Sed
Short for "Streamline Editor", sed allows you to run a file through it and either match or change data without actually editing the file itself. 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, the changes were not actually made to the file. I didn't tell it to put the changes anywhere! Easily fixed with the help of redirection:
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 :)
Another option for using sed is to have it make changes in place, but save the original version of the file(s) with a suffix (like .bak) added, using the -i argument. 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; %, _, 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
For more info, check out these pages:
http://www.student.northpark.edu/pemente/sed/
