linux week7

Week 7Working 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

Linux+ Guide to Linux Certification, 2e 2

Objectives (continued)

Describe the purpose and nature of shell scripts

Create and execute basic shell scripts Effectively use common decision

constructs in shell scripts


Command Input and Output BASH shell responsible for:

Providing user interface Interpreting commandsManipulating command input and output

○ Provided user specifies certain shell metacharacters with command

File descriptors: Numeric labels that define command input and command output


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


Command Input and Output (continued)


Figure 7-1: The three common file descriptors

Redirection

Redirect stdout and stderr from terminal screen to a file Use “>” shell metacharacterCan redirect stdout and stderr to separate

files

Use separate filenames for stdout and stderr


Redirection (continued)

Redirecting stdin to a file:Use “<“ shell metacharacter

tr command: Replace characters in a file sent via stdin


Redirection (continued)


Table 7-1: Common redirection examples

Pipes

Send stdout of one command to another command as stdin

Pipe: String of commands connected by “|” metacharactersstdout on left, stdin on right

Commonly used to reduce amount of information displayed on terminal screen


Pipes (continued)

11Linux+ Guide to Linux Certification, 2e

Figure 7-2: Piping information from one command to another

Pipes (continued)

Can use multiple pipes on command linePass information from one command to

another over a series of commands

filter commands: Commands that can take from stdin and give to stdoutCan be on either side of a pipe

tee commands: Filter commands that also send information to a file


Pipes (continued)

13Linux+ Guide to Linux Certification, 2e

Figure 7-3: Piping several commands

Pipes (continued)


Pipes (continued)

Can combine redirection and pipingInput 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


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


Environment Variables

set command: Lists environment variables and current values

echo command: View contents a specified variableUse $ shell metacharacter

Changing value of a variable:Specify variable name followed by equal

sign (=) and new value


Environment Variables (continued)


Table 8-3: Common BASH environment variables



Table 7-3 (continued): Common BASH environment variables

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 charactersMust not start with a numberTypically capitalized to follow convention


User-Defined Variables (continued) Subshell: Shell created by current shell

Most shell commands run in a subshellVariables 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


Other Variables Not displayed by set or env commands

Perform specialized functions in the shelle.g., UMASK variable

alias command: Creates shortcuts to commandsUse unique alias namesAliases stored in special variablesCan create single alias to multiple

commands○ Use ; metacharacter


Environment Files

When exiting BASH shell, all stored variables are destroyed

Environment files: Store variables and valuesExecuted each time BASH shell is startedEnsures variables are always accessible


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


Environment Files (continued) To add a variable, add a line to

environment fileUse command line syntax

Any command can be placed inside any environment filee.g., alias creation

.bashrc (BASH run-time configuration): First hidden environment file executed at login


Shell Scripts

Shell script: Text file containing a list of commands or constructs for shell to executeMay contain any command that can be

entered on command line

Hashpling: First line in a shell scriptDefines which shell is used to interpret shell

script commands


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


Escape Sequences

Character sequences having special meaning in the echo commandPrefixed by \ characterMust use –e option in echo command


Escape Sequences (continued)


Table 7-4: Common echo escape sequences

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 stdinPlaces in a variable specified by an

argument to read command


Decision Constructs

Most common type of construct used in shell scripts

Alter flow of a program:Based on whether a command completed

successfullyBased on user input


Decision Constructs (continued)


Figure 7-4: A sample decision construct

Decision Constructs (continued)


Figure 7-5: A sample decision construct

The if Construct

Control flow of program based on true/false decisions

Syntax:


The if Construct (continued) Common rules governing if constructs:

elif (else if) and else statements optionalUnlimited number of elif statementsdo 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


The if Construct (continued) test statement: Used to test a condition

Generates a true/false valueInside of square brackets ( [ … ] )

○ Must have spaces after “[” and before “]”

Special comparison operators: –o (OR) –a (AND)! (NOT)


The if Construct (continued)


Table 7-5: Common test statements

The if Construct (continued)


Table 7-6: Special operators in test statements

The case Construct

Compares value of a variable with several different patterns of text or numbers

Syntax:


The case Construct (continued) If a match is found, commands to right

of pattern are executed Must end with esac


The && and || Constructs

Time-saving shortcut constructsWhen only one decision needs to be made

during execution

Syntax:command && commandcommand || command


The && and || Constructs (continued) &&: Second command executed only if

first completes successfully ||: Second command executed only if

first fails


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


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


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 week7

Documents

linux guide

bash shell

special variables

programs started

environment variables

current shell

environment files

command input