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Shell scripting is really pretty good fun. I’m nowhere near claiming to be an expert (hell, I’ve been doing it for about three weeks now), but I kind of feel like I’m getting somewhere. Here’s some stuff I’ve been messing around with, hopefully accompanied by some clear explanations that, if nothing else, will help me get my head around it a little better. I’m playing with my chat logs because I’m obsessed with stats (hopefully you’ve figured that out by now).

Redirecting output

One of the most simple things, but it bears noting down. You can redirect the output of the bash shell by using the greater than symbol (>). Let’s build it up:

% ls

Will list all the files and folders in directory.

% ls -R

Applies the recursive -R switch, which will list all the files and folders, as well as any files inside any folders.

% ls -R > output_list.txt

Now we get on to the redirecting output bit: the > part followed by a filename will take the output and put it in the file you named. Here’s the output - not that you care, or anything…

Grep

Grep is something different: a command for searching, basically. I’m going to use it with the file that was created above in order to make it more readable.

% grep hello example.txt

Will search for (and print) the lines containing “hello” in the file text file “example”. Clearly this is a pretty basic use of the tool, but it works. To get a bit more complicated:

% grep .chatlog output_list.txt > filtered_list.txt

Will print all the lines in the output_list file that contain the phrase “.chatlog”, and save them to the filtered_list file.

% egrep .chatlog$ output_list.txt > filtered_list.txt

What’s changed? The egrep just tells grep to apply the -e flag, which will make it use regular expressions for searching. I haven’t bothered trying to understand these. For me, it’s only to add the $ sign to the end of the string to indicate it should only search for that at the end of a line. Here’s the output from the second part. Much easier to read, much easier to do stats on/with…

There you go

Hopefully that helps someone understand something a little better. If I’ve got something wrong, let me know please. I might report back if I can find it in me to manipulate the text file a little bit.

Comments

By Farhan Mannan on 09 November 2008 at 20:29:

For a long time, I tried to muster the motivation to learn stuff like this. Because of that stupid Yellow Dog Linux thing, I got my wish…