Description

Kameswari Chebrolu
Introduction
• Command-line bash vs bash script
• Scripting: take a set of commands you’d normally run by hand and put them in a file. Why?
– Automate tasks to make life easier!
– Can run them repeatedly with one command!
Example
• A sequence of commands
• Invoke a script using bash
• Invoke a script as a hash-bang executable
– #!/usr/bin/env bash (env will find the path, better!) or
– #!/bin/bash (absolute path)
– Above is called shebang
• A line of code that tells the shell what program to use
• # is used for commenting, hence ignored during execution
– Want to use python, replace bash with python
– Need to make the file executable via chmod
Variables
• Debug: use echo to print statements
• No typing, no need for declaration
– Bash variables are character strings, but, depending on context permits integer operations
• Defined using the assign operator “=”
– e.g. s=hello
– No space before or after!
– Variable name can include alphabets, digits, and underscore; can be started with alphabets and underscore only; case sensitive
– Value can be used with the expression $s
Few things to note
• Special character need escaping via single quotes (‘…’) or back slash ()
– ” or ’ or \ or $
• Single and double quotes: Help group arguments with spaces
– Single quotes (‘) preserve the literal value of each character
– Double quotes (“”) preserve the literal value of all characters with the exception of $, `,
Environment Variables
• Variables in your system that describe your environment!
– SHELL: what shell you’re running
– USER: username of the current user
– PWD: present working directory
– PATH: specifies the directories to be searched to find a command
– etc
• “env” command shows the list
Arithmetic Expressions
• Some support but not great
• let is a built-in command in Linux systems used for evaluating arithmetic expressions
– let “x = 1” or let “a = a + 1”
– let “var1 = 5” “var2 = 10” “var3=var1+var2”; echo $var3
• $((expression)), $[expression], declare i and bc are a few other options
If-then-else
Syntax:
• … represent conditions
– Conditions can be commands as well
if …
then
some statements elif …
some statements
else
some statements
fi
• Every command you run in shell returns a number
– No error: returns 0
– Error: usually 1 or -1 or some other number corresponds to the type of error that occurred
• Chained elif and the else parts are optional
Comparisons
-lt <
-gt >
-le <=
-ge >=
-eq ==
-ne !=
Loops
• Multiple types of loops: for, while, and until
• … list of things (need not be numeric)
– item is the loop variable which iterates through each item in the list
for item in [LIST]
do
[COMMANDS] done
while [CONDITION] do
[COMMANDS] done
until [CONDITION] do
[COMMANDS] done
• while executes a piece of code if the control expression is true and only stops when it is false (or a explicit break is found)
• until loop is almost equal to the while loop, except that the code is executed while the control expression evaluates to false!
Command Line Arguments and other
Special Shell Variables
$0 Name of the current shell script
$1-$9 Positional parameters 1 through 9 $# The number of positional parameters
$* All positional parameters, “$*” is one string
$@ All positional parameters, “$@” is a set of strings
$? Return status of most recently executed command
$$ Process id of current process
Functions
• In Bash, functions emulate the way commands work
– $*, $#, $1, $2 … (no explicit arguments, much like in command line arguments)
– Do not return values in usual way
• Any value sent back must be an integer which acts like the exit code of an executable
Local vs Gobal Variables
• Environment variables are global; inherited by any child shells or processes
– Shell variables are only present in the shell in which they were defined
– Shell variable can be made an environment variable by using export command
Arrays
• Arrays declared via -a command
• To access all elements in the array we can use
@
• To get the number of the elements in array we can use #
• Use “unset” to delete elements
Read from a file
See relevant file
References
1. https://linuxconfig.org/bash-scripting-tutorial
(a good beginner’s guide)
2. https://www.javatpoint.com/bash (another guide)
3. https://tldp.org/LDP/abs/html/ (more advanced)