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Shell Scripting Cheat Sheet

Shell scripting is a way to automate tasks by writing scripts (sequences of commands) in a text file, which the shell can execute.

Basic Shell Scripting Commands:

1. #!/bin/bash

  • Purpose: This is called the shebang line. It specifies the interpreter for the script (in this case, Bash). The script will be executed by the Bash shell.
  • Example: This script will print “Hello, World!” to the screen.
#!/bin/bash echo "Hello, World!"

2. echo

  • Purpose: The echo command is used to print a message or the value of a variable to the terminal.
  • Usage :Example:
  • echo “Message” – Prints a string to the terminal.
  • echo $variable – Prints the value stored in a variable.
echo "Hello, World!"   
name="Alice"   
echo "Hello, $name"

3. Variables

  • Purpose: Variables are used to store data that can be reused later in the script.
  • Usage: Example:
  • variable_name=“value” – Declare and assign a value to a variable.
  • echo $variable_name – Access the value of a variable.
name="Alice"   
echo "Hello, $name"  # Output: Hello, Alice

4. Conditionals (if/else)

  • Purpose: Conditional statements allow the execution of commands based on whether a condition is true or false.
  • Usage:
if \[ condition \]; then  
    # command if condition is true  
else  
    # command if condition is false  
fi  
  
#!/bin/bash  
  
number=5  # No spaces around '=' in variable assignment  
  
if \[ "$number" -eq 5 \]; then  
    echo "The number is 5 buddy"  
else  
    echo "Teri lowwde"  
fi
  • Example:
if \[ $name == "Alice" \]; then  
    echo "Hello, Alice!"  
else  
    echo "You're not Alice."  
fi
  • This checks if the variable name is equal to “Alice” and prints a message accordingly.

5. Loops (for/while)

  • Purpose: Loops allow repeated execution of commands. Useful for iterating over items or performing repetitive tasks.
  • For Loop:
for i in 1 2 3; do  
    echo "Loop $i"  
done
  • This will loop through the numbers 1, 2, and 3, printing each one.
  • While Loop:
count=1  
while \[ $count -le 5 \]; do  
    echo "Counter: $count"  
    ((count++))  
done
  • This will increment count from 1 to 5 and print the value of count each time.

6. Reading Input

  • Purpose: Accept user input during the execution of a script.
  • Usage:
read variable\_name  
echo "You entered: $variable\_name"
  • Example:
echo "Enter your name:"  
read name  
echo "Hello, $name!"

7. Functions

  • Purpose: Functions allow you to group related commands together and reuse them within your script.
  • Usage:
my\_function() {  
    echo "This is a function"  
}  
my\_function  # Call the function
  • Example:
greet() {  
    echo "Hello, $1!"  
}  
greet "Alice"  # Output: Hello, Alice!
  • $1 refers to the first argument passed to the function.

8. Making a Script Executable

  • Purpose: Allows you to run a script as if it were a command from the terminal.
  • Steps:

Make the script executable using chmod:

chmod +x script\_name.sh

Run the script by calling it directly:

./script\_name.sh
  • This allows you to execute the script without explicitly invoking the interpreter (e.g., bash script_name.sh).

9. Comments

  • Purpose: Comments are used to explain parts of your script and make it easier to understand.
  • Usage:
\# This is a single-line comment
  • Example:
\# This script greets the user echo "Hello, User!"

10. File Redirection and Pipes

  • Purpose: Shell scripting allows you to redirect input/output or pipe the output of one command to another.
  • Redirection:
  • > – Redirects output to a file (overwrites).
  • >> – Redirects output to a file (appends).
  • Pipes:
  • | – Pipes the output of one command to another.
  • Examples:
echo "Hello, World!" > output.txt   # Redirects to a file  
cat output.txt                     # Display the contents of output.txt  
  
ls | grep "file"                   # Pipe the output of ls to grep

11. Exit Status

  • Purpose: Every command in a script returns an exit status code, which indicates whether the command was successful.
  • 0 – Success
  • Non-zero — Error
  • Usage:
if \[ $? -eq 0 \]; then  
    echo "Last command was successful."  
else  
    echo "Last command failed."  
fi

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