Red Hat Enterprise Linux Implementation

Shell Environment Configuration

Module Topics

Environment Variables
bash Startup Scripts
alias
Functions

Environment Variables

Environment variables are variables that are passed to subprocesses
Applications and sessions use them to determine their behavior
- Common environment variables: PATH, USER, and HOSTNAME
To make variable into environment variable, flag it with export
- Will be passed, with value, to any subprocess spawned from shell
Any variable defined in shell can be environment variable
To view all environment variables, use env

Transcript:

The shell and scripts use variables to store data; some variables can be passed to sub-processes along with their content. These special variables are called environment variables.

Applications and sessions use these variables to determine their behavior. Some of them are likely familiar to administrators, such as PATH, USER, and HOSTNAME, among others. What makes variables environment variables is that they have been exported in the shell. A variable that is flagged as an export will be passed, with its value, to any sub-process spawned from the shell. Users can use the env command to view all environment variables that are defined in their shell.

Any variable defined in the shell can be an environment variable. The key to making a variable become an environment variable is flagging it for export using the export command.

Environment Variables

Example: Set MYVAR and spawn sub-shell

MYVAR variable does not exist in sub-shell

[student@DEM ~]$ MYVAR="some value"
[student@DEM ~]$ echo $MYVAR
some value
[student@DEM ~]$ bash
[student@DEM ~]$ echo $MYVAR
[student@DEM ~]$ exit

Example: Set MYVAR, tag MYVAR with export, and spawn sub-shell

MYVAR environment variable exists in sub-shell

[student@DEM ~]$ MYVAR="some value"
[student@DEM ~]$ export MYVAR
[student@DEM ~]$ echo $MYVAR
some value
[student@DEM ~]$ bash
[student@DEM ~]$ echo $MYVAR
some value
[student@DEM ~]$ exit

`bash` Startup Scripts

Upon login, environment variables initialize bash environment
Several shell scripts execute:
- Start with /etc/profile
- Follow with profile in user’s home directory, typically ~/.bash_profile
Bash shell looks for one of three files in user’s home directory:
- .bash_profile
- .bash_login
- .profile
Shell looks for files in this order and executes first file it locates
Profiles contain additional scripting that calls other shell scripts
Bash login scripting typically looks like this:

/etc/profile
           \__ /etc/profile.d/*.sh

~/.bash_profile
              \__ ~/.bashrc
                         \__ /etc/bashrc

`bash` Startup Scripts

Profiles:

Set and export environment variables
Run commands that run only upon login
Only executed in login shell

RCs:

Run commands, set aliases, define functions, and other settings that cannot be exported to sub-shells
Execute every time shell is created (login or non-login)
- File call can override default settings provided by system-wide scripts
- Many Red Hat config files contain comment about where to add user-specific changes

`alias`

To define own system command or override system command, use alias
- Aliases are parsed and substituted before shell checks PATH

Can use alias to display aliases defined in shell

[student@DEM ~]$ alias
alias egrep='egrep --color=auto'
alias fgrep='fgrep --color=auto'
alias grep='grep --color=auto'
alias l.='ls -d .* --color=auto'
alias ll='ls -l --color=auto'
alias ls='ls --color=auto'
alias vi='vim'
alias which='alias | /usr/bin/which --tty-only --read-alias --show-dot --show-tilde'

Transcript:

alias is a way administrators or users can define their own command to the system or override the use of existing system commands. Aliases are parsed and substituted prior to the shell checking PATH. alias can also be used to display all existing aliases defined in the shell.

All the default aliases defined in a user environment are in the previously shown output of alias. The definition of the ll alias means that when the user types ll as a command, the shell will expand the alias and execute ls -l --color=auto. In this way, a new command ll has been added to the shell.

In another example, the alias for grep, the alias overrides the default invocation of an existing command on the system. When a user enters the grep command, the shell will expand the alias and substitute the grep --color=auto command. Due to the alias, all calls to grep are overridden to become calls to grep with the --color option passed automatically.

`alias`

Use alias to set an alias that exists only for the duration of current shell

alias mycomm="<command to execute>"

[student@DEM ~]$ alias usercmd='echo "Hurrah!"; ls -l'
[student@DEM ~]$ usercmd
Hurrah!
total 0
-rw-rw-r--. 1 student student 0 Jun  9 13:21 file1
-rw-rw-r--. 1 student student 0 Jun  9 13:21 file2
-rw-rw-r--. 1 student student 0 Jun  9 13:21 file3

`alias`

To make alias persistent, add alias to bottom of ~/.bashrc

[student@DEM ~]$ vi ~/.bashrc
...
# User specific aliases and functions
alias usercmd='echo "Hurrah!"; ls -l'

Alias added to ~/.bashrc will be available in every shell created
To remove alias from environment, use unalias

Functions

Function isolates code segment so it can be called repeatedly

Updated code executes everywhere function is referenced

Example: pathmunge function defined in shell script in /etc/profile

 pathmunge () {
  if [ "$2" = "after" ] ; then
    PATH=$PATH:$1
  else
    PATH=$1:$PATH
  fi
}
...
if [ "$EUID" = "0" ]; then
    pathmunge /sbin
    pathmunge /usr/sbin
    pathmunge /usr/local/sbin
else
    pathmunge /usr/local/sbin after
    pathmunge /usr/sbin after
    pathmunge /sbin after
fi

Transcript:

When used in shell scripts, functions are a way of isolating a segment of code so that it can be called repeatedly without having to write the entire segment again. Additionally, if the code requires an update, the function’s content can be updated, and everywhere the function is referenced, the updated code is now executed.

An example of defining and using a function within a shell script follows, taken from /etc/profile. The pathmunge function takes two arguments; the first is a directory, the second (optional) is the word "after". Based on whether "after" is passed as $2, the directory will be added to the PATH environment variable at the front or end of the existing list of directories. Later, the function is invoked several times to build the PATH for root or regular users. Notice that for root, all the directories are prepended to PATH, where regular users have their PATH built by appending.

Functions

Can also set functions in Bash shell environment
When set in environment, can execute as commands, similar to aliases
Unlike aliases, functions can:
- Take arguments
- Perform more sophisticated actions
- Provide return code
Can type functions into current shell
- Preferable to set in user’s ~/.bashrc or global /etc/bashrc
To see default functions and variables in current shell environment, use set
To remove function from environment, use unset and function or variable name

References

Man pages: bash(1), env(1), and builtins(1)

Summary

Environment Variables
bash Startup Scripts
alias
Functions

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Red Hat Enterprise Linux Implementation

Shell Environment Configuration

Module Topics

Environment Variables

Environment Variables

bash Startup Scripts

bash Startup Scripts

alias

alias

alias

Functions

Functions

Summary

Module Completion

`bash` Startup Scripts

`bash` Startup Scripts

`alias`

`alias`

`alias`