AUC 2 – Arbitrary Unix Command #2

Today I wanted to talk about a command that we use every day: ls. Yes, this isn’t a very arbitrary pick, but I decided sharing useful commands to be more important than obscure.

If you aren’t a regular unix user, the command ‘ls’ is equivalent to the Windows ‘dir’ command when using cmd.exe.

It is so frequently used that nearly 10% of my bash history contains ‘ls’. When considering the unix mantra that every command should do only one thing, but do it well, ls is surprisingly robust.

While you may be quite familiar with this command, I always recommend checking the man page for a unix command as command switches may differ depending on your system (ie: gnu commands) and more importantly you may learn something new and useful. You can do so by typing:

man ls

This should look familiar. The -l switch produces a vertical list (just like dir /p in cmd.exe.) The -a switch includes hidden files, which are typically preceded by a period. I won’t go into detail on how to read the permissions as the internet has many such guides.

Lets say you are interested in only file names of a particular directory. The -1 switch will declutter the output quite nicely and only spit out a list of filenames and not any of the meta information such as file permissions, file ownership, size, and last file modify time.

Ok. So what? Perhaps you are interested in performing a command against every file (not directory) in your current working directory. The first of the two commands above produces a single line of output, space delimited list of filenames. (I have see Internet postings frowning upon the use of xargs to convert newlines into spaces as I have, but still need to discover a better way.)

The second command builds upon the first by embedding it in backticks (usually shared with the tilde key) in use with a for loop. The command string inside the backticks is executed first and produces a space seperated list for the x variable in the for loop to iterate through. The result is a copy and paste-able start for a bash script. I’ve used the head command here as a placeholder example.

Another useful application of ls is to produce a full, recursive directory listing.

Saving the best for last is my favorite ls command. The -t switch sorts by modify time, newest at top and the -r switch reverses the order. This is the perfect way to quickly find your recent files. It’s such a time saver, I rarely use ls -al anymore in favor of the above.

Until next time!

Related posts:

1. AUC #1: paste
2. Complete Unix Cron Job Crib Sheet
3. Quick note on sorting john.pot files

AUC 1 – Arbitrary Unix Command #1

Up first is the command that took my 15 minutes to remember at work today when I wanted to take a list with one column and seperate it into a list with two columns. Here, a visual will make this easier to explain:

This is a list of a person and their favorite fruit. You want to convert this into a table like this:

But your table is 100,000 lines long and you don’t want to do it by hand, or mess with any complex regular expressions in sed, awk or a scripting language.

The unix command paste does exactly this. Here’s how simple the syntax is:

cat 100k_file | paste – - > lookup_table

Now that’s pretty simple. Just change the number of “-”‘s to how many columns you want.

paste can do some other things but this is how I found it to be useful. If you have another tip on paste, the comment box below awaits.

Related posts:

1. AUC #2: ls
2. Complete Unix Cron Job Crib Sheet
3. Detecting bad TCP sessions