To execute a script file with the bash executable found in the PATH environment variable by using the executable env, the first line of a script file must indicate the absolute path to the env executable with the argument bash:

#!/usr/bin/env bash

The env path in the shebang is resolved and used only if a script is directly launch like this:

script.sh

The script must have execution permission.

The shebang is ignored when a bash interpreter is explicitly indicated to execute a script:

bash script.sh

To execute a script file with the bash interpreter, the first line of a script file must indicate the absolute path to the bash executable to use:

#!/bin/bash

The bash path in the shebang is resolved and used only if a script is directly launch like this:

./script.sh

The script must have execution permission.

The shebang is ignored when a bash interpreter is explicitly indicated to execute a script:

bash script.sh

There are two kinds of programs the kernel knows of. A binary program is identified by it's ELF (ExtenableLoadableFormat) header, which is usually produced by a compiler. The second one are scripts of any kind.

If a file starts in the very first line with the sequence #! then the next string has to be a pathname of an interpreter. If the kernel reads this line, it calls the interpreter named by this pathname and gives all of the following words in this line as arguments to the interpreter. If there is no file named "something" or "wrong":

#!/bin/bash something wrong
echo "This line never gets printed"

bash tries to execute its argument "something wrong" which doesn't exist. The name of the script file is added too. To see this clearly use an echo shebang:

#"/bin/echo something wrong 
# and now call this script named "thisscript" like so:
# thisscript one two
# the output will be:
something wrong ./thisscript one two

Some programs like awk use this technique to run longer scripts residing in a disk file.

Syntax

Remarks

A common mistake is to try to execute Windows end-line formatted \r\n script files on UNIX/Linux systems, in this case the used script interpreter in the shebang is:

/bin/bash\r

And is obliviously not found but can be hard to figure out.