Why Every Bash Script Starts with #! – Understanding the Shebang

What is a shebang?

The term shebang refers to the two characters # and ! that appear at the very first line of a script file. It tells the operating system which interpreter should execute the script.

How the kernel interprets the shebang

When the kernel loads an executable file that begins with #!, it reads the path that follows and launches that binary, passing the script file as the first argument. For example, #!/bin/bash causes the kernel to run /bin/bash script_name.

Common shebang lines

#!/bin/sh
#!/bin/bash
#!/usr/bin/perl
#!/usr/bin/tcl
#!/bin/sed -f
#!/usr/awk -f

Each line specifies a different interpreter. The path after #! must exist and be executable; otherwise the script fails with an error such as “Command not found”.

Example script

Assume a file named shell_script contains only the shebang line: #!/bin/bash Make the file executable and run it:

chmod u+x shell_script
./shell_script

The kernel actually executes /bin/bash shell_script, so any commands in the file are interpreted by Bash.

Behavior when the shebang is omitted

If a script lacks a shebang, the kernel falls back to the current user's default shell. On many systems this is /bin/zsh or /bin/bash:

echo $0
/bin/zsh

Thus the script runs under that interpreter.

Shebang can point to any executable

The interpreter does not have to be a shell; any executable can be used. For example, replacing the shebang with #!/bin/cat makes the script run the cat command, which simply outputs the script’s contents:

#!/bin/cat
hello world

Running the script prints “hello world” (and the script itself) because cat reads the file and writes it to standard output.

Portability note

Using #!/bin/sh invokes the system’s default POSIX‑compatible shell. On most Linux distributions this is Bash, but on other UNIX variants it may be a different shell. Selecting /bin/sh improves portability at the cost of Bash‑specific extensions.

References

https://blog.twentytwotabs.com/the-smallest-bash-program-in-the-universe/