How to select multiple lines from a file or from pipe in a script?

Question

I'd like to have a script, called lines.sh that I can pipe data to to select a series of lines.

For example, if I had the following file:

test.txt

a 
b
c
d

Then I could run:

cat test.txt | lines 2,4

and it would output

b
d

I'm using zsh, but would prefer a bash solution if possible.

Answer 1

You can use this awk:

awk -v s='2,4' 'BEGIN{split(s, a, ","); for (i in a) b[a[i]]} NR in b' file
two
four

Via a separate script lines.sh:

#!/bin/bash
awk -v s="$1" 'BEGIN{split(s, a, ","); for (i in a) b[a[i]]} NR in b' "$2"

Then give execute permissions:

chmod +x lines.sh

And call it as:

./lines.sh '2,4' 'test.txt'