1. Linux+ Guide to Linux Certification, Second Edition
Chapter 8
Working with the BASH Shell

2. Linux+ Guide to Linux Certification, 2e
Objectives
Redirect the input and output of a command
Identify and manipulate common shell environment variables
Create and export new shell variables
Edit environment files to create variables upon shell startup
Linux+ Guide to Linux Certification, 2e

3. Objectives (continued)
Describe the purpose and nature of shell scripts
Create and execute basic shell scripts
Effectively use common decision constructs in shell scripts
Linux+ Guide to Linux Certification, 2e

4. Command Input and Output
BASH shell responsible for:
Providing user interface
Interpreting commands
Manipulating command input and output
Provided user specifies certain shell metacharacters with command
File descriptors: Numeric labels that define command input and command output
Linux+ Guide to Linux Certification, 2e

5. Command Input and Output (continued)
Standard Input (stdin): File descriptor representing command input
Standard Output (stdout): File descriptor representing command output
Standard Error (stderror): File descriptor representing command error messages
Linux+ Guide to Linux Certification, 2e

6. Command Input and Output (continued)
Figure 8-1: The three common file descriptors
Linux+ Guide to Linux Certification, 2e

7. Linux+ Guide to Linux Certification, 2e
Redirection
Redirect stdout and stderr from terminal screen to a file
Use “>” shell metacharacter
Can redirect stdout and stderr to separate files
Use separate filenames for stdout and stderr
Linux+ Guide to Linux Certification, 2e

8. Redirection (continued)
Redirecting stdin to a file:
Use “<“ shell metacharacter
tr command: Replace characters in a file sent via stdin
Linux+ Guide to Linux Certification, 2e

9. Redirection (continued)
Table 8-1: Common redirection examples
Linux+ Guide to Linux Certification, 2e

10. Linux+ Guide to Linux Certification, 2e
Pipes
Send stdout of one command to another command as stdin
Pipe: String of commands connected by “|” metacharacters
stdout on left, stdin on right
Commonly used to reduce amount of information displayed on terminal screen
Linux+ Guide to Linux Certification, 2e

11. Pipes (continued)
Figure 8-2: Piping information from one command to another
Linux+ Guide to Linux Certification, 2e

12. Linux+ Guide to Linux Certification, 2e
Pipes (continued)
Can use multiple pipes on command line
Pass information from one command to another over a series of commands
filter commands: Commands that can take from stdin and give to stdout
Can be on either side of a pipe
tee commands: Filter commands that also send information to a file
Linux+ Guide to Linux Certification, 2e

13. Pipes (continued) Figure 8-3: Piping several commands
Linux+ Guide to Linux Certification, 2e

14. Pipes (continued) Table 8-2: Common filter commands
Linux+ Guide to Linux Certification, 2e

15. Linux+ Guide to Linux Certification, 2e
Pipes (continued)
Can combine redirection and piping
Input redirection must occur at beginning of pipe
Output redirection must occur at end of pipe
sed filter command: Search for and replace text strings
awk filter command: Search for text and perform specified action on it
Linux+ Guide to Linux Certification, 2e

16. Linux+ Guide to Linux Certification, 2e
Shell Variables
Variable: A reserved portion of memory containing accessible information
BASH shell has several variables in memory
Environment variables: Contain information that system and programs access regularly
User-defined variables: Custom variables define by users
Special variables
Useful when executing commands and creating new files and directories
Linux+ Guide to Linux Certification, 2e

17. Environment Variables
set command: Lists environment variables and current values
echo command: View contents a specified variable
Use $ shell metacharacter
Changing value of a variable:
Specify variable name followed by equal sign (=) and new value
Linux+ Guide to Linux Certification, 2e

18. Environment Variables (continued)
Table 8-3: Common BASH environment variables
Linux+ Guide to Linux Certification, 2e

19. Environment Variables (continued)
Table 8-3 (continued): Common BASH environment variables
Linux+ Guide to Linux Certification, 2e

20. Environment Variables (continued)
Table 8-3 (continued): Common BASH environment variables
Linux+ Guide to Linux Certification, 2e

21. User-Defined Variables
Variable identifier: Name of a variable
Creating new variables:
Specify variable identifier followed by equal sign and the new contents
Features of variable identifiers:
Can contain alphanumeric characters, dash characters, or underscore characters
Must not start with a number
Typically capitalized to follow convention
Linux+ Guide to Linux Certification, 2e

22. User-Defined Variables (continued)
Subshell: Shell created by current shell
Most shell commands run in a subshell
Variables created in current shell are not available to subshells
export command: Exports user-defined variables to subshells
Ensures that programs started by current shell have access to variables
env command: Lists all exported environment and user-defined variables in a shell
Linux+ Guide to Linux Certification, 2e

23. Linux+ Guide to Linux Certification, 2e
Other Variables
Not displayed by set or env commands
Perform specialized functions in the shell
e.g., UMASK variable
alias command: Creates shortcuts to commands
Use unique alias names
Aliases stored in special variables
Can create single alias to multiple commands
Use ; metacharacter
Linux+ Guide to Linux Certification, 2e

24. Linux+ Guide to Linux Certification, 2e
Environment Files
When exiting BASH shell, all stored variables are destroyed
Environment files: Store variables and values
Executed each time BASH shell is started
Ensures variables are always accessible
Linux+ Guide to Linux Certification, 2e

25. Environment Files (continued)
Common BASH shell environment files (in order they are executed):
/etc/profile
~/.bash_profile
~/.bash_login
~/.profile
Hidden environment files allow users to set customized variables
Linux+ Guide to Linux Certification, 2e

26. Environment Files (continued)
To add a variable, add a line to environment file
Use command line syntax
Any command can be placed inside any environment file
e.g., alias creation
.bashrc (BASH run-time configuration): First hidden environment file executed at login
Linux+ Guide to Linux Certification, 2e

27. Linux+ Guide to Linux Certification, 2e
Shell Scripts
Shell script: Text file containing a list of commands or constructs for shell to execute
May contain any command that can be entered on command line
Hashpling: First line in a shell script
Defines which shell is used to interpret shell script commands
Linux+ Guide to Linux Certification, 2e

28. Shell Scripts (continued)
Executing shell scripts with read permission:
Start another BASH shell, specify the shell script as an argument
Executing shell scripts with read/write permission:
Executed like any executable program
Linux+ Guide to Linux Certification, 2e

29. Linux+ Guide to Linux Certification, 2e
Escape Sequences
Character sequences having special meaning in the echo command
Prefixed by \ character
Must use –e option in echo command
Linux+ Guide to Linux Certification, 2e

30. Escape Sequences (continued)
Table 8-4: Common echo escape sequences
Linux+ Guide to Linux Certification, 2e

31. Reading Standard Input
Shell scripts may need input from user
Input may be stored in a variable for later use
read command: Takes user input from stdin
Places in a variable specified by an argument to read command
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32. Linux+ Guide to Linux Certification, 2e
Decision Constructs
Most common type of construct used in shell scripts
Alter flow of a program:
Based on whether a command completed successfully
Based on user input
Linux+ Guide to Linux Certification, 2e

33. Decision Constructs (continued)
Figure 8-4: A sample decision construct
Linux+ Guide to Linux Certification, 2e

34. Decision Constructs (continued)
Figure 8-5: A sample decision construct
Linux+ Guide to Linux Certification, 2e

35. Linux+ Guide to Linux Certification, 2e
The if Construct
Control flow of program based on true/false decisions
Syntax:
Linux+ Guide to Linux Certification, 2e

36. The if Construct (continued)
Common rules governing if constructs:
elif (else if) and else statements optional
Unlimited number of elif statements
do these commands section may consist of multiple commands
One per line
do these commands section typically indented for readability
End of statement must be “if”
this is true may be a command or test statement
Linux+ Guide to Linux Certification, 2e

37. The if Construct (continued)
test statement: Used to test a condition
Generates a true/false value
Inside of square brackets ( [ … ] )
Must have spaces after “[” and before “]”
Special comparison operators:
–o (OR)
–a (AND)
! (NOT)
Linux+ Guide to Linux Certification, 2e

38. The if Construct (continued)
Table 8-5: Common test statements
Linux+ Guide to Linux Certification, 2e

39. The if Construct (continued)
Table 8-6: Special operators in test statements
Linux+ Guide to Linux Certification, 2e

40. Linux+ Guide to Linux Certification, 2e
The case Construct
Compares value of a variable with several different patterns of text or numbers
Syntax:
Linux+ Guide to Linux Certification, 2e

41. The case Construct (continued)
If a match is found, commands to right of pattern are executed
Must end with esac
Linux+ Guide to Linux Certification, 2e

42. The && and || Constructs
Time-saving shortcut constructs
When only one decision needs to be made during execution
Syntax:
command && command
command || command
Linux+ Guide to Linux Certification, 2e

43. The && and || Constructs (continued)
&&: Second command executed only if first completes successfully
||: Second command executed only if first fails
Linux+ Guide to Linux Certification, 2e