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A shebang (#!) is a two-byte magic number (0x23 0x21 in hexadecimal) located at the absolute beginning of an executable text file. It instructs the operating system’s kernel program loader (specifically during the execve system call) to parse and execute the file using the specified interpreter rather than the default system shell. 
#![absolute-path-to-interpreter] [optional-argument]

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Technical Mechanics

When a script is invoked as an executable (e.g., ./script.sh), the kernel reads the first two bytes of the file. If it detects the #! signature, it extracts the remainder of the first line to identify the interpreter. The kernel then replaces the current process image by executing the specified interpreter, passing the original script’s file path as an argument. For example, if a script named app.sh contains #!/bin/bash, the kernel translates the execution to: 
/bin/bash ./app.sh

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Standard Bash Implementations

There are two primary syntaxes used to invoke the Bash interpreter: 1. Direct Path Resolution 
#!/bin/bash
This instructs the kernel to execute the bash binary located exactly at /bin/bash. It is highly performant but relies on a rigid filesystem hierarchy. 2. Dynamic Path Resolution via env 
#!/usr/bin/env bash
This instructs the kernel to execute the env utility (located at /usr/bin/env). The env program then performs a $PATH environment variable lookup to dynamically locate and execute the first instance of the bash binary it finds.

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Strict Parsing Rules

  • Byte Offset Zero: The # character must be at byte offset 0 of the file. Preceding empty lines, spaces, or Byte Order Marks (BOM) will invalidate the shebang, causing the kernel to treat the file as a standard binary or default shell script.
  • Absolute Paths: The path to the interpreter (or to env) must be absolute. Relative paths will fail.
  • Argument Limitations: The POSIX specification does not standardize how multiple arguments in a shebang line are handled. Most Unix-like kernels will group everything after the interpreter path into a single argument string. Therefore, passing multiple flags (e.g., #!/bin/bash -e -x) is generally avoided in favor of using the set command within the script.
  • Line Endings: The shebang line must be terminated by a Line Feed (LF or \n). If the file uses Carriage Return + Line Feed (CRLF or \r\n), the kernel includes the \r as part of the interpreter’s name or argument (e.g., looking for /bin/bash\r), resulting in a bad interpreter: No such file or directory error.
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