Shebang

Core Idea

A shebang is the first line of an executable script that tells the kernel which interpreter should run the file.

Shape

#!/bin/bash

The first two characters must be #!. The path after them must point to an interpreter. In this course, Bash scripts use /bin/bash.

Why It Matters

These two commands are different:

bash ~/scripts/hello.sh Makers
~/scripts/hello.sh Makers

The first command explicitly runs Bash and gives it the script. The second asks the system to execute the file directly. Direct execution needs executable permission and a valid shebang so the system knows which interpreter to start.

Safe Script Header

#!/bin/bash
set -euo pipefail

Use this at the top of course Bash scripts unless a quest says otherwise.

Common Failures

• Permission denied: the script is not executable. Run chmod +x script.sh or execute it with bash script.sh.
• bad interpreter: No such file or directory: the shebang path is wrong or the file has Windows CRLF line endings.
• Script runs under the wrong shell: check the first line and run head -n 1 script.sh.
• #!/usr/bin/env bash: common on many systems, but this course uses #!/bin/bash for predictability.

CRLF Check

If a script copied from another system behaves strangely:

head -n 1 script.sh | xxd

If the line ends in 0d 0a, it has Windows CRLF line endings. Recreate the first line in the terminal editor or ask before doing bulk conversion.

Proof Check

Create a tiny script with #!/bin/bash, add chmod +x, run it directly, then run the same script with bash script.sh. Explain which command used the shebang.

Docs Pointers

• Run man bash and search for INVOCATION.
• Run man 2 execve.
• Read Shell Scripting, Script Permissions, and File Encoding.