Sed

2010-06-28T08:19:54Z

Judy123:

Short for "Stream Editor", '''sed''' allows you to run a stream (for example, the contents of a file, or the output of a program) through it and either match or change data. This can be particularly handy if you need to make a lot of similar changes to a file -- e.g., you find out you consistently put in a wrong hostname in a configuration file, and you need to change your conf file to point to the right server....except the same change needs to be made 100 times. [[sed]] to the rescue.

Note that the change will only happen on the first instance of the phrase in each line.

The most common usage is to change all instances of a phrase to another; to wit:

dave@samizdata:~% '''cat sed_testfile'''
dave
dave
davedave
jimbo
dave
freebsd
dave@samizdata:~% '''sed s/dave/david/ sed_testfile'''
david
david
daviddave
jimbo
david
freebsd
dave@samizdata:~% '''more sed_testfile'''
dave
dave
davedave
jimbo
dave
freebsd
dave@samizdata:~%

As you can see, no changes were made to the file itself - '''sed''' merely wrote the edited stream to standard output. You can easily put the changed data in a new file with the help of [[redirection]]:

[http://www.superiorpapers.com custom term papers]
dave@samizdata:~% '''sed s/dave/david/ sed_testfile > sed_testfile_new'''
dave@samizdata:~% '''more sed_testfile_new'''
david
david
daviddave
jimbo
david
freebsd
dave@samizdata:~%

You'll also note that it only changed the first instance of the word per line; to change all instances on each line, you'll want to use the "g" modifier (to make '''global''' switches) like this:
dave@samizdata:~% sed s/dave/david/g sed_testfile > sed_testfile_new
See the man page for more details.

To give you another example of how useful [[sed]] can be, let's say that you have a DHCP server and want to change a whole segment's range:

samizdata# '''sed s/10.1.0./10.2.0./ dhcpd.conf > new_dhcpd.conf'''

Will give you a new_dhcpd.conf file with the changes made. You'll want to check the file by hand to make sure that the changes are correct, but you just saved yourself a lot of typing :)

'''sed''' can also be used to make changes to a file or files directly, while backing up the original version of the file(s). This is done using the -i argument, along with the desired suffix for backed up files. For example:

samizdata# '''sed -i .bak s/10.1.0./10.2.0./ dhcpd.conf'''
samizdata# '''ls | grep dhcpd'''
dhcpd.conf
dhcpd.conf.bak

Note that the standard delimiter is "/" but it can be just about anything; %, _, or even commas. So changing a file with lots of forward-slashes (/) can be as ugly as
> sed s/\/dev\/am0/\/dev\/gm0/g example.fstab > example.fstab.new
or as simple as
> sed s%/dev/am0%/dev/gm0%g example.fstab > example.fstab.new

If you want to use [[regular expression]]s with '''sed''', you should install the [[gsed]] [[port]] - FreeBSD's native '''sed''' does not handle anything more complicated than a wildcard the way you'd expect it to. For example, say you want to use the '''\b''' (word boundary) regex code to change instances of "david" to "dave", ''without'' breaking any instances of "davidson":

samizdata# '''echo david davidson | sed s/david/dave/g'''
dave daveson

Not what we want! So we'll use the standard '''\b''' (word boundary) regex code to make sure we don't change words that begin with "david":

samizdata# '''echo david davidson | sed "s/david\b/dave/g"'''
david davidson

Not what we expected! But if we install [[gsed]] from [[ports]] and use it instead:

samizdata# '''cd /usr/ports/textproc/gsed && make install clean && [[rehash]]'''
samizdata# '''echo david davidson | gsed "s/david\b/dave/g"'''
dave davidson

Much better! You may be tempted to simply overwrite the base sed with [[gsed]], but carefully consider it before you do so: if you have any utilities installed which depend on FreeBSD's native implementations differences in regex (or flag) handling, you could break them. This generally should not be a problem, but BACK UP THE ORIGINAL VERSION of FreeBSD's '''sed''' before overwriting it with [[gsed]] if you decide to do so.

For more info, check out these pages:

http://www.student.northpark.edu/pemente/sed/

http://www.faqs.org/faqs/editor-faq/sed/

http://www.grymoire.com/Unix/Sed.html

[[Category:System Commands]]

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Sed