How do I execute a bash script in Terminal?

How to Execute a Bash Script in Terminal

Learn the essential methods for running Bash scripts, from basic execution to making them executable and handling common scenarios.

Bash scripts are powerful tools for automating tasks and managing your system. Whether you're a beginner or an experienced user, knowing how to properly execute these scripts in your terminal is a fundamental skill. This article will guide you through the various methods, best practices, and common issues you might encounter.

Understanding Script Permissions and the Shebang

Before executing any script, it's crucial to understand file permissions and the 'shebang' line. The shebang (#!) at the beginning of a script tells the operating system which interpreter to use for executing the script. Without it, or with incorrect permissions, your script might not run as expected.

#!/bin/bash
# This is a simple bash script
echo "Hello from my first script!"
ls -l

Example of a basic Bash script with a shebang.

Methods for Executing Bash Scripts

There are several ways to execute a Bash script, each with its own use case. The most common methods involve direct execution, sourcing, and using the bash command explicitly.

flowchart TD
    A[Start]
    A --> B{Is script executable?}
    B -->|Yes| C[./script.sh]
    B -->|No| D[chmod +x script.sh]
    D --> C
    A --> E[bash script.sh]
    A --> F[. script.sh or source script.sh]
    C --> G[Script runs in new subshell]
    E --> G
    F --> H[Script runs in current shell]
    G --> I[End]
    H --> I

Flowchart illustrating different methods of executing a Bash script.

1. Method 1: Direct Execution (Recommended for most scripts)

This is the most common way to run a script. First, you need to make the script executable using chmod. Then, you can run it by specifying its path relative to your current directory or its absolute path.

2. Step 1: Make the script executable

Open your terminal and navigate to the directory where your script is located. Use the chmod +x command to add execute permissions to the script file. For example, if your script is named my_script.sh, you would type: chmod +x my_script.sh.

3. Step 2: Execute the script

Once the script is executable, you can run it using ./ followed by the script name. The ./ indicates that the script is in the current directory. For example: ./my_script.sh. If the script is in your system's PATH, you can simply type its name.

4. Method 2: Using the bash command

You can explicitly tell the shell to execute the script using the bash command, regardless of its execute permissions or shebang line. This is useful for testing or when you don't want to change file permissions.

5. Step 1: Execute with bash

Simply type bash followed by the script's filename: bash my_script.sh. This will run the script using the bash interpreter.

6. Method 3: Sourcing the script

Sourcing a script (. script.sh or source script.sh) executes it within the current shell environment, meaning any variables or functions defined in the script will become available in your current terminal session. This is different from direct execution, which runs the script in a new subshell.

7. Step 1: Source the script

To source a script, use either . (a single dot) or the source command, followed by the script's filename: . my_script.sh or source my_script.sh.

Passing Arguments to Your Script

Most useful scripts accept arguments to customize their behavior. You can pass arguments by simply listing them after the script name during execution.

#!/bin/bash

echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "All arguments: $@"
echo "Number of arguments: $#"

A script demonstrating how to access command-line arguments.

./my_script.sh arg1 "another argument" 123

Executing the script with multiple arguments.

In the example above, $0 refers to the script's name, $1, $2, etc., refer to individual arguments, $@ refers to all arguments as separate strings, and $# gives the total number of arguments.