# Shebang, Permissions, PATH and Exit Codes — Bash / Shell Scripting

Source: https://www.skillbyai.com/en/bash/f-first

> Create, run and exit a script correctly.

## Anatomy of a script

A Bash script is a text file of commands. Its first line, the **shebang**, tells the kernel which interpreter to use: `#!/usr/bin/env bash` finds `bash` on the `PATH` (portable across systems where Bash lives in different places), while `#!/bin/bash` hard-codes the location. Make the file executable with `chmod +x script.sh` and run it as `./script.sh`; the leading `./` is needed because the current directory is normally not on the **`PATH`**, the list of directories searched for commands. Every command returns an **exit status**: `0` means success, anything from 1 to 255 means failure (some values have conventions: 2 for misuse, 126 for not executable, 127 for command not found, 128 + N for death by signal N). `$?` holds the last command's status; `exit N` ends the script with a status. Scripts that report failures through exit codes compose well with other tools, CI pipelines and `&&`/`||` chains.

## A first script

Arguments, an exit code and a check of the previous command's status.

```bash
#!/usr/bin/env bash
# greet.sh - say hello and check that a directory exists

name="${1:-world}"            # first argument, default "world"
echo "Hello, $name!"

if ! ls /var/log > /dev/null 2>&1; then
    echo "cannot read /var/log" >&2   # errors go to stderr
    exit 1
fi

echo "done"
exit 0

# chmod +x greet.sh
# ./greet.sh Asha ; echo "exit status: $?"
```

## A recipe card with a chef's name on top

The shebang is the line at the top of a recipe saying which chef should cook it. Without it, whoever picks up the card cooks it their own way, and the result may be quite different.

**Quiz:** What does an exit status of 0 mean?

- [ ] The command failed
- [ ] The command was killed by a signal
- [ ] The command was not found
- [x] The command succeeded

*Answer:* The command succeeded. By convention, 0 means success and non-zero means some kind of failure.