We get back to the most powerful tool on Linux, and my favourite, the Shell. Specifically, we will have a look at some of the features that the Bourne Again Shell (bash) has to offer for command line users. Some of these features are also supported by other shells. A lot of these tricks are common place for power users, and I hope that they will be very useful for newer users.
- bash supports command completion. What this means is that you almost never have to type more than 4 characters to get to the right command. Type in the first 2 characters of your command and press the ‘tab’ key twice. The shell will list all the possible completions of your command. Type in the next couple of characters till the number of possibilities become one, and press ‘tab’ once, the shell will happily complete the command. This feature is also useful when typing out long filenames. Follow the same procedure to get to the correct file.
- Output redirection is another fantastic feature offered by all the shells. We just have to type in a chain of commands separated by a
|(a pipe, found along with the
\), to form a ‘pipeline’. Lets have a look at an example: This command will print UbuntuCodeofConduct-1.0.1.txt, send it’s output to
grep, which will print the lines containing ‘Ubuntu’, which are then piped to
wcwhich will count the number of lines in it as the option l is specified to it.
- Pipelines will simply run the commands given. However, if you wish that the commands will run with a given condition, then we can use lists. Here, the commands to be run are separated by ; , & , && and
||. The function of && and
||is the same as that in C language, i.e., AND and OR logic respectively.
Command Substitution is another useful tool. Here, you substitute the output of a command in the middle of another command. It is done by enclosing the command in
`(a backtick). Note, this is not a single quote. It is the character found on the same key as the ‘~’ (tilde). An example that I have seen often is:
$ apt-get install linux-headers-`uname -r`The output of
uname -rwill be substituted to get the proper package name, linux-headers-220.127.116.11-generic for my system.
- One more useful tool in the bash arsenal is command aliases. To put in simple, but inaccurate words, we are creating keyboard shortcuts for commands that we commonly use on the shell. Custom bash aliases are defined in the
~/.bash_aliasesfile while default settings are in the
~/.bashrcfile (atleast in Ubuntu). Some of them in my file are:
Hope that after reading this post, you are a slightly better shell user than before. If you find anything wrong, in any of my posts before, or after this, please feel free to drop a comment and correct me. I too am a noob as many of you are.