Unit 13: bash Scripts

Now that we know the basic of text editing, we will start writing some simple bash scripts.

.bashrc

First, let's write a bash script that will be executed every time we run bash. Just like .vimrc, the "run commands" for bash is called .bashrc. Recall that the rc files should be hidden and located in the home directory.

So, start vim by

$ vim ~/.bashrc

and add the following lines at the bottom of .bashrc.

alias rm="rm -i"
alias mv="mv -i"
alias cp="cp -i"
echo ".bashrc loaded"

Remember that the -i flag causes these commands to interact with you and ask to confirm whether you want to remove/override a file. The alias command is used to create a new command. Here,

alias rm="rm -i"

Go ahead and hit Shift+Z+Z to save and quit. Now, exit bash and reload bash again. You should see the message .bashrc loaded printed.

Try to create a dummy file and remove it. bash should now prompt you for confirmation.

$ touch dummy
$ rm dummy
remove dummy?

Bash Environment Variables

In addition to creating your customized commands, you can also affect the behavior of bash by configuring its environment variables. An environment variable is something that can hold a value (just like $<span><span class="MathJax_Preview">x</span><script type="math/tex">x$ in math). The naming convention for an environment variable is to use all upper cases.

The most common ones used are:

• PATH: which is where bash would search for a command to execute.
• PS1: which allows you to customize your command prompt.
• EDITOR: which allows you to configure the default text editor.

For PS1 configuration, there are many neat examples on the Internet. We recommend that you should least have your identity displayed somewhere on the command prompt so that the tutors know which terminal they are looking at via tmate.

For EDITOR, you can set it to vim.

Remember that earlier, to run hello.sh, we needed to include the prefix ./hello.sh? The ./ is to tell bash to look for the executable in the current directory. To avoid typing ./ all the time (remember our goal: minimize finger movement), we can add . to the PATH.

Now, cd into the directory where you have hello.sh, and type

$ echo $PATH
$ export PATH=$PATH:.
$ echo $PATH

The first command shows you what the current PATH variable contains. The $ sign refers to the value of that variable instead of the variable name. The PATH contains a list of paths in the directory structure, separated by :.

In the second line, we add . (the current directory) to the PATH variable. The third line checks if . is added correctly.

If . is indeed added to the path, you can now run hello.sh without the prefix ./.

Note that there is a security risk in adding . to PATH. My recommendation is only to do it on your personal Unix system (not a shared computing server) and add . only at the end of PATH. See this FAQ for more information.

If you wish, you can now edit this line into .bashrc, so that it is executed every time you launch `bash.

export PATH=$PATH:.

Simple Bash Scripts

In addition to creating aliases, you can put commonly used commands in a bash script. The advantage is that it allows us to pass in arguments so that the script is more flexible.

For instance, the following script finds out the size of a directory and the size of the largest subdirectory (in MBs)

#!/bin/bash
du -m $1 | sort -n | tail -2

You can read the man pages for what du, sort, and tail and the various options do. But the two interesting points we wish to point out are the first line:

#!/bin/bash

The #! sequence is called shebang. It is used by Unix to determine which interpreter to use to run this script. It could be replaced with /usr/bin/python or other language interpreters, for instance. But here, we meant this to be a bash script so we tell Unix to invoke bash.

The variable $1 will be replaced by the argument passed into the script on the command line. Suppose that we save the script above as dirsize.sh. Then you run:

dirsize.sh /usr/local

$1 will become /usr/local and so the size of /usr/local and its subdirectories will be determined.

If you do not pass in any argument, $1 will be empty, and the size of the current working directory will be determined instead.

Developing an Automation Mind Set

If you find yourself typing the same command or sequence of commands over and over again, it is probably good to start creating a script to automate it.

You will need to invest some time upfront to get the script written and tested, but in the long run, it will save you time.