105.1. Customize and use the shell environment

Weight: 4

Candidates should be able to customize shell environments to meet users’ needs. Candidates should be able to modify global and user profiles.

Key Knowledge Areas

  • Set environment variables (e.g. PATH) at login or when spawning a new shell
  • Write Bash functions for frequently used sequences of commands
  • Maintain skeleton directories for new user accounts

Set command search path with the proper directory

Terms

  • .
  • source
  • /etc/bash.bashrc
  • /etc/profile
  • env
  • export
  • set
  • unset
  • ~/.bash_profile
  • ~/.bash_login
  • ~/.profile
  • ~/.bashrc
  • ~/.bash_logout
  • function
  • alias
  • lists

Environment variables

This is discussed in the past. But how we should set them when we login or change shell. Oh! And what happen when we are not logged in? ;)

login vs non-login shell

Sometimes you login into the shell, say after a ssh but sometimes you just open a shell; say in GUI.

login shell

This happens when you give your user and pass to enter a shell. Many steps are involved to setup your variables and settings. These are the steps:

1- /etc/profile is run

2- A line in /etc/profile runs whatever is in /etc/profile.d/*

Now the global profile is loaded and system will go for user specific profiles:

3- /home/USERNAME/.bash_profile

4- /home/USERNAME/.bash_login

5- /home/USERNAME/.profile

Note that only one of the 3, 4 & 5 will be run. The system will go for .bash_profile and IF IT IS NOT THERE will try for .bash_login and IF IT IS NOT THERE will try to run .profile. If any of these exists, the system wont look any furthur. So if you have only 4 & 5, only the 4 will be run.

At the end, the system loads:

  1. /home/USERNAME/.bashrc

which is users information (like aliases).

Note: /etc/profile also loads /etc/bashrc or /etc/bash.bashrc

Interactive (non-login) shell

if you run a shell in an interactive mode (non-login) shell say from a GUI, only two things will be run:

  1. /etc/bash.bashrc (or in some systems /etc/bashrc)
  2. /home/USERNAME/.bashrc

adding global configs for login shell

you can add your global new config files int /etc/profile.d/ (with .sh at the end). It is cleaner and better than editing the /etc/profile because an update can overwrite your changes if you do so.

adding global configs for interactive/non-login shell

you can use /etc/bash.bashrc file (some systems /etc/bashrc). This is good for aliases and other global configs.

user specific configs

Most of the time PATH and env vars go into the in ~/.bash_profile and aliases go into the ~/.bashrc. Have a look at them!

Aliases

Most of the time they are defined in ~/.bashrc and look like this:

alias ll='ls -alF'
alias la='ls -A'
alias l='ls -CF'

It is kind of a shortcut.

/etc/skel

This directory contains files which will be used as a starting template for each new user.

.bash_logout

runs when you logout from a login shell. In many distros it only clears the screen so the next person will not be able to watch what you were doing before you logout.

. (and source)

Yes only a dot! This is a shortcut for the bash source command. You can find it in files like /etc/profile. It runs the executable in front of it as part of the current environment.

Note: If you just execute a file (without source or dot) bash creates a child, runs the executable there and then closes it.

functions

Like "real" programming languages, Bash has functions, though in a somewhat limited implementation. A function is a subroutine, a code block that implements a set of operations, a "black box" that performs a specified task. Wherever there is repetitive code, when a task repeats with only slight variations in procedure, then consider using a function.

funnyls () {
    ls -ltrh
    echo "This is a funny ls"
}

set

set allows you to change the values of shell options and set the positional parameters, or to display the names and values of shell variables.

Using set we can configure how bash works. These are some samples:

switch result
-b Cause the status of terminated background jobs to be reported immediately, rather than before printing the next primary prompt.
-e return in case a pipline, command, ... return non-zero
-n Read commands but do not execute them; this may be used to check a script for syntax errors. This option is ignored by interactive shells.
-t Exit after reading and executing one command.
-C Prevent output redirection using ‘>’, ‘>&’, and ‘<>’ from overwriting existing files.

export

Set an environment variable. Mark each name to be passed to child processes in the environment.

$ export name=jadi
$ echo $name
jadi

Note: when exporting, the variable will exists in child processes (commands you run from the current shell). If you only say name=jadi and run a new command in your shell, the name wont be jadi in that shell.

$ name=jadi
$ echo $name
jadi
$ bash
$ echo $name

$ exit
exit
$ echo $name
jadi
$ export name=jadi
$ bash
$ echo $name
jadi

unset

This command has nothing to do with set command! This can unset the defined variables or functions.

$ name=jadi
$ echo $name
jadi
$ unset name
$ echo $name

You can also unset functions.

env

can set, remove or display variables or even run a command in a modified environment.

Syntax
     env [OPTION]... [NAME=VALUE]... [COMMAND [ARGS]...]

Options

  -u NAME
  --unset=NAME
       Remove variable NAME from the environment, if it was in the
       environment.

  -
  -i
  --ignore-environment
       Start with an empty environment, ignoring the inherited
       environment.

lists

Bash even has arrays! We will see them later when scripting but you can do things like:

$ list=(salam donya man injam)
$ echo ${list[1]}
donya
$ list=("salam donya" "man injam")
$ echo ${list[1]}
man injam

Please pay attention to the syntax.


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105.2 Customize or write simple scripts →

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