Bash: Succinct way to loop over lines from stdin or command line arguments

bash

I've got a bash script I'd like to loop over the lines in stdin, or loop over each argument passed in.

Is there a clean way to write this so I don't have to have 2 loops?

#!/bin/bash

# if we have command line args... 
if [ -t 0 ]
then
  # loop over arguments
  for arg in "$@" 
  do
    # process each argument
  done
else
  # loop over lines from stdin
  while IFS= read -r line; do
    # process each line
  done
fi

EDIT: I'm looking for a generic solution that just uses a single loop, as I find I want to do this quite often, but have always wrote out 2 loops and then called a function instead. So maybe something that turns stdin into an array, so I could use a single loop instead?

Best Answer

We can also use redirection of standard input:

#!/usr/bin/env bash
test -t 0 && exec < <(printf '%s\n' "$@")
while IFS= read -r line; do
    echo "$line"
done

test with :

test.sh Hello World
test.sh < /etc/passwd

Related Solutions

Bash – In bash, feed output of one command line-by-line into another command

mysql --format csv --skip-column-names \
    -e "select xmldata from mytable limit 1;" \
    | while IFS= read -r line; do xmllint --format - <<<"$line"; done

Split for easier reading, obviously you don't need to. The <<<$line is a herestring, which feeds the contents of $line to xmllint as stdin.

Bash – Passing a bash command-line argument containing a dot

You need to use declare -A instead of declare -a. You are clearly using associative arrays with arbitrary string arguments as indices, but declare -a is only for integer indexed arrays. arg.txt does not evaluate to a valid integer, hence your error.

Edit

You seem to be using bash version 3. Unfortunately, associative arrays are not available until version 4. I recommend you post a sanitized version of your original deploy.sh script with sensitive personal information removed so you can get ideas from other people about alternative approaches.

Edit 2

Just to summarize a bit of exchange in the chat:

The easiest way to do some action over all the arguments is to just iterate over them with a for loop:

    for arg; do
        scp login1@host1:"$arg" login2@host2:/dest/
    done

Remember to double-quote all instances of "$arg". You do not need to put the arguments in an array yourself, as they already exist in the array $@, which is what for uses by default when you don't give an explicit in list....