Command Line Utilities

This is a collection of random useful command line tools.

Network Related

rsync

• --no-p: this will disable permissions synchronization.
• --no-o: this will disable owner synchronization.

nmap

• This program allows you to test if a port is open on a remote box.

netstat

• netstat -altp will list all the open ports on your machine, along with the process that opened it. Very useful!

File & I/O

• To obtain the space available on a HD: df -h

tar

• Use the -C option to switch to a directory before compressing. Very useful.
• Use the -h option to follow symbolic links.

cpio

• This seems to be something similar to tar.
• Allows you to do nice stuff like

find . -name '*.txt' print0 | cpio -o -0 | ssh elvanor@otherbox 'cd dirB && cpio -iduv -0'

This would copy the *.txt files to another box, preserving the directory hierarchy.

Administration

gpasswd

• This is the command used on Gentoo to change an user's groups. You can also edit /etc/group, but this is not the right way of doing things. It will usually fail because you also need to change /etc/gshadow. Note that /etc/group- is just a backup of /etc/group.

Other

ps

• The legend for the status letters of processes can be found in the man page. R basically means running, and S means sleeping.
• Something very tricky can happen with ps: ps lists a process, but ps | grep will not. This is because if processes are always spawned and short lived (which means you have a problem on your system), ps when started on a shell will get the time to catch the process but ps | grep will run faster (less text to output) and so won't catch the process. A way to check for signs of this problem is to launch htop and see if the CPU is at 100% usage.
• ps can display the environment of the current running processes, with the e option.

Tricks and tips

• Redirecting all outputs to a file:

foo &> bar

To redirect only stderr:

foo 2> bar