Guide Linux+ to Linux Certification, Second Edition
Chapter 8 Working with the BASH Shell
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
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Objectives (continued)
Describe the purpose and nature of shell scripts Create and execute basic shell scripts Effectively use common decision constructs in shell scripts
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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
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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
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Command Input and Output (continued)
Figure 8-1: The three common file descriptors
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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
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Redirection (continued)
Redirecting stdin to a file:
Use "<" shell metacharacter
tr command: Replace characters in a file sent via stdin
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Redirection (continued)
Table 8-1: Common redirection examples
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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
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Pipes (continued)
Figure 8-2: Piping information from one command to another
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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
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Pipes (continued)
Figure 8-3: Piping several commands
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Pipes (continued)
Table 8-2: Common filter commands
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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
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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
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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
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Environment Variables (continued)
Table 8-3: Common BASH environment variables
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Environment Variables (continued)
Table 8-3 (continued): Common BASH environment variables
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Environment Variables (continued)
Table 8-3 (continued): Common BASH environment variables
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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
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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
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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
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Environment When Files
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
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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
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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
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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
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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
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Escape Sequences
Character sequences having special meaning in the echo command
Prefixed by \ character Must use e option in echo command
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Escape Sequences (continued)
Table 8-4: Common echo escape sequences
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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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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
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Decision Constructs (continued)
Figure 8-4: A sample decision construct
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Decision Constructs (continued)
Figure 8-5: A sample decision construct
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The if Construct
Control flow of program based on true/false decisions Syntax:
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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
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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)
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