Relexec

relexec is a helper program to enable "relative shebangs".

Relative shebangs are not natively supported on POSIX systems. Paths specified as relative in shebangs are resolved relative to the current working directory, behaviour which is almost useless.

To work around this, a shebang can be specified with the relexec program, installed in a well-known path. relexec constructs a path relative to the script and execs it.

This allows scripts to be defined that run with interpreters installed with the script. These can then run from anywhere in the filesystem, or be moved to another location without breaking (as long as the relative path between the script and interpreter does not change).

Unlike other solutions to this problem, it is easy to programmatically rewrite shebangs to use relexec (or rewrite them back).

Usage

The intended usage is in the shebang line of a script:

#!/usr/lib/relexec python3.9
<Python 3.9 code>

which would define that the script should be executed with an executable named python3.9 in the same directory as the script.

Specifically, if the script is in /path/to/file, then executing /path/to/file arg1 arg2 would run relexec with this command line:

/usr/lib/relexec python3.9 /path/to/file arg1 arg2

which relexec transforms to

/path/to/python3.9 /path/to/file arg1 arg2

and exec()s the result.

Installation

We provide C, Rust, and for reference, Python versions of relexec.

We specify that the relexec binary, if installed, must be installed to /usr/lib/relexec so that this path can be hard coded in shebangs.

/usr/lib is the appropriate place for internal binaries according to the Filesystem Hierarchy Standard 3.0, as /usr/libexec does not exist on all systems.