📜 Shell Scripting Notes

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𞷂 Table of Contents

1. Introduction to Shell Scripting
2. Basic Shell Commands
3. Variables and Constants
4. Input and Output
5. Conditional Statements
6. Loops
7. Functions
8. Arrays
9. String Manipulation
10. File Handling
11. Command-line Arguments
12. Exit Status & Return Codes
13. Error Handling & Debugging
14. Signals and Traps
15. Process Management
16. Scheduling Jobs (cron & at)
17. Advanced Topics
18. Environment Variables & PATH
19. Best Practices
20. Useful Built-in Commands
21. Resources & Further Reading

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1 Introduction to Shell Scripting

• Shell: Interface between user and OS
• Shell Script: File containing a sequence of shell commands
• Common Shells: bash, zsh, sh, fish

• First script:

  #!/bin/bash
  echo "Hello, World!"
  

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2 Basic Shell Commands

Command	Description
ls	List files
cd	Change directory
pwd	Show current directory
touch	Create empty file
mkdir	Make new directory
rm	Remove files/folders
cp	Copy files
mv	Move/Rename
cat	Display file contents
man	Manual/help pages

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3 Variables and Constants

name="Adarsh"
readonly college="VIT"
unset name

• $name, ${name} — access variable
• No spaces around =
• readonly makes variable constant
• unset deletes a variable

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4 Input and Output

echo "Enter your name:"
read user
echo "Hello, $user!"

• read -p "Prompt" var — prompt user
• read -s var — silent input (e.g., passwords)

• Redirects:

  • > : overwrite
  • >>: append
  • < : input file

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5 Conditional Statements

if [ "$a" -gt 10 ]; then
  echo "Greater"
elif [ "$a" -eq 10 ]; then
  echo "Equal"
else
  echo "Lesser"
fi

• [ condition ] or [[ condition ]]
• test expr is same as [ expr ]

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6 Loops

For Loop

for i in {1..5}; do
  echo "Number: $i"
done

While Loop

while [ $n -lt 5 ]; do
  echo $n
  ((n++))
done

Until Loop

until [ $n -ge 5 ]; do
  echo $n
  ((n++))
done

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7 Functions

greet() {
  echo "Hello $1"
}
greet "Adarsh"

• $1, $2 — positional params
• return — return status code (0–255)

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8 Arrays

arr=(apple banana mango)
echo ${arr[1]}      # banana
echo ${arr[@]}      # all
echo ${#arr[@]}     # size

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9 String Manipulation

str="hello world"
echo ${#str}           # length
echo ${str:0:5}        # slice
echo ${str/world/Adarsh} # replace

• ${var#pattern} — remove prefix
• ${var%pattern} — remove suffix

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10 File Handling

if [ -f "file.txt" ]; then
  echo "File exists"
fi

• -f file exists, regular
• -d is directory
• -r readable
• -w writable
• -x executable

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11 Command-line Arguments

echo $0  # script name
echo $1  # first arg
echo $#  # number of args
echo $@  # all args

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12 Exit Status & Return Codes

• $? — last command exit code
• exit 1 — custom exit
• 0 is success, non-zero is error

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13 Error Handling & Debugging

set -e      # exit on error
set -x      # debug mode (print commands)
trap 'echo "Error at $LINENO"' ERR

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14 Signals and Traps

trap "echo Caught SIGINT" SIGINT

• Used to intercept signals like SIGINT, SIGTERM
• Useful for cleanup scripts

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15 Process Management

Command	Use
ps	View processes
top	Live process monitor
kill	Kill process by PID
&	Run in background
wait	Wait for background job
jobs	List background jobs

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16 Scheduling Jobs (cron & at)

cron – Repeated Jobs

crontab -e
# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of week (0 - 6) (Sunday=0)
# │ │ │ │ │
# * * * * * command_to_execute

at – One-time Jobs

echo "bash backup.sh" | at now + 2 minutes

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17 Advanced Topics

• Here Documents:

  cat <<EOF
  Multi-line
  text block
  EOF
  

• Subshells:

  (cd dir && ls)
  

• Arithmetic Expressions:

  ((a = b + 5))
  

• Eval:

  cmd="ls -l"
  eval $cmd
  

• Sourcing Files:

  source config.sh
  

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18 Environment Variables & PATH

• Environment Variable: Global variable accessible by all processes (e.g., PATH, HOME)

• Set Temporarily:

  export EDITOR=nano
  

• PATH Variable: Tells the shell where to look for executables

  echo $PATH
  export PATH="$PATH:/my/custom/bin"
  

• To persist changes, modify ~/.bashrc or ~/.zshrc

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19 Best Practices

• Use #!/bin/bash shebang
• Quote all variables: "${var}"
• Check for errors: if [ $? -ne 0 ]; then ...
• Use functions for reuse
• Comment complex logic
• Prefer [[ ]] over [ ]

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20 Useful Built-in Commands

Command	Description
alias	Create shortcut
type	Show command type
source	Execute file in current shell
which	Show command path
basename, dirname	Path parsing

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21 Resources & Further Reading

• Bash Reference Manual
• ShellCheck Linter
• Advanced Bash-Scripting Guide
• Explainshell