How to detect and remove indentation of a piped text

Question

I'm looking for a way to remove the indentation of a piped text. Below is a solution using cut -c 9- which assumes the indentation is 8 character wide.

I'm looking for a solution which can detect the number of spaces to remove. This implies going through the whole (piped) file to know the minimum number of spaces (tabs?) used to indent it, then remove them on each line.

run.sh

help() {
    awk '
    /esac/{b=0}
    b
    /case "\$arg" in/{b=1}' \
    "$me" \
    | cut -c 9-
}

while [[ $# -ge 1 ]]
do
    arg="$1"
    shift
    case "$arg" in
        help|h|?|--help|-h|'-?')
            # Show this help
            help;;
    esac
done

$ ./run.sh --help

help|h|?|--help|-h|'-?')
    # Show this help
    help;;

---

Note: echo $' 4\n 2\n 3' | python3 -c 'import sys; import textwrap as tw; print(tw.dedent(sys.stdin.read()), end="")' works but I expect there is a better, way (I mean, one which doesn't only depends on software more common than python. Maybe awk? I wouldn't mind seeing a perl solution either.

Note2: echo $' 4\n 2\n 3' | python -c 'import sys; import textwrap as tw; print tw.dedent(sys.stdin.read()),' also works (Python 2.7.15rc1).

Answer 1

The following is pure bash, with no external tools or command substitutions:

#!/usr/bin/env bash
all_lines=( )
min_spaces=9999 # start with something arbitrarily high
while IFS= read -r line; do
  all_lines+=( "$line" )
  if [[ ${line:0:$min_spaces} =~ ^[[:space:]]*$ ]]; then
    continue  # this line has at least as much whitespace as those preceding it
  fi
  # this line has *less* whitespace than those preceding it; we need to know how much.
  [[ $line =~ ^([[:space:]]*) ]]
  line_whitespace=${BASH_REMATCH[1]}
  min_spaces=${#line_whitespace}
done

for line in "${all_lines[@]}"; do
  printf '%s\n' "${line:$min_spaces}"
done

Its output is:

  4
2
 3

Answer 2

Suppose you have:

$ echo $'    4\n  2\n   3\n\ttab'
    4
  2
   3
    tab

You can use the Unix expand utility to expand the tabs to spaces. Then run through an awk to count the minimum number of spaces on a line:

$ echo $'    4\n  2\n   3\n\ttab' | 
expand | 
awk 'BEGIN{min_indent=9999999}
     {lines[++cnt]=$0
      match($0, /^[ ]*/)
      if(RLENGTH<min_indent) min_indent=RLENGTH
     }
     END{for (i=1;i<=cnt;i++) 
               print substr(lines[i], min_indent+1)}'
  4
2
 3
      tab