Shell Scripts

A shell script is a text file that contains a sequence of commands for a Unix-based operating system. It is called a shell script because it combined a sequence of commands in a file that would otherwise have to be typed in one at a time into a single script.

Shell Script Basics

What's a Shebang? 

#!/bin/zsh
#!/bin/bash
#!/bin/sh

A shebang is a special comment at the top of shell scripts that tells the system which shell to use when executing a file.

Adding Comments 

# This is a comment in a shell script!

Variables 

name="Grant"
age=28

echo "$name is $age years old."
  • -eq
  • -ne
  • -gt - Greater than
  • -lt - Less than
  • -ge
  • -le
  • = - Equal to
  • != - Not equal to

Getting User Input 

echo "Enter a number:"
read number

Conditionals

Some popular comparison operators include: 

if [ $num -gt 10 ]; then
  echo "The number is greater than 10"
else
  echo "The number is 10 or less"
fi
if [ condition ]; then
  # Code for true condition
fi
if [ condition ]; then
  # Code for true condition
else
  # Code for false condition
fi
if [ condition1 ]; then
  # Code for condition1 true
elif [ condition2 ]; then
  # Code for condition2 true
else
  # Code if none of the above conditions are true
fi

Arrays 

fruits=("apple" "banana" "cherry")

echo "First fruit {fruits[0]}"
echo "All fruits {fruits[@]}"

Loops 

# For Loop

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

# While Loop

count=1

while [$count -le 5]; do
  echo "Count: $count"
  ((count++))
done

Functions 

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

greet "World"

Command-Line Arguments

Some popular comparison operators include:

  • $@ - All arguments
  • $? - Exit status of the last command
echo "Script name: $0"
echo "Script name: $1"
echo "Script name: $2"
echo "Script name: $#"

Exit Status and Error Handling 

mkdir new_folder

if [ $? -eq 0 ]; then
  echo "Directory created successfully!"
else
  echo "Failed to create directory"
fi
mkdir new_folder || { echo "Error creating directory!"; exit 1 }

Using Quotes 

name="Grant"
echo 'Hello $name' # Prints Hello $name
echo "Hello $name" # Prints Hello Grant

Cleanup 

trap function INT

This means to run the function when I receive Ctrl+C (SIGINT). 

trap function EXIT

This means to run the function whenever the shell script exits.

Debugging 

bash -x script.sh
#!/bin/zsh
#!/bin/zsh -x
#!/bin/zsh -e

Sleep 

sleep 2 # waits 2 seconds before resuming

ANSI

See more ANSI details. 

# Define color variables

RED='\e[31m'
GREEN='\e[32m'
YELLOW='\e[33m'
BLUE='\e[34m'
ENDCOLOR='\e[0m' # Clear text formatting

# Use variables in your script

echo -e "${GREEN}Success:${ENDCOLOR} The operation completed successfully."
echo -e "${YELLOW}Warning:${ENDCOLOR} Disk space is getting low."
echo -e "${RED}Error:${ENDCOLOR} Process failed."

Creating and Running a New Shell Script

Creating the File 

touch shell_script.sh

Adding a Script 

#!/bin/zsh

echo "This is a new shell script!"

echo "Creating a directory..."

mkdir shell_script_example

cd shell_script_example

echo "Adding a file..."

touch number_list.txt

echo "1234567890" >> number_list.txt

echo "Script executed successfully!"

Changing Permissions to Execute the Script 

ls -l shell_script.sh

chmod +x shell_script.sh

Executing the Script 

./shell_script.sh