What Does #!/bin/bash Do? Uncovering the Shebang Mystery

We usually see script files beginning with #!/bin/bash. This article explains what that line means and why it is written.

What is a shebang?

The characters #! are called a shebang , a term derived from “sharp” ( #) and “bang” ( !), which together sound like “shebang”.

Interpreter path

The part after #!, such as /bin/bash, specifies the absolute path to the interpreter binary. When the kernel sees the shebang line, it launches that interpreter to run the script.

Common variations

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

Each line selects a different command interpreter; the path must exist or the script will fail with “Command not found”.

If you use /bin/sh , the default shell on most Linux systems is Bash, but on other UNIX systems it may be the Bourne shell, allowing the script to run on non‑Linux machines at the cost of losing Bash‑specific features.

Example

Consider a file shell_script containing only the shebang line:

#!/bin/bash

Make it executable and run it:

$ chmod u+x shell_script

$ ./shell_script

The kernel translates the execution to /bin/bash shell_script. If the shebang is omitted, the system uses the currently running shell (e.g., zsh) as the interpreter.

Interesting uses

The shebang does not have to point to a shell; it can be any executable. Replacing #!/bin/bash with #!/bin/cat makes the script’s contents printed by cat:

#!/bin/cat

hello world

As long as the path points to a valid command, the script will run; an invalid command yields an error. The underlying mechanism is implemented in the kernel’s C code.

Reference

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