How to iterate through an array of numbers in shell-script?

Question

I am trying to iterate through an array created after some command execution. The code used is:

#!/bin/bash mailx -H|grep '^ [UN]'>ListOfMessages.txt msgNumbers=`cut -c 4-5 ListOfMessages.txt` echo $msgNumbers for msg in "${msgNumbers[@]}"; do echo $msg; echo $msg|mailx; done 

The ListOfMessages.txt look like:

 U 5 Sender1 Thu Aug 23 14:28 179/10454 Incident U 7 Sender2 Thu Aug 23 15:20 179/10456 Incident U 8 Sender3 Thu Aug 23 15:41 192/10801 Incident N 9 Sender4 Thu Aug 23 15:45 197/11011 Incident 

The array is getting updated with the numbers 5 7 8 9 as required. I keep getting "Bad Substitution" error(at the line where for loop starts).

Please advise me on the she-bang to be used.

Answer 1

As far as I can see, you have no array in your code. The variable msgNumbers is a string that holds the output of your cut command.

To iterate over the output of cut, use

#!/bin/bash mailx -H | grep '^ [UN]' | cut -c 4-5 | while read msg; do print 'msg = %s\s' "$msg" done 

This sends the output of cut into the while loop immediately following it, through the pipe (|). The while loop will iterate with msg set to each individual line of output from cut.

The cut gets its data directly from the grep command, which removes the need for storing the data in an intermediate file or variable.

I removed the echo $msg|mailx; command because it did not make much sense to me (the mailx utility needs an address to send the data to).

The grep+cut could also be replaced by a single call to awk where we let awk do the work of both tools and output the second whitespace-delimited column when the regular expression matches:

#!/bin/bash mailx -H | awk '/^ [UN]/ { print $2 }' | while read msg; do print 'msg = %s\s' "$msg" done 

I'm not commenting further on the use of mailx as it is a non-standard utility which is implemented slightly differently across Unix systems (my version does not have a -H option, for example).

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The #!-line looks ok to me, if you want the script to be executed by bash and if the bash executable is located at that path (which it commonly is on Linux systems, for example, but check with command -v bash on your system to be sure). The code I have posted above is compatible with /bin/sh, so bash isn't really needed to run it (it would run in any sh-like shell).

Just make sure that the script is executable and that you run it without specifying an explicit interpreter.

Answer 2

Don't double quote the msgNumbers reference, and, msgNumbers is not an array, so the indexing is pointless.

Answer 3

Use () to store the value as an array.

msgNumbers=(`cut -c 4-5 ListOfMessages.txt`) echo "${msgNumbers[@]}" for msg in "${msgNumbers[@]}"; do echo $msg; done 

Answer 4

The issue is solved. The problem was that msgNumbers was not an indexed array. So I changed the for msg in "${msgNumbers[@]}";to for msg in ${msgNumbers};.Now its working. The rest of the code is same as in my question.