I need to make some changes to a whole bunch of systems and on of the things to do is alter systems files... files such as fstab & iscsi.conf

What I'm simply looking to do is have a shell script take out a uncomment symbol, "#", out of fstab. How can I do that in a shell script?

Using sed


Cheers,
Tink

For instance, for a file testfile.conf, with the contents:

key1:yes
key2:no
#key3:no


The following command will create a backup file testfile.conf.old, and uncomment the third line:

Rob

Ok. That makes more sense! Do I have to include all the text from the certain line I want to uncomment? Also I don't really need a backup so what part of the code do I take out? Please forgive me I've never used sed for anything.

OK - to break down that command:

sed
#erm, sed
-i
#edit the file in place, rather than having to direct
#output to another file
\.old
#this is the suffix for the backup file. The backslash
#"escapes" the full stop (stops it having its special #meaning)
's/<pattern>/<substituted pattern>/'
#the sed substitute command
\(...\)
#defines a block to be matched
# and key3
#the literal strings
.
#matches any single character
*
#matches the previous character 0 or more times. Since the
#preceding character was . it means "match any number of
#any characters"
\2
#Replay what was matched by the second block - i.e.
#key3 followed by any other characters.
testfile.conf
#The file to be processed

No, you don't have to include all the text from the line to be matched. You just have to specify a pattern that will match that line (and no line that you do not want uncommenting!). So (in this very simple example) #key3.* matches #key3:no, but would also match #key31957:fnord! It's worth having a look at man regex, and googling for help on regular expressions (this form of pattern matching). Once you get your head around them, regexes are one of the most powerful tools for working with strings, and are used with many other linux tools (most notably grep) as well as the majority of programming languages.

And as for backups - if you omit the suffix following -i, one won't be created. I usually feel a lot happier if I back up a config file before changing it, though.

Rob

*Especially* when "I need to make some changes to a whole bunch of systems ..."

Make the backup(s) - easy enough to clean up later. Better you never need a backup you've got than need one you didn't take.

So for this script to work I need to be in the conf file's specific directory right?

Can I do this then?

sed -i\.old 's/\(#\)\(key3.*\)/\2/' /etc/testfile.conf

Of course you can - stick it in a script and pass the name on invocation if it changes on some/all of the sysytems if you like.

Tink was gently trying to nudge you in the direction of reading some doco - start with the man page then have a look at this for a tutorial. The fella that wrote it knows a few things about Linux.

Ok, will do! Thanks everybody for your help!