The shebang (#!/bin/bash) is the script's first line; it tells the kernel which interpreter to run the file with.

* How the kernel uses #! to pick the interpreter when you run a script. *

When you execute a script directly (./script.sh), the kernel reads the first two bytes. If they're #!, it runs the program named after them and feeds it the script. So #!/bin/bash means "run this file through /bin/bash." Without a shebang, the file is handed to whatever shell happens to be calling it — which may not be bash, so bash-specific syntax (arrays, [[ ]]) can mysteriously break.

Format:

#!/bin/bash
# Your script starts here
echo "Hello World"

Key points:

• Must be the very first line (no spaces before #!)
• #! followed by the path to the interpreter
• Without it, the script runs in the current shell (may behave differently)

Common shebangs:

Tip: Use #!/usr/bin/env bash for portability - it finds bash in the user's PATH.

Go deeper:

• Shebang (Unix) — Wikipedia — the #! interpreter directive's kernel mechanics, including the env portability trick.

Two steps: chmod +x script.sh to grant execute permission, then ./script.sh to run it from the current directory.

A fresh script is just a text file — Linux won't run it until the execute bit is set, which is what chmod +x does. Then you launch it with ./script.sh. The ./ matters: the shell only auto-searches directories in $PATH, and your current directory normally isn't one of them (a deliberate security choice — otherwise a malicious ls dropped in a folder could hijack the real command). ./ says "the file is right here."

1. Make executable:

chmod +x myscript.sh

2. Run the script:

./myscript.sh              # From current directory
/home/user/myscript.sh     # Full path
bash myscript.sh           # Without execute permission

Why ./ is needed:

• Current directory is usually NOT in PATH
• ./ explicitly means "current directory"

Alternative - add to PATH:

# Add script directory to PATH
export PATH="$PATH:$HOME/bin"
# Now you can run without path
myscript.sh

Tip: Store personal scripts in ~/bin and add it to your PATH.