Is there a way to run a command (e.g. ps aux|grep someprocess) for n times?

Something like:

run -n 10  'ps aux|grep someprocess'

I want to use it interactively, please do not post scripts.

Update: The reason I am asking this is, that I do work on a lot of machines and I don't want to import all my adaped scripts etc into every box to get the same functionality accross every machine.

up vote 160 down vote accepted

I don't think a command or shell builtin for this exists, as it's a trivial subset of what the Bourne shell for loop is designed for and implementing a command like this yourself is therefore quite simple.

For starters you can use a dummy for loop:

for i in `seq 10`; do command; done

Or equivalently as per JimB's suggestion, using the Bash builtin for generating sequences:

for i in {1..10}; do command; done

This iterates ten times executing command each time - it can be a pipe or a series of commands separated by ; or &&. You can use the $i variable to know which iteration you're in.

If you consider this one-liner a script and so for some unspecified (but perhaps valid) reason undesireable you can implement it as a command, perhaps something like this on your .bashrc (untested):

#function run
run() {
    number=$1
    shift
    for i in `seq $number`; do
      $@
    done
}

Usage:

run 10 command

Example:

run 5 echo 'Hello World!'
  • 15
    @mahatmanich, A loop is not a script. There is nothing preventing you from using for... at an interactive terminal. – Zoredache May 24 '11 at 18:10
  • 3
    Well, the one-liner above is the kind of standard way to do it and it is fairly simple. Why is it not good for you? Maybe you are asking the wrong question? What is the main goal of your scripts or your actions that you want to repeat them a number of times? Maybe there is a better solution if we put the problem in a different way. – Patkos Csaba May 25 '11 at 8:43
  • 5
    @mahatmanich - for is the bash built-in for iterating. – JimB May 25 '11 at 12:58
  • 3
    @Eduardo Ivanec - FYI, bash has built-in range like seq: {1..10} – JimB May 25 '11 at 13:03
  • 1
    It seems the inbuilt bracket sequence does not support variable substitution (see stackoverflow.com/a/3737773/713554). The function you gave works perfectly when placed in .bashrc if {1..$number} is exchanged for `seq $number` though. – Leo Mar 29 '12 at 13:36

ps aux | grep someprocess looks like you want to watch changes of a program for a fixed time. Eduardo gave an answer that answer your question exactly but there is an alternative: watch:

watch 'ps aux | grep someprocess'

Note that I've put the command in single quotes to avoid the shell from interpreting the command as "run watch ps aux" and pipe the result through grep someprocess. Another way to do the previous command would be:

watch ps aux \| grep someprocess

By default, watch refreshes every two seconds, that can be changed using the -n option. For instance, if want to have an interval of 1 second:

watch -n 1 'ps aux | grep someprocess'
  • Yes this works like a charm. – Jānis Gruzis Dec 21 '12 at 14:08

Just for fun

pgrep ssh ;!!;!!;!!;!!;!!;!!
  • Can someone explain this please? Looks very intriguing. – Daniel Vartanov Feb 24 '14 at 16:31
  • 4
    ; is just command separator. !! replay last command in bash. So this run 'pgrep ssh' and then replay it 6 times. – Piotr Kukielka Sep 21 '14 at 19:11
  • While that is instructive is not vry intuitive. One-liner for seems the best way. Anyway thanks for the answer. – erm3nda Mar 21 '15 at 6:27
  • Won't cmd ;!! on one line just repeat the command entered before the current line? It'd have to be two separate history entries. – Joel Purra Apr 28 '15 at 7:16

Try this:

yes ls | head -n5 | bash

This requires the command to be executed in a sub-shell, a slight performance penalty. YMMV. Basically, you get the "yes" command to repeat the string "ls" N times; while "head -n5" terminated the loop at 5 repeats. The final pipe sends the command to the shell of your choice.

Incidentally csh-like shells have a built-in repeat command. You could use that to execute your command in a bash sub-shell!

similar to previous replies, but does not require the for loop:

seq 10 | xargs -I -- echo "hello"

pipe output of seq to xargs with no arguments or options

  • 2
    This will just do one execution echo "hello" 1 2 3 4 5 6 7 8 9 10. You need to run xargs with -n 1 for one process for each input (number) and with -P 10 for parallel execution (10 parallel processes). Ending up with seq 10 | xargs -n 1 -P 10 echo. – Alexander Klimetschek Apr 26 '17 at 22:08

POSIX way

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04_01

x=10
while [ $x -gt 0 ]; do
    command
    x=$(($x-1))
done

which could of course be made into a one liner:

x=10; while [ $x -gt 0 ]; do command; x=$(($x-1)); done

and seems to be the most portable way, and thus less likely to make you install programs.

The following are less portable:

  • brace expansion {1..10}: bash specific, and would generate a huge line if the argument is large
  • seq: GNU
  • yes: GNU

And if you get tired of portability, consider GNU parallel:

sudo apt-get install parallel
seq 100 | parallel echo {}

which runs commands in parallel and has many cool options.

See also: https://stackoverflow.com/questions/169511/how-do-i-iterate-over-a-range-of-numbers-defined-by-variables-in-bash

A fish shell implementation of @eduardo-ivanec's run function above

function run
  set number $argv[1]
  for i in (seq $number)
    eval $argv[2..-1]
  end
end

That would be 'watch' - should be part of almost every unix system using procps

You can see fs changes or otherwise interesting movement in a system. For example,

watch "df -hP | grep /tmp" 

Just put your commands as an argument (see man watch)

  • There is already an answer with watch ... see right below ... – mahatmanich Jun 22 at 6:57

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