RH033 Unit12 Configuring the Bash Shell
Upon completion of this unit, you should be able to:
- Know how to use local and environment variables
- Know how to inhibit variable expansion
- Know how to create aliases
- Understand how shell parses a command line
- Know how to configure startup files
- Know how to handle input with the read command and positional parameters
Bash Variables
1) Variables are named values
- useful for storing data or command output
2) Set with VARIABLE=VALUE
3) Referenced with $VARIABLE
$ File=$(ls /etc/)
Environment Variables
1) Variables are local to a single shell by default
2) Environment variables are inherited by child shells
- Set with export VARIABLE=VALUE
- Accessed by some programs for configuration
The set, env and echo commands can be used to display all variables, environment variables and a single variable
- set | less
- env | less
- echo $PATH
Some Common Variables
1) Configuration Variables
- PS1: Appearance of the bash prompt
- PATH: Directories to look for executables in
- EDITOR: Default text editor
- HISTFILESIZE: Number of commands in bash history
2) Information variables
- HOME: User’s home directory
- EUID: User’s effective UID
Aliases
1) Aliases let you create shortcut to commands
- alias dir=’ls �Claf’
2) Use alias by itself to see all set aliases
3) Use alias followed by an alias name to see alias value
$ alias dir
alias dir=’ls �ClaF’
How bash Expands a Command Line
1) Split the line into words
2) Expand aliases
3) Expand curly-brace statements ({})
4) Expand tilde statement (~)
5) Expand variables ($)
6) Command-substituation ($() and ``)
7) Split the line into word again
8) Expand file globs (*, ?, [abc], etc)
9) Prepare I/O redirections (<, >)
10) Run the command!
Preventing Expansion
1) Backslash ( \ ) makes the next character literal
2) Quoting prevents expansion
- Single quotes (‘) inhibit all expansion
- Double quotes (“) inhibit all expansion, except
- $ (dollar sign) �C variable expansion
- ` (backquotes) �C comand substitution
- \ (backslash) �C single character inhibition
- ! (exclamation point) �C history substitution
Login vs non-login shells
1) Startup is configured differently for login and non-login shells
2) Login shells are:
- Any shell created at login (include X login)
- su -
3) Non-login shells are:
- su
- graphical terminals
- executed scripts
- any other bash instances
Bash startup tasks: profile
1) Stored in /etc/profile (global) and ~/.bash_profile (user)
2) Run for login shells only
3) Used for
- Setting environment variables
- Running commands (eg mail-checker script)
Bash startup tasks: bashrc
1) Stored in /etc/bashrc (global) and ~/.bashrc (user)
2) Run for al shells
3) Used for
- Setting local variables
- Defining aliases
Bash exit tasks
1) Stored in ~/.bash_logout (user)
2) Run when a login shell exits
3) Used for
- Creating automatic backups
- Cleaning out temporary files
Scripting: Taking input with positional parameters
1) Positional parameters are special variables that hold the command-line arguments to the script
2) The positional parameters available are $1, $2, $3, etch.. These are normally assigned to more meaningful variable names to improve clarity
3) $* holds all command-line arguments
4) $# holds the number of command-line arguments
Scritping: Taking input with read command
1) Use read to assign input values to one or more shell variables:
- -p designates prompt to display
- read reads input from standard input and assigns one word to each variable
- Any leftover words are assigned to the last variable
- read �Cp “Eneter a filename:” FILE
End of Unit12
1) Questions and Answers
2) Summary
- local and environment variables
- command line parsing
- configuring the shell environment
- positional parameters and the read command