Working with the BASH shell

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Working with the BASH shell

• Unit objectives
• Redirect the input and output of a command
• Identify, manipulate, create and export shell
  variables and edit environment files to create
  variables
• Describe the purpose of shell scripts, create and
  execute them, and use common decision constructs
  in shell scripts
• Use and customize features of the BASH shell

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Topic A Command input and output
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The role of a shell

• Provides a user interface
• Interprets commands
• Can manipulate command input and output using
  shell metacharacters

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File descriptors

• Standard Input (stdin)
• Represent information inputted to a command
  during execution
• Standard Output (stdout)
• Represents the desired output from a command
• Standard Error (stderror)
• Represents any error message generated by a
  command

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Three common file descriptors
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Redirection

• Redirect Standard Output and Standard Error to a
  file using the gt
• You can redirect both Standard Output and
  Standard Error to separate files at the same time
• You can redirect both Standard Output and
  Standard Error to the same file using the
  character

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Redirection

• Append data
• date gtgt dateoutput
• Adds output of date command to dateoutput file
• Redirect to St. Input
• tr 1 L lt/etc/hosts
• tr 1 L lt/etc/hosts gtnewhosts

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Types of redirection
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Pipes

• Sends Standard Output of one command as Standard
  Input to another command
• Pipe
• A string of commands connected by
  metacharacter
• Example
• Ls l /etc less

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Piping information
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Using multiple pipes
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Common filter commands
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Using Filters

• What does this command do?
• cat prologue tr a A sort pr d less

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The sed and awk commands

• sed
• Can search for text string
• Can replace text with another string
• awk
• Treats each line as a database record
• Treats each word as a database field

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The sed and awk commands

• sed
• cat prologue sed s/the/THE/

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Any Questions?
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• A user wants the script 'name1' to process the
  contents of the file 'name2', then redirect the
  output to the program 'name3'. Which of the
  following shell commands will do this?
• A. name1 name2 gt name3
• B. name1 lt name2 name3
• C. name1 gt name2 name3
• D. name1 name2 lt name3

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• A user wants the script 'name1' to process the
  contents of the file 'name2', then redirect the
  output to the program 'name3'. Which of the
  following shell commands will do this?
• A. name1 name2 gt name3
• B. name1 lt name2 name3
• C. name1 gt name2 name3
• D. name1 name2 lt name3
• Answer A

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• How could a user substitute all instances of PC
  with Computer in a file named instructions and
  display the results in the terminal window?
• A. sed s/PC/computer/g instructions
• B. sed s PC r computer instructions
• C. cat instructions awk C omputer
• D. cat instructions awk '/PC/ print omputer'

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• How could a user substitute all instances of PC
  with Computer in a file named instructions and
  display the results in the terminal window?
• A. sed s/PC/computer/g instructions
• B. sed s PC r computer instructions
• C. cat instructions awk C omputer
• D. cat instructions awk '/PC/ print omputer'
• Answer A

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• A user wants the script name1 to process the
  contents of the file "name2", then redirect the
  output to the program name3. Which if the
  following shell commands will do this?
• A. name1 name2 gt name3
• B. name1 lt name2 name3
• C. name1 gt name2 name 3
• D. name1 name2 lt name3

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• A user wants the script name1 to process the
  contents of the file "name2", then redirect the
  output to the program name3. Which if the
  following shell commands will do this?
• A. name1 name2 gt name3
• B. name1 lt name2 name3
• C. name1 gt name2 name 3
• D. name1 name2 lt name3
• Answer B.

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• An administrator needs to append the list of
  files in the /tmp directory to the existing file
  "DoNotCreateBackup". What command would
  accomplish this goal?
• A. ls /tmp DoNotCreateBackup
• B. ls /tmp DoNotCreateBackup
• C. ls /tmp gtgt DoNotCreateBackup
• D. ls /tmp DoNotCreateBackup

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• An administrator needs to append the list of
  files in the /tmp directory to the existing file
  "DoNotCreateBackup". What command would
  accomplish this goal?
• A. ls /tmp DoNotCreateBackup
• B. ls /tmp DoNotCreateBackup
• C. ls /tmp gtgt DoNotCreateBackup
• D. ls /tmp DoNotCreateBackup
• Answer C.

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• If the command below is executed in a user's home
  directory, which of the following would be
  accomplished? echo hello gtgt allusers
• A. Sends the message 'hello' to all users on the
  system.
• B. Creates a new file called 'hello' and sends it
  to all users.
• C. Sends the message 'hello' to all currently
  logged in users.
• D. Appends 'hello' to the end of the file
  llusers? if it exists.

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• If the command below is executed in a user's home
  directory, which of the following would be
  accomplished? echo hello gtgt allusers
• A. Sends the message 'hello' to all users on the
  system.
• B. Creates a new file called 'hello' and sends it
  to all users.
• C. Sends the message 'hello' to all currently
  logged in users.
• D. Appends 'hello' to the end of the file
  llusers? if it exists.
• Answer D

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Topic B Shell variables
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Shell variables

• Variable
• A reserved portion of memory containing
  information that may be accessed
• Environment variables
• Typically set by the system
• Contain information that the system and programs
  access regularly
• User-defined variables
• Your own custom variables

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Environment variables

• Set by default in the BASH shell
• The set command
• Displays a list of environment variables and
  their current values

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The PATH variable

• One of the most important variables
• Helps shell find location of commands
• Saves you having to enter full path to commands
• Most commands located in /bin or /sbin

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BASH environment variables
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User-defined variables

• Variable identifier
• The name of a variable
• Features of variable identifiers
• Can contain alphanumeric characters, the dash
  character, or the underscore character
• Must not start with a number
• Typically capitalized

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Subshells

• Most shell scripts that are run by the shell are
  run in a separate subshell
• The subshell is created by the current shell
• Variables created in the current shell are not
  available to subshells or the commands running
  within them

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Built-in commands

• Other variables not displayed by the set or env
  commands
• Perform specialized functions in the shell
• The UMASK variable
• A special variable is set by the umask command

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Environment files

• Store defined variables
• Can define different variables for different
  users
• Load variables at log in

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Environment file execution

• Order
• /etc/profile
• /.bash_profile
• /.bash_login
• /.profile

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Topic C Shell scripts
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Shell scripts

• Text files that contain a list of commands or
  constructs for the shell to execute in order
• Can enter commands just as a user would

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Escape sequences

• Character sequences that have special meaning
  inside the echo command

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Reading standard input

• Shell script may need input from the user
• Input may be stored in a variable for later use
• The read command
• Takes input from Standard Input and places it in
  a variable specified by an argument to the read
  command

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Decision constructs
continued
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Decision constructs, continued
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if construct rules

• elif (else if) and else statements are optional
• Can have unlimited number of elif statements
• do these commands section can consist of multiple
  commands
• do these commands section is typically indented
  from the left
• End of statement is backwards if, fi
• this is true part of syntax may be command or
  test statement

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Common test statements
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Special operators
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The case construct

• The case statement
• Compares variable value with several patterns of
  text or numbers
• If there is a match, commands to the right are
  executed
• Ended by a backwards case, esac

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The and constructs

• Shortcut constructs that take less time form
  making only one decision
• Syntax
• command command
• command command

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Topic D BASH command history
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The bash history feature

• Recalls previous commands
• Reduces the number of number of keystrokes

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Customizing BASH history

• Update the bash_history file
• Specify number of commands to remember
• Set HISTSIZE environment variable

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Unit summary

• Redirected the input and output of commands
• Identified, manipulated, created, and exported
  shell variables and edited environment files to
  create variables
• Described the purpose of shell scripts, executed
  scripts, and used common decision constructs in
  shell scripts
• Used and learned how to customize the BASH shell