Essential Bash Crash Course for Software Engineers

Reference

Bash is a powerful scripting language essential for automating tasks in Linux. This crash course covers the core elements that every software engineer should know to become effective at Bash scripting.

Basics of Bash

Variables

name="Alice"
echo "Hello, $name"

• Variables in bash are defined without = spaces.
• Use $ to reference a variable.

Conditional Statements

if [ "$name" == "Alice" ]; then
  echo "Hello, Alice!"
else
  echo "You are not Alice."
fi

• [ ... ] is used to test conditions.
• == compares strings, -eq is for integers.

Loops

• For Loop

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

  Loops over a sequence of numbers or list of strings.
• While Loop

  count=0
  while [ $count -lt 3 ]; do
    echo "Count is: $count"
    count=$((count+1))
  done
  

Functions

function greet() {
  local name="$1"
  echo "Hello, $name!"
}
greet "Bob"

• Use function keyword (optional) and local for scope isolation.
• Call with greet "argument".

File Operations

• Redirect Output

  echo "Hello World" > output.txt
  

  > writes (overwrites), >> appends.
• Read File Line by Line

  while read -r line; do
    echo "Line: $line"
  done < input.txt
  

Command Substitution

current_date=$(date)
echo "Today's date is $current_date"

• Wrap a command in $(...) to assign its output to a variable.

Control Operators

• AND (&&) and OR (||)

  mkdir my_folder && echo "Folder created"
  

  Executes second command only if the first succeeds.
• Pipes (|)

  cat file.txt | grep "search_term"
  

  Passes the output of one command to another.

Wildcards & Globbing

echo *.txt  # Lists all files ending with .txt

• * matches zero or more characters, ? matches exactly one.

Debugging Scripts

• Run with Debug Mode

  bash -x script.sh
  

  Enables debug output for step-by-step tracing.

Practical Tips

• Environment Variables: Use $HOME or $USER for user-specific paths.
• Shebang (#!): Always start your script with #!/bin/bash to specify the interpreter.
• Exit Codes: Use $? to get the status of the last command executed.

Example Script: File Backup

#!/bin/bash
src_folder="/path/to/source"
dest_folder="/path/to/backup"
date=$(date +%Y-%m-%d)
backup_file="$dest_folder/backup-$date.tar.gz"

tar -czf "$backup_file" "$src_folder" && echo "Backup successful: $backup_file"

• Create a compressed backup using tar.
• && ensures echo runs only on success.

FAQ

1. How do you run a bash script?

   • Use ./script.sh after making it executable (chmod +x script.sh).

2. What's the difference between [ ... ] and [[ ... ]]?

   • [[ ... ]] supports more complex expressions and is safer for string comparisons.

3. How do you pass arguments to a script?

   • Use $1, $2, etc., for positional arguments (script.sh arg1 arg2).

4. How do you check if a file exists?

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

5. What is #!/bin/bash?

   • It specifies the interpreter used to execute the script.

6. How do you set a variable as read-only?

   readonly my_var="value"
   

7. How do you loop through files in a directory?

   for file in /path/to/dir/*; do echo "File: $file"; done
   

8. How can you handle errors in bash?

   • Use set -e to exit the script when any command fails.

9. How do you create an alias?

   • alias ll='ls -la' for convenient command shortcuts.

10. How do you declare arrays in bash?

    my_array=("item1" "item2" "item3")
    echo "First item: ${my_array[0]}"