bash Cookbook

Looping Over Arguments Passed to a Script

Problem

You want to take some set of actions for a given list of arguments. You could write your shell script to do that for one argument and use $1 to reference the parameter. But what if you’d like to do this for a whole bunch of files? You would like to be able to invoke your script like this:

actall *.txt

knowing that the shell will pattern match and build a list of filenames that match the *.txt pattern (any filename ending with .txt).

Solution

Use the shell special variable $* to refer to all of your arguments, and use that in a for loop like this:

#!/usr/bin/env bash
# cookbook filename: chmod_all.1
#
# change permissions on a bunch of files
#
for FN in $*
do
    echo changing $FN
    chmod 0750 $FN
done

Discussion

The variable $FN is our choice; we could have used any shell variable name we wanted there. The $* refers to all the arguments supplied on the command line. For example, if the user types:

$ ./actall abc.txt another.txt allmynotes.txt

the script will be invoked with $1 equal to abc.txt and $2 equal to another.txt and $3 equal to allmynotes.txt, but $* will be equal to the entire list. In other words, after the shell has substituted the list for $* in the for statement, it will be as if the script had read:

for FN in abc.txt another.txt allmynotes.txt
do
  echo changing $FN
  chmod 0750 $FN
done

The for loop will take one value at a time from the list, assign it to the variable $FN and proceed through the list of statements between the do and the ...