Unit 11 configuring the bash shell – shell script

RedHat Enterprise Linux Essential
Unit 11: Configuring the Bash Shell – Shell
script

Objectives

Upon completion of this unit, you should be able to:

 Know how to use local and environment variables

 Know how to inhibit variable expansion

 Know how to create aliases

 Understand how the shell parses a command line

 Know how to configure startup files

 Know how to handle input with the read command and
positional parameters

Bash Variables

 Variables are named values
 Useful for storing data or command output

 Set with VARIABLE=VALUE

 Referenced with $VARIABLE

$ HI="Hello, and welcome to $(hostname)."
$ echo $HI
Hello, and welcome to stationX.

Environment Variables

 Variables are local to a single shell by default

 Environment variables are inherited by child shells
 Set with export VARIABLE=VALUE

 Accessed by some programs for configuration

Some Common Variables

 Configuration variables
 PS1: Appearance of the bash prompt

 PATH: Directories to look for executables in

 EDITOR: Default text editor

 HISTFILESIZE: Number of commands in bash history

 Information variables
 HOME: User's home directory

 EUID: User's effective UID

example PS1

syntax: PS1='[display content]'
 ! Display history number
 # Display number of current command
 $ Display $ or #
 Display symbol
 d Display current date
 h Display hostname
 s Display shell
 t Display current time
 u Display user
 W Display home work current
 w Display full path home work current

PS1='t u@h s $'

Aliases

 Aliases let you create shortcuts to commands

$ alias dir='ls -laht'

 Use alias by itself to see all set aliases

 Use alias followed by an alias name to see alias value

$ alias dir

alias dir='ls -laht'

How bash Expands a Command Line
 Split the line into words
 Expand aliases
 Expand curly-brace statements ({})
 Expand tilde statements (~)
 Expand variables ($)
 Command-substituation ($() and ``)
 Split the line into words again
 Expand file globs (*, ?, [abc], etc)
 Prepare I/O redirections (<, >)
 Run the command!

Preventing Expansion

 Backslash ( ) makes the next character literal
$ echo Your cost: $5.00
Your cost: $5.00
 Quoting prevents expansion
 Single quotes (') inhibit all expansion

 Double quotes (") inhibit all expansion, except:
• $ (dollar sign) - variable expansion

• ` (backquotes) - command substitution

• (backslash) - single character inhibition

• ! (exclamation point) - history substitution

Login vs non-login shells

 Startup is configured differently for login and non-login shells
 Login shells are:
 Any shell created at login (includes X login)

 su –
 Non-login shells are:
 su

 graphical terminals

 executed scripts

 any other bash instances

Bash startup tasks: profile

 Stored in /etc/profile (global) and ~/.bash_profile (user)

 Run for login shells only

 Used for
 Setting environment variables

 Running commands (eg mail-checker script)

Bash startup tasks: bashrc

 Stored in /etc/bashrc (global) and ~/.bashrc (user)

 Run for all shells

 Used for
 Setting local variables

 Defining aliases

Bash exit tasks

 Stored in ~/.bash_logout (user)

 Run when a login shell exits

 Used for
 Creating automatic backups

 Cleaning out temporary files

Scripting: Taking input
with positional Parameters
 Positional parameters are special variables that hold the
command-line arguments to the script.

 The positional parameters available are $1, $2, $3, etc. .
These are normally assigned to more meaningful variable
names to improve clarity.

 $* holds all command-line arguments

 $# holds the number of command-line arguments

Scripting: Taking input with the read command

 Use read to assign input values to one or more shell variables:
 -p designates prompt to display

 read reads from standard input and assigns one word to each variable

 Any leftover words are assigned to the last variable

 read -p "Enter a filename: " FILE
#!/bin/bash
read -p "Enter several values:" value1 value2
value3
echo "value1 is $value1"

While
 #!/bin/bash
# SCRIPT: method1.sh
# PURPOSE: Process a file line by line with PIPED while-
read loop.

FILENAME=$1
count=0
cat $FILENAME | while read LINE
do
let count++
echo "$count $LINE"
done

echo -e "nTotal $count Lines read"

While with redirect
 #!/bin/bash
#SCRIPT: method2.sh
#PURPOSE: Process a file line by line with redirected
while-read loop.

FILENAME=$1
count=0

while read LINE
do
let count++
echo "$count $LINE"

done < $FILENAME

echo -e "nTotal $count Lines read"

Unit 11 configuring the bash shell – shell script

Unit 11 configuring the bash shell – shell script

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Unit 11 configuring the bash shell – shell script