Run user defined command with NSTask

Question

I would like to execute a terminal command specified by the user. For example, the user might write killall "TextEdit" or say "Hello world!" in a text field, and I want to execute that command.

NSTask is the way to go, except I have two problems with it:

First: the arguments. Right now I'm doing this:

NSArray* args = [commandString componentsSeparatedByString: @" "];
[task setArguments: [args subarrayWithRange: NSMakeRange(1, [args count] - 1)]]; // First one is the command name

Is this the way to do it? I don't think I've had problems with this yet, but I doesn't look like it's safe. Imagine this: the user writes killall 'Address Book' but the command receives as arguments 'Address and Book'?? That doesn't work. So, what should I do instead? How can I safely parse the arguments?

Second: the launch path. It's much more user-friendly to only have to write the name of the command, instead of the complete path to it. So I want to support that, which means finding out programmatically the full path for a command having only it's name. For that I wrote a category on NSTask like this:

+ (NSString*)completePathForExec: (NSString*)exec
{
    NSTask* task = [[NSTask alloc] init];
    NSPipe* pipe = [[NSPipe alloc] init];

    NSArray* args = [NSArray arrayWithObject: exec];
    [task setLaunchPath: @"/usr/bin/which"];
    [task setArguments: args];
    [task setStandardOutput: pipe];
    [task setStandardError: pipe];

    [task launch];    
    [task waitUntilExit];

    NSFileHandle* file = [pipe fileHandleForReading];
    NSString* result = [[NSString alloc] initWithData: [file readDataToEndOfFile] encoding: NSASCIIStringEncoding];

    if ([result length]) {
        if ([result hasSuffix: @"\n"]) { result = [result substringWithRange: NSMakeRange(0, [result length] - 1)]; }

        return result;
    }
    else { return exec; }
}

This seems to works well. However, how can I be sure that this path: /usr/bin/which will always work? I mean: will it work on 10.6, 10.7, 10.8, etc? I think I had a problem once where the path to a shell command changed with the system version, and you can never be too careful.

If the path is guaranteed to stay the same, then this isn't a problem. If it changes, then how can I know the 'path to the path-finder'?

score 3 · Accepted Answer · answered Sep 12 '12 at 16:34

3

It'll be far easier for you to not re-invent the command line parsing wheel. But, of course, going down the route of executing arbitrary user entered code is a security nightmare (tempered by the fact that the user has access to the system and, thus, could probably just run Terminal directly).

Specifically, have NSTask wrap an invocation of one of the shells with the command line option to have it execute an arbitrary string.

sh -c "ls -alF"

This would allow you to pass the path to sh as your launch path, which is in a fixed location on every system. The @"-c" argument tells sh to parse the next argument as a script and, of course, the next argument is whatever the user entered.

Note, this will also give the user the ability to pipe stuff, too.

answered Sep 12 '12 at 16:34

bbum

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That is awesome! Much simpler than I expected. But I do have one question: what characters should I use to close off the command entered by the user, so that it becomes a string? If I use `"` or `'` the command might be split into multiple arguments, because the user might be using those characters as well. – Alex Sep 13 '12 at 12:01
Actually, I think I've figured it out: create a temporary file with the user command, create an NSTask with the path to that file as an argument and run the task. In the end delete the temporary file. – Alex Sep 13 '12 at 13:50

Run user defined command with NSTask

1 Answers1