4. Some Sample Scripts

Here are some sed scripts to guide you in the art of mastering sed.

Some exotic examples: 4.1 Centering Lines 

Emulating standard utilities:

4.1 Centering Lines

This script centers all lines of a file on a 80 columns width. To change that width, the number in \{…\} must be replaced, and the number of added spaces also must be changed.

Note how the buffer commands are used to separate parts in the regular expressions to be matched--this is a common technique.

#!/usr/bin/sed -f

# Put 80 spaces in the buffer
1 {
  x
  s/^$/          /
  s/^.*$/&&&&&&&&/
  x
}

# del leading and trailing spaces
y/tab/ /
s/^ *//
s/ *$//

# add a newline and 80 spaces to end of line
G

# keep first 81 chars (80 + a newline)
s/^\(.\{81\}\).*$/\1/

# \2 matches half of the spaces, which are moved to the beginning
s/^\(.*\)\n\(.*\)\2/\2\1/

4.2 Increment a Number

This script is one of a few that demonstrate how to do arithmetic in sed. This is indeed possible,(7) but must be done manually.

To increment one number you just add 1 to last digit, replacing it by the following digit. There is one exception: when the digit is a nine the previous digits must be also incremented until you don't have a nine.

This solution by Bruno Haible is very clever and smart because it uses a single buffer; if you don't have this limitation, the algorithm used in Numbering lines, is faster. It works by replacing trailing nines with an underscore, then using multiple s commands to increment the last digit, and then again substituting underscores with zeros.

#!/usr/bin/sed -f

/[^0-9]/ d

# replace all leading 9s by _ (any other character except digits, could
# be used)
:d
s/9\(_*\)$/_\1/
td

# incr last digit only.  The first line adds a most-significant
# digit of 1 if we have to add a digit.
#
# The tn commands are not necessary, but make the thing
# faster

s/^\(_*\)$/1\1/; tn
s/8\(_*\)$/9\1/; tn
s/7\(_*\)$/8\1/; tn
s/6\(_*\)$/7\1/; tn
s/5\(_*\)$/6\1/; tn
s/4\(_*\)$/5\1/; tn
s/3\(_*\)$/4\1/; tn
s/2\(_*\)$/3\1/; tn
s/1\(_*\)$/2\1/; tn
s/0\(_*\)$/1\1/; tn

:n
y/_/0/

4.3 Rename Files to Lower Case

This is a pretty strange use of sed. We transform text, and transform it to be shell commands, then just feed them to shell. Don't worry, even worse hacks are done when using sed; I have seen a script converting the output of date into a bc program!

The main body of this is the sed script, which remaps the name from lower to upper (or vice-versa) and even checks out if the remapped name is the same as the original name. Note how the script is parameterized using shell variables and proper quoting.

#! /bin/sh
# rename files to lower/upper case... 
#
# usage: 
#    move-to-lower * 
#    move-to-upper * 
# or
#    move-to-lower -R .
#    move-to-upper -R .
#

help()
{
        cat << eof
Usage: $0 [-n] [-r] [-h] files...

-n      do nothing, only see what would be done
-R      recursive (use find)
-h      this message
files   files to remap to lower case

Examples:
       $0 -n *        (see if everything is ok, then...)
       $0 *

       $0 -R .

eof
}

apply_cmd='sh'
finder='echo "$@" | tr " " "\n"'
files_only=

while :
do
    case "$1" in 
        -n) apply_cmd='cat' ;;
        -R) finder='find "$@" -type f';;
        -h) help ; exit 1 ;;
        *) break ;;
    esac
    shift
done

if [ -z "$1" ]; then
        echo Usage: $0 [-h] [-n] [-r] files...
        exit 1
fi

LOWER='abcdefghijklmnopqrstuvwxyz'
UPPER='ABCDEFGHIJKLMNOPQRSTUVWXYZ'

case `basename $0` in
        *upper*) TO=$UPPER; FROM=$LOWER ;;
        *)       FROM=$UPPER; TO=$LOWER ;;
esac

eval $finder | sed -n '

# remove all trailing slashes
s/\/*$//

# add ./ if there is no path, only a filename
/\//! s/^/.\//

# save path+filename
h

# remove path
s/.*\///

# do conversion only on filename
y/'$FROM'/'$TO'/

# now line contains original path+file, while
# hold space contains the new filename
x

# add converted file name to line, which now contains
# path/file-name\nconverted-file-name
G

# check if converted file name is equal to original file name,
# if it is, do not print nothing
/^.*\/\(.*\)\n\1/b

# now, transform path/fromfile\n, into
# mv path/fromfile path/tofile and print it
s/^\(.*\/\)\(.*\)\n\(.*\)$/mv "\1\2" "\1\3"/p

' | $apply_cmd

4.4 Print bash Environment

This script strips the definition of the shell functions from the output of the set Bourne-shell command.

#!/bin/sh

set | sed -n '
:x

# if no occurrence of `=()' print and load next line
/=()/! { p; b; }
/ () $/! { p; b; }

# possible start of functions section
# save the line in case this is a var like FOO="() "
h

# if the next line has a brace, we quit because
# nothing comes after functions
n
/^{/ q

# print the old line
x; p

# work on the new line now
x; bx
'

4.5 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.(8)

#!/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

4.6 Reverse Lines of Files

This one begins a series of totally useless (yet interesting) scripts emulating various Unix commands. This, in particular, is a tac workalike.

Note that on implementations other than GNU sed this script might easily overflow internal buffers.

#!/usr/bin/sed -nf

# reverse all lines of input, i.e. first line became last, ...

# from the second line, the buffer (which contains all previous lines)
# is *appended* to current line, so, the order will be reversed
1! G

# on the last line we're done -- print everything
$ p

# store everything on the buffer again
h

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

4.8 Numbering Non-blank Lines

Emulating `cat -b' is almost the same as `cat -n'--we only have to select which lines are to be numbered and which are not.

The part that is common to this script and the previous one is not commented to show how important it is to comment sed scripts properly...

#!/usr/bin/sed -nf

/^$/ {
  p
  b
}

# Same as cat -n from now
x
/^$/ s/^.*$/1/
G
h
s/^/      /
s/^ *\(......\)\n/\1  /p
x
s/\n.*$//
/^9*$/ s/^/0/
s/.9*$/x&/
h
s/^.*x//
y/0123456789/1234567890/
x
s/x.*$//
G
s/\n//
h

4.9 Counting Characters

This script shows another way to do arithmetic with sed. In this case we have to add possibly large numbers, so implementing this by successive increments would not be feasible (and possibly even more complicated to contrive than this script).

The approach is to map numbers to letters, kind of an abacus implemented with sed. `a's are units, `b's are tens and so on: we simply add the number of characters on the current line as units, and then propagate the carry to tens, hundreds, and so on.

As usual, running totals are kept in hold space.

On the last line, we convert the abacus form back to decimal. For the sake of variety, this is done with a loop rather than with some 80 s commands(9): first we convert units, removing `a's from the number; then we rotate letters so that tens become `a's, and so on until no more letters remain.

#!/usr/bin/sed -nf

# Add n+1 a's to hold space (+1 is for the newline)
s/./a/g
H
x
s/\n/a/

# Do the carry.  The t's and b's are not necessary,
# but they do speed up the thing
t a
: a;  s/aaaaaaaaaa/b/g; t b; b done
: b;  s/bbbbbbbbbb/c/g; t c; b done
: c;  s/cccccccccc/d/g; t d; b done
: d;  s/dddddddddd/e/g; t e; b done
: e;  s/eeeeeeeeee/f/g; t f; b done
: f;  s/ffffffffff/g/g; t g; b done
: g;  s/gggggggggg/h/g; t h; b done
: h;  s/hhhhhhhhhh//g

: done
$! {
  h
  b
}

# On the last line, convert back to decimal

: loop
/a/! s/[b-h]*/&0/
s/aaaaaaaaa/9/
s/aaaaaaaa/8/
s/aaaaaaa/7/
s/aaaaaa/6/
s/aaaaa/5/
s/aaaa/4/
s/aaa/3/
s/aa/2/
s/a/1/

: next
y/bcdefgh/abcdefg/
/[a-h]/ b loop
p

4.10 Counting Words

This script is almost the same as the previous one, once each of the words on the line is converted to a single `a' (in the previous script each letter was changed to an `a').

It is interesting that real wc programs have optimized loops for `wc -c', so they are much slower at counting words rather than characters. This script's bottleneck, instead, is arithmetic, and hence the word-counting one is faster (it has to manage smaller numbers).

Again, the common parts are not commented to show the importance of commenting sed scripts.

#!/usr/bin/sed -nf

# Convert words to a's
s/[ tab][ tab]*/ /g
s/^/ /
s/ [^ ][^ ]*/a /g
s/ //g

# Append them to hold space
H
x
s/\n//

# From here on it is the same as in wc -c.
/aaaaaaaaaa/! bx;   s/aaaaaaaaaa/b/g
/bbbbbbbbbb/! bx;   s/bbbbbbbbbb/c/g
/cccccccccc/! bx;   s/cccccccccc/d/g
/dddddddddd/! bx;   s/dddddddddd/e/g
/eeeeeeeeee/! bx;   s/eeeeeeeeee/f/g
/ffffffffff/! bx;   s/ffffffffff/g/g
/gggggggggg/! bx;   s/gggggggggg/h/g
s/hhhhhhhhhh//g
:x
$! { h; b; }
:y
/a/! s/[b-h]*/&0/
s/aaaaaaaaa/9/
s/aaaaaaaa/8/
s/aaaaaaa/7/
s/aaaaaa/6/
s/aaaaa/5/
s/aaaa/4/
s/aaa/3/
s/aa/2/
s/a/1/
y/bcdefgh/abcdefg/
/[a-h]/ by
p

4.11 Counting Lines

No strange things are done now, because sed gives us `wc -l' functionality for free!!! Look:

#!/usr/bin/sed -nf
$=

4.12 Printing the First Lines

This script is probably the simplest useful sed script. It displays the first 10 lines of input; the number of displayed lines is right before the q command.

#!/usr/bin/sed -f
10q

4.13 Printing the Last Lines

Printing the last n lines rather than the first is more complex but indeed possible. n is encoded in the second line, before the bang character.

This script is similar to the tac script in that it keeps the final output in the hold space and prints it at the end:

#!/usr/bin/sed -nf

1! {; H; g; }
1,10 !s/[^\n]*\n//
$p
h

Mainly, the scripts keeps a window of 10 lines and slides it by adding a line and deleting the oldest (the substitution command on the second line works like a D command but does not restart the loop).

The "sliding window" technique is a very powerful way to write efficient and complex sed scripts, because commands like P would require a lot of work if implemented manually.

To introduce the technique, which is fully demonstrated in the rest of this chapter and is based on the N, P and D commands, here is an implementation of tail using a simple "sliding window."

This looks complicated but in fact the working is the same as the last script: after we have kicked in the appropriate number of lines, however, we stop using the hold space to keep inter-line state, and instead use N and D to slide pattern space by one line:

#!/usr/bin/sed -f

1h
2,10 {; H; g; }
$q
1,9d
N
D

Note how the first, second and fourth line are inactive after the first ten lines of input. After that, all the script does is: exiting on the last line of input, appending the next input line to pattern space, and removing the first line.

4.14 Make Duplicate Lines Unique

This is an example of the art of using the N, P and D commands, probably the most difficult to master.

#!/usr/bin/sed -f
h

:b
# On the last line, print and exit
$b
N
/^\(.*\)\n\1$/ {
    # The two lines are identical.  Undo the effect of
    # the n command.
    g
    bb
}

# If the N command had added the last line, print and exit
$b

# The lines are different; print the first and go
# back working on the second.
P
D

As you can see, we mantain a 2-line window using P and D. This technique is often used in advanced sed scripts.

4.15 Print Duplicated Lines of Input

This script prints only duplicated lines, like `uniq -d'.

#!/usr/bin/sed -nf

$b
N
/^\(.*\)\n\1$/ {
    # Print the first of the duplicated lines
    s/.*\n//
    p

    # Loop until we get a different line
    :b
    $b
    N
    /^\(.*\)\n\1$/ {
        s/.*\n//
        bb
    }
}

# The last line cannot be followed by duplicates
$b

# Found a different one.  Leave it alone in the pattern space
# and go back to the top, hunting its duplicates
D

4.16 Remove All Duplicated Lines

This script prints only unique lines, like `uniq -u'.

#!/usr/bin/sed -f

# Search for a duplicate line --- until that, print what you find.
$b
N
/^\(.*\)\n\1$/ ! {
    P
    D
}

:c
# Got two equal lines in pattern space.  At the
# end of the file we simply exit
$d

# Else, we keep reading lines with N until we
# find a different one
s/.*\n//
N
/^\(.*\)\n\1$/ {
    bc
}

# Remove the last instance of the duplicate line
# and go back to the top
D

4.17 Squeezing Blank Lines

As a final example, here are three scripts, of increasing complexity and speed, that implement the same function as `cat -s', that is squeezing blank lines.

The first leaves a blank line at the beginning and end if there are some already.

#!/usr/bin/sed -f

# on empty lines, join with next
# Note there is a star in the regexp
:x
/^\n*$/ {
N
bx
}

# now, squeeze all '\n', this can be also done by:
# s/^\(\n\)*/\1/
s/\n*/\
/

This one is a bit more complex and removes all empty lines at the beginning. It does leave a single blank line at end if one was there.

#!/usr/bin/sed -f

# delete all leading empty lines
1,/^./{
/./!d
}

# on an empty line we remove it and all the following
# empty lines, but one
:x
/./!{
N
s/^\n$//
tx
}

This removes leading and trailing blank lines. It is also the fastest. Note that loops are completely done with n and b, without relying on sed to restart the the script automatically at the end of a line.

#!/usr/bin/sed -nf

# delete all (leading) blanks
/./!d

# get here: so there is a non empty
:x
# print it
p
# get next
n
# got chars? print it again, etc... 
/./bx

# no, don't have chars: got an empty line
:z
# get next, if last line we finish here so no trailing
# empty lines are written
n
# also empty? then ignore it, and get next... this will
# remove ALL empty lines
/./!bz

# all empty lines were deleted/ignored, but we have a non empty.  As
# what we want to do is to squeeze, insert a blank line artificially
i\

bx

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