How is git able to run C scripts in the current directory as a command on terminal?

问题

I have been looking into bash and shell recently and been trying to work out how git is able to make a terminal command that runs C scripts in the current directory e.g git init, git push/pull etc.

I have been trying to simulate it in python, by making an executable python script in my home directory,

#!/usr/bin/env python
import os
print("current_dir: ",os.getcwd()) #prints /Users/usr/folder/script.py

Then I create a .command file that calls the python script

cd
cd FOLDER/
python3 SCRIPT.py

and editing the bash profile to export a variable to run the .command file.

export mycommand=/Users/urn/folder/command.command

Although this is not even nearly close to the way git achieves its command line. For example, when I run my script is not actually a terminal command it is just an environment variable, hence the $.

$mycommand

Secondly, this goes to the directory of the python file and runs the script from with that directory, therefore the output will always be the same

/Users/usr/folder/script.py

Although in git it runs the file in the current directory. Therefore the print statement would change depending on the terminal directory.

How am I able to create my own 'terminal command' such as git init to run my python script in whatever directory I'm in. Ps, I'm on mac.

Any help would be greatly appreciated :)

回答1:

It sounds like you are missing at least two basic concepts:

• The search path: when you issue a command that is not a shell function or built-in and that has an unqualified name (one with no / characters), the shell searches a list of zero or more directories for a matching executable file. This list of directories is the "path", and it is stored in the PATH environment variable. You can change it if you wish. This is how the shell finds the git program when you do not specify a path to it.

• Executable scripts: Python, shell, Perl, etc. programs that must be run via an interpreter can be made executable by name alone by including an appropriate shebang line as the very first line and assigning an executable mode. You include an appropriate shebang line in your example Python program, in fact, but you seem not to understand its significance, because you explicitly launch the script via the python3 command. The shebang line is just another comment to Python, but it is meaningful to the system.

It seems like you probably also are missing some other concepts, like the fact that your script doesn't need to be in the current working directory for you to run it via the python3 launcher, path notwithstanding. Just specify its full pathname. Alternatively, Python has its own variation on a path, PYTHONPATH, by which it can locate modules and packages. These are alternatives, however -- the first two points are enough to achieve your apparent objective. Specifically,

1. Keep the shebang line in your script, though your default python is probably v2.7, so if you really want to run it specifically via python3 then modify the shebang line to say so:

   #!/usr/bin/env python3
   

2. Make sure the file has executable mode. For example,

   chmod 0755 /Users/urn/bin/SCRIPT.py
   

   Then you should be able to execute the script from anywhere via its full pathname.

3. To access it from anywhere via its simple name, ensure that the directory containing it is in your path. For this purpose, it would be wise to choose an appropriate directory, such as /usr/local/bin or /Users/urn/bin (you may need to create the directory first). Whichever you choose, ensure that that directory is in your PATH. For example, edit /Users/urn/.bash_profile, creating it if necessary, and ensure that it contains (say) the commands

   PATH=$PATH:/Users/urn/bin
   export PATH
   

   That will take effect in new Terminal windows you open afterward, but not automatically in any that are already open. In those windows, you will be able to run the script, from anywhere, via its simple name.

来源：https://stackoverflow.com/questions/50243373/how-is-git-able-to-run-c-scripts-in-the-current-directory-as-a-command-on-termin