1. Linux Shell Script Programming
Chapter 3: Shell Basics
Linux Shell Script Programming

2. Linux Shell Script Programming
Chapter Objectives
In this chapter, you will:
Understand the shell login and logout files
Learn about shell variables
Become familiar with the shell environment
Learn about shell builtin commands
Additional Commands
Learn about shell grammar
Understand command types
Linux Shell Script Programming

3. Understanding the Shell Login and Logout Files
When you login, the shell executes and provides you with a shell prompt
The default shell is the Bourne-Again Shell, or bash
There are startup files in the Linux directory hierarchy used to set up the user’s environment
Startup files are read and executed as the user logs in
A logout file is read and and executed when a user logs out
Linux Shell Script Programming

4. Understanding the Shell Login and Logout Files
If present, the shell reads and executes the /etc/profile script file
The /etc/profile script file is read and executed for each user
Place commands here that you want to affect all users
Place command statements at the end so you won’t interfere with existing commands
The permissions on /etc/profile are read and write for root and read for all other users
Linux Shell Script Programming

5. Understanding the Shell Login and Logout Files
After the shell reads /etc/profile, for all users, ~/.bash_profile, in each user’s home directory, $HOME, is read
The ~ represents each user’s own home directory
Another script file, ~/.bashrc, is executed if it exists
The ~/.bash_logout script file is executed as a user logs out
Linux Shell Script Programming

6. Learning about Shell Variables
A shell variable is a keyword that is set by the shell
Each shell variable is used for a specific purpose
A shell variable is typically entered in uppercase
You precede the shell variable name with a $, as in $PATH
Linux Shell Script Programming

7. Learning about Shell Variables
Examples of shell variables:
PWD – the most recent working directory set with the cd command
OLDPWD – the previous working directory set with the cd command
BASH – the full path of the bash shell
RANDOM – generates a random number between 0 and 32,767
HOSTNAME – the current hostname of a Linux system
Linux Shell Script Programming

8. Learning about Shell Variables
Examples of shell variables
IFS – Internal Field Separator, which is used as a separator between shell words
PATH – contains the list of directories used to search for commands when executing them
HOME – each user’s home directory, typically /home/username, where username is a user’s login name
PS1 – shell prompt
TMOUT – the amount of time the shell will wait without user input before exiting the shell
Linux Shell Script Programming

9. Learning about Shell Variables
Linux Shell Script Programming

10. Becoming Familiar with the Shell Environment
When you execute a command using a partial path, the shell looks to the PATH shell variable
Each directory on the path is separated by a colon
Search order is left to right
Search stops when a match is found
If the directory is not found, then an error message is displayed
Linux Shell Script Programming

11. Becoming Familiar with the Shell Environment
When using the full path, the shell does not use the PATH variable
Instead the shell goes directly to the directory using the specified
Example: /bin/cp
Linux Shell Script Programming

12. Becoming Familiar with the Shell Environment
The File System Hierarchy standard was developed by several vendors
Used to standardize system directories
Important use was to use /usr/local/bin to store executable programs and scripts used by all users
Use of $HOME/bin to execute a script or program in a user’s home directory
Linux Shell Script Programming

13. Learning about Shell Builtin Commands
A builtin command is part of the bash shell
Compiled into the shell
You cannot access them directly
Perform man on the bash to get their manual help pages
Linux Shell Script Programming

14. Learning about Shell Builtin Commands
Examples:
./filename – where filename is a script that is read and executed
cd – changes the current directory
declare – defines a variable
echo – displays output
exec – causes a command to replace the current shell with no new PID
exit – exits a shell with an exit status
Linux Shell Script Programming

15. Learning about Shell Builtin Commands
Examples:
export – exports a variable to a spawned shell
history – displays previous commands
kill – sends a signal to a PID
let – evaluates an arithmetic expression
logout – exits a login shell
pwd – displays current working directory
read – read characters from the keyboard
readonly – makes a variable readonly so it cannot be changed
Linux Shell Script Programming

16. Learning about Shell Builtin Commands
Examples:
return – returns an exit status
set – set and display a shell variable
shift – moves positional parameters
test – evaluates expressions
trap – catches a signal sent to the shell
Linux Shell Script Programming

17. Linux Shell Script Programming
Additional Commands
Accessed as command contained in a directory (eg. /bin, /usr/bin, etc…)
Examples:
Shell Programs: sh, csh, ksh, bash
Directories / Files: mkdir, rmdir, rm, cd, ls, pwd, cp, mv
Files: cat, lpr, touch, more, vi, uniq, sort, grep, cut, paste, diff
Misc: cal, date, echo, history, man, who
Linux Shell Script Programming

18. Learning about Shell Grammar
Defined as rules that are required for proper shell operation
Uses strict syntax
Fundamental building blocks
Blank
Word
Name
Metacharacter
Control operator
Reserved word
Linux Shell Script Programming

19. Learning about Shell Grammar
A blank is a space or tab used to separate items
A word is a sequence of characters considered to be a single unit to the shell
A name is a word consisting of letters, numbers and underscore
A metacharacter is a character used for a unique and specific purpose by the shell
Linux Shell Script Programming

20. Learning about Shell Grammar
Metacharacters
| - the pipe symbol allows you to pass output to another command
& - the ampersand allows you to run a process in the background
; - the semicolon allows you to separate commands
() – the left and right parentheses allow you to run commands in a subshell
< - less than redirects input
> - greater than redirects output
Linux Shell Script Programming

21. Learning about Shell Grammar
Control operators:
|| - executes a command depending upon failure of another
&& - executes a command depending upon success of another
& - execute in background
; - execute in sequence as separate commands
() – execute in a subshell
| - pass output to another command
A reserved word has special meaning to the shell and cannot be used for another purpose. Example: if, then, else, fi
Linux Shell Script Programming

22. Understanding Command Types
Command types rely upon an understanding of shell grammar
Command types:
Simple command – set of words separated by blanks
Examples: ls, touch, pwd, clear
Pipelines – sequence of simple commands separated by the pipe symbol
Examples: ls | more, who | sort | more
List – sequence of one or more pipelines
separated by one of these ; & && ||
terminated by one of these ; & or newline
Linux Shell Script Programming

23. Understanding Command Types
Linux Shell Script Programming

24. Understanding Command Types
The ; Operator
Causes commands to be executed sequentially in the same shell
Example: ls; who
The & Operator
Causes commands to be executed in a subshell
Subshell – spawned from the parent shell
Example: ls & pwd
Linux Shell Script Programming

25. Understanding Command Types
The && Operator
Causes a command to execute only if the preceding command completes successfully (exit status of 0)
Example: rm file4.txt && ls
The ls command will only execute if rm file4.txt completes successfully
The || Operator
Causes a command to execute only if the preceding command completes unsuccessfully (exit status of 1)
Example: rm file4.txt || pwd
The ls command will only execute if rm file4.txt completes unsuccessfully
Linux Shell Script Programming

26. Understanding Command Types
Command Types – compound command
Allow you to perform calculations, assign variables, perform decision tests and create loops
Includes previous command types and
Group commands
Expressions
Decision constructs
Looping structures
Linux Shell Script Programming

27. Understanding Command Types
Group commands – two forms:
(list) – execute list in a subshell
Example: (pwd; ls)
{ list; } – execute list in the current shell
Example: { rm file5.txt; pwd; }
Expressions used to assign values, perform calculations and test for values – two forms:
((expression)) – evaluates an arithmetic operation
Example: ((y=5 + 4))
[[expression]] – tests file attributes, perform string and numeric comparisons
Linux Shell Script Programming

28. Understanding Command Types
Linux Shell Script Programming

29. Understanding Command Types
Linux Shell Script Programming

30. Understanding Command Types
Linux Shell Script Programming

31. Linux Shell Script Programming
Chapter Summary
The shell can be invoked by either logging in or by entering the name of the shell at the shell prompt. The shell uses startup files to customize the user’s environment.
The shell has its own variables that are reserved specifically for its use. They must be entered in uppercase.
The File System Hierarchy Standard supports placing scripts used by all users in the /usr/local/bin directory and having users place their own scripts in ~/bin.
Linux Shell Script Programming

32. Linux Shell Script Programming
Chapter Summary
Shell builtin commands are commands that are compiled as part of the shell.
The basic building blocks of shell grammar are: blanks, words, names, metacharacters, control operators, and reserved words.
The shell provides for several command types which are simple commands, pipelines, lists, and compound commands
Linux Shell Script Programming