How to indent lines in text files using sed, awk, perl

Suppose I have a text file named input.txt, and I want to indent each line in the file by 5 spaces.

$ cat input.txt
12
34
56
78

The following are different ways to do the same thing:

• sed
  

  
  $ sed  's/^/     /'  input.txt
       12
       34
       56
       78

  
  s/^/ / searches for the beginning of a line (^), and "replaces" that with 5 spaces.
  
  
• awk
  

  
  $ awk  '{ print "     " $0 }'  input.txt
       12
       34
       56
       78

  
  
  
• perl
  

  $ perl -pe  's/^/     /' input.txt
       12
       34
       56
       78

  
  

To indent a range of lines, say lines 1 to 3, inclusive:

• sed
  

  $ sed  '1,3s/^/     /' input.txt
       12
       34
       56
  78

  
  Note that the comma specifies a range (from the line before the comma to the line after).
  
  
• awk
  

  $  awk  'NR==1,NR==3 {$0 = "     "$0} {print }' input.txt
       12
       34
       56
  78

  
  An awk program consists of condition/action pairs. The first pair has the condition "if NR (current line number) is from 1 to 3", and the action is to append 5 spaces to $0 (the current line). The second pair has a null condition which means it applies to every line, and the action is just to print the line.
  
  
• perl
  

  $ perl  -pe '$_ = "     " . $_ if  1 <= $. and $. <=3' input.txt
       12
       34
       56
  78

  
  $. is the current input line number.
  $_ is the current line.
  . (dot) is the concatenate operator.