Overview
This lecture explains the role of the shebang (#!) in Bash scripts, covering its syntax, effects, options, and best practices for specifying interpreters.
What is Shebang?
- Shebang (#!) is placed at the top of a script to specify which interpreter should execute the script.
- The format is
#! followed by the absolute path to the interpreter (e.g., #!/bin/bash).
- Shebang was introduced to allow non-compiled scripts to be executed as if they were executables.
Script Execution with and without Shebang
- Without a shebang, scripts are executed using the user’s default shell, which may cause unexpected behavior across different systems.
- The default shell can be checked with
echo $0.
- Specifying a shebang ensures consistent interpretation of script syntax, regardless of the current shell.
How Shebang Works
- The kernel uses the shebang line to identify the interpreter when running a script with executable permissions.
- The
execve system call reads the first line for "#!" and uses the following interpreter to run the script.
- Shebang syntax:
#! /path/to/interpreter [arguments]
Shebang Options and Variations
- Shebang can include interpreter options, e.g.,
#!/bin/bash -ex for enabling script termination on errors and execution tracing.
- Common options:
-e (exit on error), -x (trace execution), -v (print input lines).
- Common shebang types:
#!/bin/sh (Bourne shell or compatible)#!/bin/bash (Bash shell)#!/usr/bin/env bash (uses the first bash in $PATH)#!/usr/bin/python3, #!/usr/bin/env python3 (Python 3)#!/bin/false (script always fails)
Importance of Shebang
- Ensures scripts run with the intended interpreter for consistent behavior.
- Makes scripts behave like executables by specifying the interpreter explicitly.
- Omitting a shebang can result in scripts running with the default shell, which might not support all syntax/features used.
Overriding or Omitting Shebang
- Shebang can be ignored if the script is explicitly run with an interpreter (e.g.,
bash script-name).
- Overriding the shebang reduces consistency and can lead to errors; it is not recommended.
Key Terms & Definitions
- Shebang — The
#! character sequence at the start of a script specifying the interpreter.
- Interpreter — The program (e.g., Bash) that reads and executes the script.
- execve — System call used to execute programs in Linux.
- /usr/bin/env — Command to locate and execute the interpreter from the user's
PATH.
Action Items / Next Steps
- Always start scripts with an appropriate shebang line.
- Avoid overriding the shebang to maintain consistent script behavior.
- Practice writing scripts with various interpreters and shebang formats.