What does the watch command do in Linux?

What does the watch command do in Linux?

Last Updated : 27 May, 2019 watch command in Linux is used to execute a program periodically, showing output in fullscreen. This command will run the specified command in the argument repeatedly by showing its output and errors. By default, the specified command will run every 2 seconds and watch will run until interrupted.

How often should I run the watch command?

The watch command can be used to monitor any file or script periodically. It runs every 2 seconds by default and it will run until interrupted.

Which is the Lt switch in the watch command?

-lt switch in the ls command displays the latest modified file at the top. Example 3 : Removing the title/header from the output. In case you do not want to print the header in the output of watch command, you can use the –no-title or -t option.

How to run a command every 10 seconds?

Every 10.0s : is the time interval to run the watch command. i.e. 10 seconds. ls -lt : is the command to be executed every 10 seconds. Tue Feb 14 12:27:43 2017 : is the current date and time. There is a way to monitor any file on the system with the command watch.

How often do I run the watch command?

When used without any option watch will run the specified command every two seconds. On the top left side of the screen header you can see the watch update interval and the executed command ( Every 2.0s: date ), while on the top left side watch shows the current time and date.

How to change time interval in Linux Watch?

The -n ( –interval) option followed by the desired number of seconds allows you to change the time interval between updates: For example, to monitor your disk space usage with the df command and refresh the screen every five seconds on you would run:

How to strip color in Linux with watch?

Some newer versions of watch now support color. For example watch –color ls -ahl –color. Related. Do not use watch When you use watch programs can detect they’re not writing to a terminal and then strip the color.

How to set non default interval in watch?

The WATCH_INTERVAL environment can be used to persistently set a non-default interval (following the same rules and formatting). -p, –precise Make watch attempt to run command every –interval seconds.

How to watch the output of a command?

Commands with Pipes. If you want to execute a command that contains pipes you need to enclose the command in single or double quotes. If you don’t enclose the full command watch will run just the first command and then pipe its output to the next command in the pipeline. watch ‘COMMAND_1 | COMMAND_2’.

How to watch any command output in real time?

To do this automatically, just add a watch command at the beginning of the previous command like this By default, watch command refresh every 2 seconds. If you want to change the interval, add a -n argument like this

When do I exit from the watch command?

You can specify your watch command to exit whenever there is an error produced by the command it is running. Simply add the -e option, and watch will exit when the output contains an error. The below command illustrates this using a very simple example. $ watch -e exit 99

Why is the watch command wrapped in quotes?

Notice that in the command above, the command to be watched is wrapped inside quotes. This is because without the quotes, the grep command is confused which output stream to consume: watch command or netstat command. To clear out the confusion, we have to put the command we want to “watch” in quotes

How to capture the exit code of a command?

Otherwise, you can capture the exit code of the command using the shell variable $? . This only works if the command adopts standard behaviour of returning an exit code of 0 when the command is successful, and non-zero when there are any errors during the execution of the command. # Copies a local file to a remote location.

How to check the success of a command?

For what it’s worth, a shorter way to write code to check each command for success is: It’s still tedious but at least it’s readable. An alternative is simply to join the commands together with && so that the first one to fail prevents the remainder from executing:

What to do when an external command fails?

If the external command returns a message upon success or failure, then you can store the output of the external command into a variable using a backtick operator. Otherwise, you can capture the exit code of the command using the shell variable $? .

How to use watch with a piped chain of commands?

If I do that, watch is really watching ls -ltr and the output is being passed to tail -n 1 which doesn’t output anything. I see no change in the output at the given interval because the command inside $ () is run just once and the resulting output string is always printed (“watched”) as a literal.

How can I make a Bash command run periodically?

There could be true timer object bash function, implementing timer event by just ‘echo’ call, then piped to loop with read cmd, like this: I was faced with this challenge recently.

How to periodically run a command with very short?

The watch command is appropriate in your case, i would use a terminal multiplexer (e.g. tmux, screen, etc.) and then run the command using In that case even if you close the terminal or ssh connection to the machine the script would still be running.

How to use Bash to repeat a command?

First, open your .bash_aliases file: Et voilà ! You can now use it like this: $ repeat 5 echo Hello World !!! you can use crontab. run the command crontab -e and open it with your preferred text editor, then add this line X number of times to repeat. Y time to wait to repeat.

How to monitor disk I / O with Watch Command on?

By default, watch command refresh every 2 seconds. If you want to change the interval, add a -n argument like this This will make the command refresh every 1 second, showing the last 10 lines of the log file. You can monitor disk I/O with iostat command. First, you have to install iostat, which is included in sysstat package