Shell Customization

CS155 Customization

Automatic scripts

When your shell starts, it reads commands, just as if you’d typed them, from a special file.
Which file? It depends on which shell you’re using:
- csh reads from ~/.cshrc
- bash reads from ~/.bashrc
- ksh reads from ~/.kshrc
- zsh reads from ~/.zshrc
Do you see a pattern?

What’s the syntax?

What’s the syntax for one of these rc files?
It depends on the shell.
~/.cshrc contains C-shell syntax, ~/.bashrc contains commands in the style of bash, etc.
From now on, we will devote ourselves to bash.

Environment Variables

Customization usually means setting environment variables.

    export var="value"
    export TZ="America/Denver"

Remember, all examples from now on are bash only.

Inheritance

Environment variables are inherited, so your children see the changes.
By “children”, we mean sub-processes, or commands. date is often spawned by the shell, so it’s a “child” of the shell.
If you were mutated by a gamma bomb, and became the Incredible Hulk, your future children would inherit your mutation, but your existing children and your parents would be unaffected.

Changing your PATH

To help the shell find programs in another directory, add it to the beginning of the environment variable PATH:

% ruler 50
/tmp/pmwiki-bash-scriptxX5QQd: line 10: ruler: command not found
% echo $PATH
/bin
% export PATH="~/bin:$PATH"
% echo $PATH
~/bin:/bin
% ruler 50
12345678901234567890123456789012345678901234567890
         1         2         3         4         5

Why did we use "double quotes" as opposed to 'single quotes'?

Changing your prompt

To change your interactive prompt, set the appropriate variable:

    PS1="$USER> "

Why didn’t we use export? That’s because this variable is private to the shell, and not for the use of other programs.

Creating new commands

To create a new command, use alias:

% date
Fri Apr 26 10:44:57 MDT 2024
% alias date="echo How about a date?"
% date
How about a date?
% unalias date
% date
Fri Apr 26 10:44:57 MDT 2024
% alias mkx="chmod u+x"
% touch zulu
% ls -l zulu
-rw------- 1 cs155 class 0 Apr 26 10:44 zulu
% mkx zulu
% ls -l zulu
-rwx------ 1 cs155 class 0 Apr 26 10:44 zulu

Non-recursive Aliases

Aliases are not self-recursive:

% touch a b c d e f g h i j k l m n o p
% rm *
% touch a b c d e f g h i j k l m n o p
% alias rm='rm -I'
% rm *
rm: remove 16 arguments? y
% touch x y z
% rm *

rm -I prompts if more than three files are to be removed.

umask

% touch x
% umask ugo=rwx
% touch y
% umask u=rwx,g=r,o=
% touch z
% ls -l
total 0
-rw------- 1 cs155 class 0 Apr 26 10:44 x
-rw-rw-rw- 1 cs155 class 0 Apr 26 10:44 y
-rw-r----- 1 cs155 class 0 Apr 26 10:44 z

umask sets the default permissions for new files.
trusting: umask ugo=rwx
cautious: umask u=rwx,go=
Put the umask command in ~/.bashrc.

CS155: Introduction to Unix

Spring 2018

Customization