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Wednesday, February 24, 2016

Repeat a Linux Command Every X Seconds Forever

Run Linux Command Every Second

In this Article, you will learn a simple scripting techniques to monitor or keep a eye on a particular command in continuously running state  for every 3 seconds by default.

1. Use watch Command

Watch is a Linux command that allows you to execute a command or program periodically and also shows you output on the screen. This means that you will be able to see the program output in time. By default watch re-runs the command/program every 2 seconds. The interval can be easily changed to meet your requirements.
Monitor Memory Usage

“Watch” is extremely easy to use, to test it, you can fire up a Linux terminal right away and type the following command:

# watch free -m

The above command will check your system free memory and update the results of the free command every two seconds.

Monitor Memory Usage in Linux

As seen per the above output, you have a header, displaying information about (from left to right) update interval, command that is being executed and current time. If you wish to hide this header, you can use the -t option.

The next logical question is – how to change the execution interval. For that purpose, you can use the -n option, that specifies the interval with which the command will be executed. This interval is specified in seconds. So let’s say you want to run your file every 10 seconds, you can do it like this:

# watch -n 10

Monitor Logged-In Users, Uptime and Load Average Let’s say you want to monitor logged-in users, server uptime and load average output in continuously phase every few seconds, then use following command as shown:

# watch uptime

Watch Linux Load Average

To exit the command, press CTRL+C.

Here, the 'uptime' command will run and display the updated results every 2 seconds by default.
Monitor Progress of Copy Command

In Linux, while copying files from one location to other using cp command, the progress of data is not shown, to see the progress of data being copied, you can use the watch command along with  du -s command to check disk usage in real time.

2. Use sleep Command

Sleep is often used to debug shell scripts, but it has many other useful purposes as well. For example, when combined with for or while loops, you can get pretty awesome results.

In case this is the first time you hear about the "sleep" command, it is used to delay something for a specified amount of time. In scripts, you can use it to tell your script to run command 1, wait for 10 seconds and then run command 2.

With the above loops, you can tell bash to run a command, sleep for N amount of seconds and then run the command again.

Below you can see examples of both loops:
for loop Example

# for i in {1..10}; do echo -n "This is a test in loop $i "; date ; sleep 5; done

The above one liner, will run the echo command  and display the current date, total of 5 times, with 5 seconds sleep between executions. Here is a sample output:

This is a test in loop 1 Wed Feb 17 20:49:47 EET 2015
This is a test in loop 2 Wed Feb 17 20:49:52 EET 2015
This is a test in loop 3 Wed Feb 17 20:49:57 EET 2015
This is a test in loop 4 Wed Feb 17 20:50:02 EET 2015

You can change the echo and date commands with your own commands or script and change the sleep interval per your needs.
while loop Example

# while true; do echo -n "This is a test of while loop";date ; sleep 5; done

Here is sample output:

This is a test of while loopWed Feb 17 20:52:32 EET 2015
This is a test of while loopWed Feb 17 20:52:37 EET 2015
This is a test of while loopWed Feb 17 20:52:42 EET 2015
This is a test of while loopWed Feb 17 20:52:47 EET 2015
This is a test of while loopWed Feb 17 20:52:52 EET 2015
This is a test of while loopWed Feb 17 20:52:57 EET 2015

The above command will run until it is either killed or interrupted by user It can come in handy if you need to run a command running in the background and you don’t want to count on cron.

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