1. Week 7
Working with the BASH Shell

2. 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 2

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 3

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 4

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 5

6. Command Input and Output
(continued)
Figure 7-1: The three common file descriptors
Linux+ Guide to Linux Certification, 2e 6

7. 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 7

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 8

9. Redirection (continued)
Table 7-1: Common redirection examples
Linux+ Guide to Linux Certification, 2e 9

10. 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 10

11. Pipes (continued)
Figure 7-2: Piping information from one command to another
Linux+ Guide to Linux Certification, 2e 11

12. 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 12

13. Pipes (continued)
Figure 7-3: Piping several commands
Linux+ Guide to Linux Certification, 2e 13

14. Pipes (continued)
Linux+ Guide to Linux Certification, 2e 14

15. 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 15

16. 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 16

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 17

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

19. Environment Variables
(continued)
Table 7-3 (continued): Common BASH environment variables
Linux+ Guide to Linux Certification, 2e 19

20. Environment Variables
(continued)
Table 7-3 (continued): Common BASH environment variables
Linux+ Guide to Linux Certification, 2e 20

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 21

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 22

23. 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 23

24. 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 24

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 25

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 26

27. 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 27

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 28

29. 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 29

30. Escape Sequences (continued)
Table 7-4: Common echo escape sequences
Linux+ Guide to Linux Certification, 2e 30

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
Linux+ Guide to Linux Certification, 2e 31

32. 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 32

33. Decision Constructs (continued)
Figure 7-4: A sample decision construct
Linux+ Guide to Linux Certification, 2e 33

34. Decision Constructs (continued)
Figure 7-5: A sample decision construct
Linux+ Guide to Linux Certification, 2e 34

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

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 36

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 37

38. The if Construct (continued)
Table 7-5: Common test statements
Linux+ Guide to Linux Certification, 2e 38

39. The if Construct (continued)
Table 7-6: Special operators in test statements
Linux+ Guide to Linux Certification, 2e 39

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

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 41

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 42

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 43

44. Summary
 Three components are available to
commands: Standard Input, Standard
Output, and Standard Error
 Standard Input is typically user input
taken from the keyboard; Standard
Output and Standard Error are sent to
the terminal screen
 You can redirect the Standard Output
and Standard Error of a command to a
file using redirection symbols
Linux+ Guide to Linux Certification, 2e 44

45. Summary (continued)
 Use the pipe symbol to redirect the
Standard Output from one command to
the Standard Input of another
 Most variables available to the BASH
shell are environment variables that are
loaded into memory after login from
environment files
 You can create your own variables in the
BASH shell and export them so that they
are available to programs started by the
shell
Linux+ Guide to Linux Certification, 2e 45

46. Summary (continued)
 The UMASK variable and command
aliases are special variables that must
be set using a certain command
 Shell scripts can be used to execute
several Linux commands
 Decision constructs can be used in shell
scripts to execute certain Linux
commands based on user input or the
results of a certain command
Linux+ Guide to Linux Certification, 2e 46