulated by sleep 2 seconds. After finishing, the trap removes the lock file so future runs can proceed. This method prevents multiple instances from running simultaneously and causing conflicts.

Practice

1. What is the main purpose of using a lock file in a bash script?

easy

A. To prevent multiple instances of the script from running at the same time

B. To speed up the script execution

C. To store user input temporarily

D. To log errors during script execution

Solution

1. Step 1: Understand the role of a lock file

   A lock file is used to signal that a script is already running, preventing duplicates.

2. Step 2: Identify the main purpose

   This prevents multiple script instances from running simultaneously, avoiding conflicts.

3. Final Answer:

   To prevent multiple instances of the script from running at the same time -> Option A

4. Quick Check:

   Lock file purpose = prevent duplicates [OK]

Hint: Lock files stop duplicate script runs quickly [OK]

Common Mistakes:

• Thinking lock files speed up scripts
• Confusing lock files with log files
• Using lock files to store data

2. Which of the following bash commands correctly creates a lock file only if it does not already exist?

easy

A. touch /tmp/myscript.lock

B. echo $$ > /tmp/myscript.lock

C. mkdir /tmp/myscript.lock

D. if [ ! -e /tmp/myscript.lock ]; then touch /tmp/myscript.lock; fi

Solution

1. Step 1: Check for existence before creating

   The command must create the lock file only if it does not exist, so a test with [ ! -e ] is needed.

2. Step 2: Identify the correct syntax

   if [ ! -e /tmp/myscript.lock ]; then touch /tmp/myscript.lock; fi uses an if statement to check and then create the file, which is correct.

3. Final Answer:

   if [ ! -e /tmp/myscript.lock ]; then touch /tmp/myscript.lock; fi -> Option D

4. Quick Check:

   Check existence before touch = if [ ! -e /tmp/myscript.lock ]; then touch /tmp/myscript.lock; fi [OK]

Hint: Use if with [ ! -e ] to create lock file safely [OK]

Common Mistakes:

• Using touch without checking file existence
• Using mkdir which creates a directory, not a file
• Overwriting lock file without check

3. Given this script snippet:

if [ -e /tmp/myscript.lock ]; then
  echo "Script already running"
  exit 1
else
  touch /tmp/myscript.lock
  echo "Running script"
  rm /tmp/myscript.lock
fi

What will be the output if the lock file exists?

medium

A. Running script

B. Script already running

C. No output

D. Error: lock file missing

Solution

1. Step 1: Check the condition when lock file exists

   If /tmp/myscript.lock exists, the if condition is true and the script echoes "Script already running" and exits.

2. Step 2: Understand the output

   Since the script exits early, it does not run or remove the lock file.

3. Final Answer:

   Script already running -> Option B

4. Quick Check:

   Lock file exists = "Script already running" [OK]

Hint: If lock file exists, script exits with message [OK]

Common Mistakes:

• Assuming script runs even if lock file exists
• Expecting no output when lock file exists
• Confusing lock file presence with absence

4. This script is intended to prevent multiple runs using a lock file:

if [ -e /tmp/myscript.lock ]; then
  echo "Already running"
  exit 1
fi

touch /tmp/myscript.lock
# script work here
rm /tmp/myscript.lock

But sometimes multiple instances run simultaneously. What is the likely problem?

medium

A. The lock file is not checked before creation

B. The script does not remove the lock file

C. The lock file is created after the check, causing a race condition

D. The script uses rm instead of rmdir

Solution

1. Step 1: Analyze the timing of lock file creation

   The script checks for the lock file, then creates it. Between these steps, another instance can start and pass the check.

2. Step 2: Identify the race condition

   This gap causes multiple instances to run simultaneously because the lock file is not created atomically with the check.

3. Final Answer:

   The lock file is created after the check, causing a race condition -> Option C

4. Quick Check:

   Race condition from delayed lock creation = The lock file is created after the check, causing a race condition [OK]

Hint: Create lock file atomically to avoid race conditions [OK]

Common Mistakes:

• Ignoring race conditions in lock file creation
• Forgetting to remove lock file after script ends
• Confusing file removal commands

5. You want to write a bash script that runs only one instance at a time using a lock file. Which approach is best to avoid race conditions?

hard

A. Use 'mkdir /tmp/myscript.lock' and check if it succeeds before running

B. Check if lock file exists, then create it with 'touch' if not

C. Create lock file with 'echo $$ > /tmp/myscript.lock' without checking

D. Remove lock file at script start, then create it

Solution

1. Step 1: Understand atomic operations for locking

   Using 'mkdir' to create a directory as a lock is atomic; it either succeeds or fails immediately.

2. Step 2: Compare with other methods

   Other methods like checking then touching can cause race conditions; removing lock at start is unsafe.

3. Final Answer:

   Use 'mkdir /tmp/myscript.lock' and check if it succeeds before running -> Option A

4. Quick Check:

   Atomic mkdir lock = safest method [OK]

Hint: Use mkdir for atomic lock creation to avoid race [OK]

Common Mistakes:

• Using touch without atomic check
• Removing lock file before checking
• Ignoring race conditions