Cron jobs are a fundamental part of automating tasks in Unix-based systems. However, one common problem with cron jobs is multiple executions, where overlapping job runs can cause serious issues like data corruption, race conditions, or unexpected system load.

In this blog, we’ll explore why multiple executions happen, the potential risks, and how flock provides an elegant solution to ensure that a cron job runs only once at a time.

The Problem: Multiple Executions of Cron Jobs

Cron jobs are scheduled to run at fixed intervals, but sometimes a new job instance starts before the previous one finishes.

This can happen due to

• Long-running jobs: If a cron job takes longer than its interval, a new instance starts while the old one is still running.
• System slowdowns: High CPU or memory usage can delay job execution, leading to overlapping runs.
• Simultaneous executions across servers: In a distributed system, multiple servers might execute the same cron job, causing duplication.

Example of a Problematic Cron Job

Let’s say we have the following cron job that runs every minute:

* * * * * /path/to/script.sh

If script.sh takes more than a minute to execute, a second instance will start before the first one finishes.

This can lead to:

Duplicate database writes → Inconsistent data

Conflicts in file processing → Corrupt files

Overloaded system resources → Performance degradation

Real-World Example

Imagine a job that processes user invoices and sends emails

* * * * * /usr/bin/python3 /home/user/process_invoices.py

If the script takes longer than a minute to complete, multiple instances might start running, causing

1. Users to receive multiple invoices.
2. The database to get inconsistent updates.
3. Increased server load due to excessive email sending.

The Solution: Using flock to Prevent Multiple Executions

flock is a Linux utility that manages file locks to ensure that only one instance of a process runs at a time. It works by locking a specific file, preventing other processes from acquiring the same lock.

Using flock in a Cron Job

Modify the cron job as follows

* * * * * /usr/bin/flock -n /tmp/myjob.lock /path/to/script.sh

How It Works

• flock -n /tmp/myjob.lock → Tries to acquire a lock on /tmp/myjob.lock.
• If the lock is available, the script runs.
• If the lock is already held (i.e., another instance is running), flock prevents the new instance from starting.
• -n (non-blocking) ensures that the job doesn’t wait for the lock and simply exits if it cannot acquire it.

This guarantees that only one instance of the job runs at a time.

Verifying the Solution

You can test the lock by manually running the script with flock

/usr/bin/flock -n /tmp/myjob.lock /bin/bash -c 'echo "Running job..."; sleep 30'

Open another terminal and try to run the same command. You’ll see that the second attempt exits immediately because the lock is already acquired.

Preventing multiple executions of cron jobs is essential for maintaining data consistency, system stability, and efficiency. By using flock, you can easily enforce single execution without complex logic.

Simple & efficient solution. No external dependencies required. Works seamlessly with cron jobs.

So next time you set up a cron job, add flock and sleep peacefully knowing your tasks won’t collide.

Why I Created This Function

At first, I used an alias

alias gitdir="cd ~/Git/" (This means gitdir switches to the ~/Git/ directory, but I wanted it to switch directly to a repository.)
So, I wrote a Bash function.

Write Code to .bashrc File

The .bashrc file runs when a new terminal window is opened.
So, we need to write the function inside this file.

Code

gitrepo() {
    # Exact Match
    repoList=$(ls $HOME/Git)
    if [ -n "$(echo "$repoList" | grep -w $1)" ]; then
	cd $HOME/Git/$1
    else
	# Relevant Match
	getRepoName=$(echo "$repoList" | grep -i -m 1 $1)
	
	if [ -n "$getRepoName" ]; then
	    cd "$HOME/Git/$getRepoName"
	else
	    echo "Repository Not Founded"
	    cd $HOME/Git
	fi
	
    fi   
}

Code Explanation

The $repoList variable stores the list of directories inside the Git folder.

Function Logic Has Two Parts:

• Exact Match
• Relevant Match

Exact Match

if [ -n "$(echo "$repoList" | grep -w $1)" ]; then
	cd $HOME/Git/$1

If condition: The $repoList variable parses input for grep.

• grep -w matches only whole words.
• $1 is the function’s argument in bash.
• -n checks if a variable is not empty. Example syntax:
  [ a != "" ] is equivalent to [ -n a ]

Relevant Match

getRepoName=$(echo "$repoList" | grep -i -m 1 $1)
	if [ -n "$getRepoName" ]; then
	    cd "$HOME/Git/$getRepoName"

Relevant search: If no Exact Match is found, this logic is executed next.

getRepoName="$repoList" | grep -i -m 1 $1

• -i ignores case sensitivity.
• -m 1 returns only the first match.

Example of -m with grep:
ls | grep i3
It returns i3WM and i3status, but -m 1 ensures only i3WM is selected.

No Match

If no match is found, it simply changes the directory to the Git folder.

	else
	    echo "Repository Not Founded"
	    cd $HOME/Git

What I Learned

• Basics of Bash functions
• How to use .bashrc and reload changes.

Introduction:
Linux is one of the most powerful and widely-used operating systems in the world, found everywhere from mobile devices to high-powered servers. Known for its stability, security, and open-source nature, Linux is an essential skill for anyone interested in IT, programming, or system administration.
In this blog , we are going to see What is linux and Why choose linux.

1) What is linux
Linux is an open-source operating system that was first introduced by Linus Torvalds in 1991. Built on a Unix-based foundation, Linux is community-driven, meaning anyone can view, modify, and contribute to its code. This collaborative approach has led to the creation of various Linux distributions, or "distros," each tailored to different types of users and use cases. Some of the most popular Linux distributions are:

• Ubuntu: Known for its user-friendly interface, great for beginners.
• Fedora: A cutting-edge distro with the latest software versions, popular with developers.
• CentOS: Stable and widely used in enterprise environments. Each distribution may look and function slightly differently, but they all share the same core Linux features.

2) Why choose linux
Linux is favored for many reasons, including its:

1. Stability: Linux is well-known for running smoothly without crashing, even in demanding environments.
2. Security: Its open-source nature allows the community to detect and fix vulnerabilities quickly, making it highly secure.
3. Customizability: Users have complete control to modify and customize their system.
4. Performance: Linux is efficient, allowing it to run on a wide range of devices, from servers to small IoT devices.

Conclusion
Learning Linux basics is the first step to becoming proficient in an operating system that powers much of the digital world. We can see more about some basics and interesting things about linux in further upcoming blogs which I will be posting.

Follow for more and happy learning :)

when we open terminal or Konsole in Linux we used to think by using many utilities why calculators was not there interactively in it ?

Now here is the solution for it just by cloning a repository from GitHub on your PC. Here is the GitHub link to clone it.

Just open your terminal and make a directory for safety purpose for that type,

mkdir calculator;cd calculator

the above command explain about making and changing directory from home directory.

then type,

git clone https://github.com/vishnumur777/simplecalculatorbash

this command will clone all the files present in the repository.

but please make sure that you have installed git on your PC.

Then change directory to simplecalculatorbash using

cd simplecalculatorbash/

then type,

chmod +x calci.sh

this command changes the permission to run on user without depending on root . So that many PC cause this error that permission was denied while executing the file.

For execution run type,

./calci.sh

and press enter key

which will run very colourful calculator interactive prompt by installing dependencies which I posted on GitHub.

Today Learned below commands.

List Commands

ls -> it listed out all the files
ls -l -> it gave long listing items
ls -lh -> it returns items with human readable file size
ls -lS -> it sorted out the files
ls -lSh -> it sorted the files also it returns file with human readable format
ls -R -> returned all the files in the directory

whoami -> shows the current user
hostname -> shows the hostname
hostname -I -> shows the IP address of the user

Date Commands

date -> displayed current date
date –date=”tomorrow” -> prints the tomorrow date
date –date=”3 years ago” -> prints the date 3 years ago.

CAT Commands : Concatenate commands

cat > test1.text -> creates a new file
cat test1.text | less -> show the file in page fize
q -> to quit

ECHO Commands

echo “Hello world” -> it usually prints the data.

History Commands

history -> it displays the last 1000 commands executed in our machine. we can increase the limit
history 10 -> executes last 10 commands
history | head -> shows first 10 commands
history | tail -> last 10 commands
history !1000(event) -> executed the specified event command.

Remove command

rm -i test1.text -> it asks user permission whether to delete this or not
rm test1.text -> removes the files
rm * text -> removes all the files which are text file type.

Manual command

man ls -> shows all the information about ls command
man date -> displayes all the info about date command
z -> to down the page.
w -> to go up in the page.