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4.7 Numbering Lines

This script replaces ‘cat -n’; in fact it formats its output exactly like GNU cat does.

Of course this is completely useless and for two reasons: first, because somebody else did it in C, second, because the following Bourne-shell script could be used for the same purpose and would be much faster:

#! /bin/sh
sed -e "=" $@ | sed -e '
  s/^/      /
  N
  s/^ *\(......\)\n/\1  /
'

It uses sed to print the line number, then groups lines two by two using N. Of course, this script does not teach as much as the one presented below.

The algorithm used for incrementing uses both buffers, so the line is printed as soon as possible and then discarded. The number is split so that changing digits go in a buffer and unchanged ones go in the other; the changed digits are modified in a single step (using a y command). The line number for the next line is then composed and stored in the hold space, to be used in the next iteration.

#!/usr/bin/sed -nf

# Prime the pump on the first line
x
/^$/ s/^.*$/1/
# Add the correct line number before the pattern
G
h
# Format it and print it
s/^/      /
s/^ *\(......\)\n/\1  /p
# Get the line number from hold space; add a zero
# if we're going to add a digit on the next line
g
s/\n.*$//
/^9*$/ s/^/0/
# separate changing/unchanged digits with an x
s/.9*$/x&/
# keep changing digits in hold space
h
s/^.*x//
y/0123456789/1234567890/
x
# keep unchanged digits in pattern space
s/x.*$//
# compose the new number, remove the newline implicitly added by G
G
s/\n//
h

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