What Is the Use of "#!/Usr/Local/Bin/Ruby -W" at the Start of a Ruby Program

what is the use of #!/usr/local/bin/ruby -w at the start of a ruby program

The Shebang line is optional, and if you run the ruby interpreter and pass the script to it as a command line argument, then the flags you set on the command line are the flags ruby runs with.

A Shebang line is not ruby at all (unless you want to call it a ruby comment). It's really shell scripting. Most linux and unix users are running the BASH shell (stands for Borne Again SHell), but pretty much every OS has a command interpreter that will honor the Shebang.

“#!/usr/local/bin/ruby -w”

The "she" part is the octothorp (#), aka pound sign, number sign, hash mark, and now hash tag (I still call it tic-tac-toe just cuz).

The "bang" part is the exclaimation mark (!), and it's like banging your fist on the table to exclaim the command.

On Windows, the "Shell" is the command prompt, but even without a black DOS window, the command interpreter will run the script based on file associations. It doesn't really matter if the command interpreter or the programming langue is reading the shebang and making sure the flags are honored, the important point is, they are honored.

The "-w" is a flag. Basically it's an instruction for ruby to follow when it runs the script. In this case "-w" turns on warnings, so you'll get extra warnings (script keeps running) or errors (script stops running) during the execution of the script. Warnings and exceptions can be caught and acted upon during the program. These help programmers find problems that lead to unexpected behavior.

I'm a fan of quick and dirty scripts to get a job done, so no -w. I'm also a fan of high quality reusable coding, so definitely use -w. The right tool for the right job. If you're learning, then always use -w. When you know what you're doing, and stop using -w on quick tasks, you'll start to figure out when it would have helped to use -w instead of spending hours trouble shooting. (Hint, when the cause of a problem isn't pretty obvious, just add -w and run it to see what you get).

"-w" requires some extra coding to make it clear to ruby what you mean, so it doesn't immediately solve things, but if you already write code with -w, then you won't have much trouble adding the necessary bits to make a small script run with warnings. In fact, if you're used to using -w, you're probably already writing code that way and -w won't change anything unless you've forgotten something. Ruby requires far less "plumbing code" then most (maybe all) compiled languages like C++, so choosing to not use -w doesn't allow you to save much typing, it just lets you think less before you try running the script (IMHO).

-v is verbose mode, and does NOT change the running of the script (no warnings are raised, no stopping the script in new places). Several sites and discussions call -w verbose mode, but -w is warning mode and it changes the execution of the script.

/usr/bin/ruby vs. rvm

the only sane way to go is:

#!/usr/bin/env ruby

it will always use current selected ruby in the environment, not depending on any tool

When trying to install pg gem: Brew bad interpreter error and not existing ruby version

Okay, I got it - I've edited the file: /usr/local/bin/brew and edited ther (first line) the path: from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin to /usr/local/bin/ruby/ and it works now..

Anyone knows how to change default system Ruby path to this one? (to avoid problems with other programs in the future).

Which ruby version am I using?

If ruby -v shows 2.6.3, then you are using the ruby which ships with the system by default. You can confirm this in several ways. Running the shell command which ruby will show /usr/bin/ruby. Checking your PATH will show /usr/bin earlier than the location of the homebrew or rbenv installations.

If you wish to run one of the others, you can put it earlier on the PATH or invoke it explicitly by giving the fully qualified name such as /usr/local/opt/ruby/bin/ruby myscript.rb. Another alternative which avoids twiddling the PATH variable is to use a shebang line at the beginning of different scripts pointing explicitly to the version to use with that script.

Ruby 1.9 If I have a shebang pointing to ruby, why doesn't the system see it?

You need to first make your script executable:

chmod +x todo.rb

then you need to run it like so:

./todo.rb

You cannot run it by just saying todo.rb, unless you place it in your PATH, in which case you can do so from anywhere.

Use non-system ruby to execute ruby scripts within macOS App

Due to Martin R's help I was able to solve the issue.
For now I deactivated macOS sandboxing in the projects capabilities.

Now I can use any ruby I want.

Thank you

bash: /usr/bin/ruby: No such file or directory

Per the comments on the question, your /usr/bin/ruby binary is a symlink to /etc/alternatives/ruby.

lrwxrwxrwx 1 root root 22 Aug 12 20:11 /usr/bin/ruby -> /etc/alternatives/ruby

You should confirm that path exists (run ls -la /etc/alternatives/ruby to check if that path exists) and if it does not, you'll need to reinstall Ruby using your system package manager (e.g., apt-get), download and install Ruby from https://www.ruby-lang.org, or use a tool like RVM.

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