6.ShellScripting.md

Shell Scripting

Why Shell Scripting?

• When administering servers, you need to execute tons of commands
  • useradd dip
  • groupadd devops
  • mkdir temp
  • touch 1.txt
  • chmod 750 /temp
  • sudo apt docker
  • docker run ...
• Instead of executing the command one by one, you can save the commands in a file and execute that file.
• Such file is called a shell script.
• File extension is .sh
• Automate all your work:
  • Backup scripts -> Monitoring scripts -> Server Configuration scripts
• Avoid repetitive work
• Keep history of configuration
• Share the instructions
• Logic & Bulk Operations

How to execute Shell Scripts?

Shell

• The program that interprets and executes the various commands that we type in the terminal
• Translates our command that the OS Kernel can understand

Different Shell implementations

• sh

  • Bourne Shell - /bin/sh

• Bash

  • Bourne again shell - /bin/bash
  • Improved version of sh
  • Default shell program for most UNIX like systems
  • Bash is a shell program and a programming language

• Shell and Bash terms are often used interchangeable

• All shell script files have the same .sh file extension

How OS knows which shell to use?

• We need to tell the OS
• SH - #!/bin/sh
• BASH - #!/bin/bash
• ZSH - #!/bin/zsh

This is called "shebang" because of the first 2 characters "#!"

• # = in musical notation, also called "sharp"
• ! = also called "bang"
• Shebang became a shortening of sharp-bang

Make shell script executable

• To execute a shell script, you need to make it executable first, by adding executing permission

sudo chmod u+x first.sh # execute permission to user(owner)
sudo chmod g+x first.sh # execute permission to group
sudo chmod o+x first.sh # execute permission to other

Writing Shell Scripts

Variables

• Used to store data and can be referenced later
• Similar to variables in general programminglanguages
• Store output of a command in a variable: variable_name=$(command)

Conditionals

• Allow you to alter the control flow of the program
• E.g. execute a command only when a certain condition is true

  if [ condition ]
  then
    statement
  elif [ condition ]
  then
    statement
  else
    statement
  fi

[ ... ] builtin command

• Square brackets enclose expressions
• It's a shorthand notation for the "test" command
  • if test -d "config" is the same

Boolean

• A datatype that can only have 2 values:
  • True
  • False
• Bash does have Boolean expressions in terms of comparison and conditions

File Test Operators

• Test various properties associated with a file:

Operator	Description	Example
-b file	Checks if file is a block special file; if yes, then the condition becomes true.	[ -b $file ]
-c file	Checks if file is a character special file; if yes, then the condition becomes true.	[ -c $file ]
-d file	Checks if file is a directory; if yes, then the condition becomes true.	[ -d $file ]
-f file	Checks if file is an ordinary file as opposed to a directory or special file; if yes, then the condition becomes true.	[ -f $file ]
-g file	Checks if file has its set group ID (SGID) bit set; if yes, then the condition becomes true.	[ -g $file ]
-k file	Checks if file has its sticky bit set; if yes, then the condition becomes true.	[ -k $file ]
-p file	Checks if file is a named pipe; if yes, then the condition becomes true.	[ -p $file ]
-t file	Checks if file descriptor is open and associated with a terminal; if yes, then the condition becomes true.	[ -t $file ]
-u file	Checks if file has its Set User ID (SUID) bit set; if yes, then the condition becomes true.	[ -u $file ]
-r file	Checks if file is readable; if yes, then the condition becomes true.	[ -r $file ]
-w file	Checks if file is writable; if yes, then the condition becomes true.	[ -w $file ]
-x file	Checks if file is executable; if yes, then the condition becomes true.	[ -x $file ]
-s file	Checks if file has size greater than 0; if yes, then condition becomes true.	[ -s $file ]
-e file	Checks if file exists; is true even if file is a directory but exists.	[ -e $file ]

Relational Operators

• Works only for numeric values
• Assume variable a holds 10 and variable b holds 20 then:

Operator	Description	Example
-eq	Checks if the value of two operands are equal or not; if yes, then the condition becomes true.	[ $a -eq $b ] is not true
-ne	Checks if the value of two operands are equal or not; if values are not equal, then the condition becomes true.	[ $a -ne $b ] is true
-gt	Checks if the value of left operand is greater than the value of right operand; if yes, then the condition becomes true.	[ $a -gt $b ] is not true
-lt	Checks if the value of left operand is less than the value of right operand; if yes, then the condition becomes true.	[ $a -lt $b ] is true
-ge	Checks if the value of left operand is greater than or equal to the value of right operand; if yes, then the condition becomes true.	[ $a -ge $b ] is not true
-le	Checks if the value of left operand is less than or equal to the value of right operand; if yes, then the condition becomes true.	[ $a -le $b ] is true

String Operators

• Test strings for equality and more
• Assume variable a holds "abc" and variable b holds "efg" then:

Operator	Description	Example
=	Checks if the value of two operands are equal or not; if yes, then the condition becomes true	[ $a = $b ] is not true
!=	Checks if the value of two operands are equal or not; if values are not equal then the condition becomes true	[ $a != $b ] is true
-z	Checks if the given string operand size is zero; if it is zero length, then it returns true	[ -z $a ] is not true
-n	Checks if the given string operand size is non-zero; if it is nonzero length, then it returns true	[ -n $a ] is not false
str	Checks if str is not the empty string; if it is empty, then it returns false	[ $a ] is not false

Passing arguments to a script

• You can pass arguments when executing a script

  read -p "Please enter your name: " user_name
  echo "You name is $username"

• We can read these passed parameters using special variables

Positional Parameters

• Arguments passed to script are processed in the same order in which they're sent
• The indexing of arguments starts at 1: $1
• $* = represent all the arguments as a single string
• $# = Total number of arguments provided
• $? - captures value returned by last command Loops
• Enables you to execute a set of commands repeatedly
• There are different types of loops:
  • while loop
  • for loop
  • until loop
  • select loop

For Loop

• Operates on lists of items
• Repeats a set of commands for every item in the list

for var in word1 word2 ...
do
  statement(s) to be executed for every word
done

# Example
for param in $*
do
  if [ -d "$param" ]
    then
      echo "directory $param exist"
  fi
  echo $param
done

While Loop

• Executes a set of commands repeatedly until some condition is matched

while condition
do
  statement(s) to be executed if command is true
done

# Example
count=0
while [ "$count" -lt "$#" ]
do
  echo "count: $count"
  count=$((count+1))
done

Functions

• Enables you to break down the overall functionality of a script into smaller, logical code blocks
• This code block can then be referenced ("called") anywhere in the script multiple times
• You can accept parameters in a function
• The parameters passed in are represented by $1, $2, etc.

function iterate {
  count=0
  while [ "$count" -lt 5 ]
  do
    echo "count: $count"
    count=$((count+1))
  done
}
iterate

Function Paramaters

function create_file() {
  filename=$1
  touch $filename
  echo "$filename created."
}

create_file hello.txt
create_file bye.sh

function sum() {
  total=$(($1+$2))
  return $total
}

sum 2 10
result=$?
echo "Result: $result"