Sed

Introduction

Sed stands for stream editor and it does just that, it edits a stream. There is many uses for it for example in the well known substitution mode.

sed and substitution

Using the s allows the results to be substituted like so:

echo one two three four five six seven eight nine | sed -e 's/one/bleep/g' -e 's/five/bleep/g'

would result in:

bleep two three four bleep six seven eight nine

In a directory where files were backed up as "file-save" the renaming from the backup to the original file went like this:

ls *save| while read i ; do NEW=`echo $i | sed -e 's/-save//g'`; mv -f $i $NEW; done

sed and deleting

If I wanted to delete lines 1 through 46 the syntax would be so:

francisco$ sed -e '1,46d' /etc/passwd   
_postfix:*:507:507:Postfix Daemon:/var/empty:/sbin/nologin
testuser:*:1001:1001:test:/usr/home/testuser:/bin/ksh

You see the last two entries of a 48 line file. This can also be achieved with head.

sed and printing certain rows

Pretend you need to work on the first entry in the /etc/passwd file:

francisco$ sed -n 1p /etc/passwd
root:*:0:0:Charlie &:/root:/bin/ksh

And this is how you print rows 2 through 5:

francisco$ sed -n 2,5p /etc/passwd
daemon:*:1:1:The devil himself:/root:/sbin/nologin
operator:*:2:5:System &:/operator:/sbin/nologin
bin:*:3:7:Binaries Commands and Source,,,:/:/sbin/nologin
smmsp:*:25:25:Sendmail Message Submission Program:/nonexistent:/sbin/nologin

sed delimiters

pretend you see this:

sed -e 's=rat=/raven/=g' file

it looks difficult because you may not know that sed can use any delimiter between a substitute.

In the example given every instance of rat is changed to /raven/ with the slashes. Here is another one

sed -e s,/bin,/newbin,g file

Here the delimiter is a comma (,) and / is part of the substitute text. This is totally ok.