Your first script

A shell script is just a text file listing commands to run in order, the same commands you'd type interactively.

The shebang

Every script starts with a shebang line telling the kernel which interpreter to run it with:

#!/usr/bin/env bash

echo "Hello, $(whoami)"

#!/usr/bin/env bash finds bash on your PATH rather than hardcoding /bin/bash, more portable across systems where bash lives somewhere else.

Making it executable

chmod +x hello.sh
./hello.sh

Hello, user

chmod +x sets the executable permission bit (see File permissions explained for the full model). Without it, the kernel refuses to run the file directly:

./hello2.sh

bash: ./hello2.sh: Permission denied

That failure is exit status 126, specifically "found the file, couldn't execute it," not to be confused with 127 ("command not found") a moment below. You can still run an unexecutable script by handing it to the interpreter directly: bash hello2.sh works with no chmod at all, since you're not asking the kernel to execute the file, just asking bash to read and run it.

Why the `./` is required

cd /tmp
hello.sh

bash: hello.sh: command not found

Without a path, bash searches only the directories listed in $PATH, and your current directory usually isn't one of them, deliberately: if it were, a malicious script dropped into any directory you cd into could shadow a real command by sharing its name. ./hello.sh sidesteps the search entirely by naming the file's exact location, so this isn't a bug to work around, it's the safe default working as intended.

Arguments

#!/usr/bin/env bash
echo "Script name: $0"
echo "First argument: $1"
echo "All arguments: $@"
echo "Argument count: $#"

./greet.sh alice bob

Script name: ./greet.sh
First argument: alice
All arguments: alice bob
Argument count: 2

$0 is the script's own invocation path, $1, $2, and so on are positional arguments, $@ expands to all of them, and $# counts them. The next lesson covers why $@ almost always needs to be written as "$@" (quoted) once arguments might contain spaces.

Exercises

Write a script that prints today's date and the current working directory, then make it executable and run it with ./.
Answer
```
#!/usr/bin/env bash
echo "Today: $(date +%F)"
echo "Directory: $(pwd)"
```
Save as whereami.sh, then chmod +x whereami.sh && ./whereami.sh.
Without running chmod +x, find two different ways to execute a script.

Answer

Run it via the interpreter directly (bash script.sh or sh script.sh), or chmod +x it first and then invoke it with ./script.sh. Without either the execute bit or an explicit interpreter, the kernel refuses to run the file and reports "Permission denied" (exit 126).
Write a script that prints an error and exits with status 1 if it's called with no arguments.
Answer
```
#!/usr/bin/env bash
if [ $# -eq 0 ]; then
  echo "Usage: $0 <name>" >&2
  exit 1
fi
echo "Hello, $1"
```
$# is the argument count; printing the usage message to stderr (>&2) rather than stdout is the convention for error output, covered further in Exit codes and error handling.

What's next

The next lesson covers the single biggest source of bash bugs: variables and quoting.