7.6 Reverse Characters of Lines

This script can be used to reverse the position of characters in lines. The technique moves two characters at a time, hence it is faster than more intuitive implementations.

Note the tx command before the definition of the label. This is often needed to reset the flag that is tested by the t command.

Imaginative readers will find uses for this script. An example is reversing the output of banner.¹⁰

#!/usr/bin/sed -f

/../! b

# Reverse a line.  Begin embedding the line between two newlines
s/^.*$/\
&\
/

# Move first character at the end.  The regexp matches until
# there are zero or one characters between the markers
tx
:x
s/\(\n.\)\(.*\)\(.\n\)/\3\2\1/
tx

# Remove the newline markers
s/\n//g

#! /bin/sh

banner -w $1 $2 $3 $4 |
  sed -e :a -e '/^.\{0,'$1'\}$/ { s/$/ /; ba; }' |
  ~/sedscripts/reverseline.sed