Repeat a command every x interval of time in terminal?

Question

How can I repeat a command every interval of time , so that it will allow me to run commands for checking or monitoring directories ?

There is no need for a script, i need just a simple command to be executed in terminal.

Answer 1

You can use watch command, watch is used to run any designated command at regular intervals.

Open Terminal and type:

watch -n x <your command>

change x to be the time in seconds you want.

For more help using the watch command and its options, run man watch or visit this Link.

For example : the following will list, every 60s, on the same Terminal, the contents of the Desktop directory so that you can know if any changes took place:

watch -n 60 ls -l ~/Desktop

Answer 2

You can also use this command in terminal, apart from nux's answer:

while true; do <your_command>; sleep <interval_in_seconds>; done

Example:

while true; do ls; sleep 2; done

This command will print the output of ls at an interval of 2 sec.

Use Ctrl+C to stop the process.

There are few drawbacks of watch:

• It cannot use any aliased commands.
• If the output of any command is quite long, scrolling does not work properly.
• There is some trouble to set the maximum time interval beyond a certain value.
• watch will interpret ANSI color sequences passing escape characters using -c or --color option. For example output of pygmentize will work but it will fail for ls --color=auto.

In the above circumstances this may appear as a better option.

Answer 3

Just wanted to pitch in to sourav c.'s and nux's answers:

1. While watch will work perfectly on Ubuntu, you might want to avoid that if you want your "Unix-fu" to be pure. On FreeBSD for example, watch is a command to "snoop on another tty line".

2. while true; do command; sleep SECONDS; done also has a caveat: your command might be harder to kill using Ctrl+C. You might prefer:

   while sleep SECONDS; do command; done
   

   It's not only shorter, but also easier to interrupt. The caveat is that it will first sleep, then run your command, so you'll need to wait some SECONDS before the first occurrence of the command will happen.

Answer 4

Sounds like the ideal task for the cron daemon which allows for running periodic commands. Run the crontab -e command to start editing your user's cron configuration. Its format is documented in crontab(5). Basically you have five time-related, space-separated fields followed by a command:

The time and date fields are:

       field          allowed values
       -----          --------------
       minute         0-59
       hour           0-23
       day of month   1-31
       month          1-12 (or names, see below)
       day of week    0-7 (0 or 7 is Sunday, or use names)

For example, if you would like to run a Python script on every Tuesday, 11 AM:

0 11 * * 1 python ~/yourscript.py

There are also some special names that replace the time, like @reboot. Very helpful if you need to create a temporary directory. From my crontab (listed with crontab -l):

# Creates a temporary directory for ~/.distcc at boot
@reboot ln -sfn "$(mktemp -d "/tmp/distcc.XXXXXXXX")" "$HOME/.distcc"

Answer 5

you can use crontab. run the command crontab -e and open it with your preferred text editor, then add this line

*/10 * * * *  /path-to-your-command

This will run your command every 10 minutes

* */4 * * *  /path-to-your-command

This will run your command every 4 hours

Another possible solution

$ ..some command...; for i in $(seq X); do $cmd; sleep Y; done

X number of times to repeat.

Y time to wait to repeat.

Example :

$ echo; for i in $(seq 5); do $cmd "This is echo number: $i"; sleep 1;done

Answer 6

If you are monitoring the file system, then inotifywait is brilliant and certainly adds less load on your system.

Example :

In 1st terminal type this command :

$ inotifywait .

Then in 2nd terminal, any command that affects the current directory,

$ touch newfile

Then in original terminal inotifywait will wake up and report the event

./ CREATE newfile2

Or in a loop

$ while true ; do inotifywait . ; done
Setting up watches.  
Watches established.
./ OPEN newfile2
Setting up watches.  
Watches established.
./ OPEN newfile2
Setting up watches.  
Watches established.
./ DELETE newfile
Setting up watches.  
Watches established.
./ CREATE,ISDIR newdir
Setting up watches.  
Watches established.

Answer 7

You can create your own repeat command doing the following steps; credits here:

First, open your .bash_aliases file:

$ xdg-open ~/.bash-aliases

Second, paste these lines at the bottom of the file and save:

repeat() {
n=$1
shift
while [ $(( n -= 1 )) -ge 0 ]
do
    "$@"
done
}

Third, either close and open again your terminal, or type:

$ source ~/.bash_aliases

Et voilà ! You can now use it like this:

$ repeat 5 echo Hello World !!!

or

$ repeat 5 ./myscript.sh

Answer 8

Another concern with the "watch" approach proposed above is that it does display the result only when the process is done. "date;sleep 58;date" will display the 2 dates only after 59 seconds... If you start something running for 4 minutes, that display slowly multiple pages of content, you will not really see it.

On the other hand, the concern with the "while" approach is that it doesn't take the task duration into consideration.

while true; do script_that_take_between_10s_to_50s.sh; sleep 50; done

With this, the script will run sometime every minutes, sometime might take 1m40. So even if a cron will be able to run it every minutes, here, it will not.

So to see the output on the shell as it's generated and wait for the exact request time, you need to look at the time before, and after, and loop with the while.

Something like:

while ( true ); do
  echo Date starting `date`
  before=`date +%s`
  sleep `echo $(( ( RANDOM % 30 )  + 1 ))`
  echo Before waiting `date`
  after=`date +%s`
  DELAY=`echo "60-($after-$before)" | bc`
  sleep $DELAY
  echo Done waiting `date`
done

This will output this:

As you can see, the command runs every minutes:

Date starting Mon Dec 14 15:49:34 EST 2015
Before waiting Mon Dec 14 15:49:52 EST 2015
Done waiting Mon Dec 14 15:50:34 EST 2015

Date starting Mon Dec 14 15:50:34 EST 2015
Before waiting Mon Dec 14 15:50:39 EST 2015
Done waiting Mon Dec 14 15:51:34 EST 2015

So just replace the "sleep echo $(( ( RANDOM % 30 ) + 1 ))" command with what ever you want and that will be run, on the terminal/shell, exactly every minute. If you want another schedule, just change the "60" seconds with what ever you need.

Shorter version without the debug lines:

while ( true ); do
  before=`date +%s`
  sleep `echo $(( ( RANDOM % 30 )  + 1 ))` # Place you command here
  after=`date +%s`
  DELAY=`echo "60-($after-$before)" | bc`
  sleep $DELAY
done