Use of “bash” or “zsh” depending on OS and installation

Question

I have a small function which executes a command line tool like 7za. I don't know where this tool is installed by the user, so I use this function:

func execute(commandName: String, arguments: [String]) -> String? {
        guard var bashCommand = execute(command: "/bin/bash" , arguments: ["-l", "-c", "which \(commandName)"]) else { return "\(commandName) not found" }   // find path of command itself, e.g. "/usr/local/bin/7za"
        bashCommand = bashCommand.trimmingCharacters(in: NSCharacterSet.whitespacesAndNewlines)   // remove newline character at the end
        if bashCommand == "" { return nil }  // command not even found! => nil
        return execute(command: bashCommand, arguments: arguments)   // now we have: /usr/local/bin/7za e -pPW -o/Users/Shared/File.JPG.7z
    }

On OSX 10.15 there is a new shell zsh. I can switch my command from bash to zsh, but my program should run on any system. How can I find out which command to use? I don't want to scan the entire harddisk to simply run an installed command line tool lile "7za". Any idea?

Answer 1

How can I find out which (shell) command to use?

Ideas:

1. Use System.getenv("SHELL") to find out the shell that the user is using, and use that.

2. Use /bin/sh . It should be a link to a POSIX compatible shell; either bash or ksh ... or possibly something else.

3. Run ls -l /bin/ and parse the output looking for commands that match *sh .

4. Read the /etc/shells file.

(See How can I check the available shells in Mac OSX? )

Apple have not (yet) said that bash is going away. At this stage all they have changed is what the default shell is.

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But I'm not convinced it is necessary to do this in the first place:

• You don't need to use a shell to run which . It should be available as an ordinary (not shell built-in) command. (It is on Linux...)

• You should not need to use which 7za . What that does is to find the 7za command using the PATH variable. But that exactly the same command that will be found and executed if you simply run 7za .

• If the user installs 7za and puts it in a strange place that is not on the PATH, they are being perverse. They need to update their PATH setting, or put the command in a more conventional place.