Command Line Program Examples

I was thinking it might be a good idea to have a resource in the Unix/Linux forum where people can post how to use command line programs. More so interesting ways to use them, or just things you can do with them that the beginning Linux user might not know and maybe looking for.

For example:

You can run the following command to see memory usage of currently running process for a specific user.



You can add the following to remove certain commands with the given phrase in the command.



You can also add the following to sum the first column and return the results, in this case its the memory used and it returns the value in MB.



All elements together:


Do you think this would be a good resource to add to the forum?

I wouldn't expect this to be a sub category, more of just an organized thread. I just wanted to get some feedback on the idea.

I think its a good idea and something that could be useful. I use the command line all the time to run and administer linux servers for websites which include ozzu. Some of the most important things I do involve using the command line and pico (not into vi).

NOT into vi?! j/k. I do like the idea. Perhaps I can throw in a few pointers here and there as well. All of our VM servers @ work are linux, so I'm in the same boat.

...will tell you how many users are logged onto a system. I put that into a little shell script then put that script in /usr/local/bin so I can call it up faster than you can snap your fingers.

The most used for me would be tar -xvzf filename.tar

whoami is nice for determining the user a script interpreter or similar application able to pass commands to a shell is logged in as.

whereis is nice for determining where an application is.

nice topic (I didn't know for that usage of ps, and will need it)..
I collect linux commandline tricks that I need and find here and there on some wiki page because otherwise I would always forget them when I needed them. Here are few more interesting ones:

Count files with <error> in them:


Remove files that don't have this string in them


Detach script from your shell session (& does that) so you can run other apps and so that it keeps executing after you close the ssh console (nohup does that):

A majority of these are probably linux specific. That's all I really use, so it's what I know. I didn't list all the flags associated with each command, but I tried to include the useful ones, which brings us to the first in my list:

man - the built-in help manual. Specify a command or app that you want info on. Format: man grep

ls - List contents of current directory (useful flags: -l, -a). Format: ls -a

df - Show used disk space and mounted drives (default returns size in blocks, use -h flag to specify common units). Format: df -h

locate - search utility. Keeps a db listing of files - generally faster returns than find. Format: locate [filename]

find - The locate db may not always be current enough to find what you need. In that case, use find. Format: find [path] -name [filename]

cat - displays the entire contents of a file. Not very useful for large files. Format: cat [filename]

head - view the top portion of a file. (-n flag sets the number of lines to view - default is 10 lines). Format: head -60 [filename]

tail - reverse of head - view the end of a file. (-n flag is the same, -f will follow a file live as it changes). Format: tail -n 60 -f

sudo - used when superuser rights are required for a command. Format: sudo tail -n 60 [filename]

ifconfig - Display the IP's that the system is using and the amount of traffic sent/received (-a flag to view all). Format: ifconfig eth0

top - Graphical real time process monitor. Format: top

ps - Overview of all running processes. Format: ps

kill - Kill a process (use top or ps to identify the PID and then plug that in). Format: kill -9 or killall -9

cp - Copy a file. Format: cp [orig filename] [destination path]

mv - Move a file. Format: mv [orig filename] [destination path]

scp - securely copy file over ssh. Format: scp [filename] [user]@[remote host]:/[path to file]

shutdown - Shut down the system (-r flag to set delay to 'now' or a number of minutes). Format: shutdown -r now

I've excluded grep from my list since it's been mentioned a few times, but I highly recommend that anyone using command line take a hard look at grep and play around with it until you're familiar with its basic capabilities. Entire books can (and have!) been written about grep. It's a powerful tool!

change string ocurance in all files

Count the lines in a directory full of files, listing per-file and total results.



Count the unique lines in a file

If you use RSA keys to login via SSH instead of being prompted for your password, you can use tab-completion while typing a remote file path for SCP.

If you want to delete files older than certain number of days then you can use the following command :-




where -mtime is used to specify the number of days old that the file is. If you enter +10 as shown in the command , it will find files older than 10 days.

And -exec, allows you to pass in a command such as rm(remove). The {} \; at the end is required to end the command.

The -atime flag can be substituted for the -mtime flag there to delete files that haven't been accessed in N days. Works great for cache folders.

Interesting never used that one before. I always use which: