Question

I want create a ruby script that I can run on the command line that will:

1. run a command line command like e.g.  "ls -l"
2. then I pass an argument for a specific column from the output of #1
3. for each column specified in #2, it will run another command

ls -l was just an example, I want this to be generic.

I’m newish to ruby, still not getting some of this so if you could commend what and how you did it that would be very helpful for me.

Answer 1

I frequently use |awk '{print $5;}' (replace 5 with the one-indexed column of your choice) in pipelines. You can get similarly easy one-liners with ruby if you use some funny command-line switches. Compare:

$ ls -l | awk '{print $5;}'

404
16
10
8733

vs

$ ls -l | ruby -ne "a = split; puts a[4];"
nil
404
16
10
8733

ruby -n places an implicit while gets() do .. end loop around your code, and ruby -e passes script elements on the command line. Combine the two and you get some easy one-liner scripting ability, similar to perl -ne or awk(1).

The manpage mentions another option that looks very useful:

 -a             Turns on auto-split mode when used with -n or
                -p.  In auto-split mode, Ruby executes
                      $F = $_.split
                at beginning of each loop.

Juan points out how to use it (single quotes prevents my shell, bash(1), from interpreting the $F[3] as a variable reference).:

$ ls -l | ruby -ane 'puts $F[3]'
nil
root
root
root
root
sarnold
sarnold
...