Iterating through a list of commands in a file in Shell Programming

Question

I have a file containing a list of unix commands like "ls -la /etc/passwd". I want to iterate through this list of commands, and take appropriate action.

Currently, my code is something like:

#!/bin/bash
clear
for cmds in `cat /tmp/cmd`
do
        if [  $cmds ] ; then
                echo $cmds;
        fi
done

But, what I get is an output where the commands are broken up in different lines like

ls 
-la 
/etc/passwd
ls 
-la
/etc/shadow

and not

ls -la /etc/passwd
ls -la /etc/shadow

Any ideas?

Answer 1

Add this before your for loop:

IFS='
'

Or in Bash:

IFS=$'\n'

IFS contains a list of characters which are used to split input into fields; it defaults to including spaces, tabs, and linefeeds, which was why you were getting the incorrect behavior. Setting it to a newline will only split on newlines, which is what you want.

Answer 2

I believe you can also do this (surround your backticks with double-quotes):

"`cat /tmp/cmds`"

Maybe try something like this (not sure what your "if" is supposed to be checking - I modified it to make sure the $cmds var was not empty string):

#!/bin/bash

clear

for cmds in "`cat cmds.txt`"
do
    if [ "$cmds" != '' ]; then
        echo "$cmds"
    fi
done

Answer 3

If each line in the file is a complete command - no continuations over multiple lines - then you can use read:

while read cmd
do
    if [ -z "$cmd" ]
    then : Empty
    elif $cmd
    then : OK
    else : Oops
    fi
done < cmds.txt

Or, if you prefer linear construction and don't need anything from the sub-shell that the while loop represents in what follows, you can use:

cat cmds.txt |
while read cmd
do
    if [ -z "$cmd" ]
    then : Empty
    elif $cmd
    then : OK
    else : Oops
    fi
done

Note the careful use of quotes around the tested string. I still prefer to use an explicit '-z' operator; there are those who argue it is not 100% necessary. I still use single square brackets rather than double - mainly out of habit built up over 20+ years. Those who learned shell using bash or ksh often prefer the double square bracket operator instead.

Beware I/O redirections, of course.

Also, if you don't have anything to do in the various then and else clauses, you should simply feed cmds.txt to the shell:

sh cmds.txt

This is by far the most reliable way of dealing with multi-line commands.