使用NSTask执行cd命令无输出，如何验证命令执行成功？

我正尝试使用NSTask通过standardInput和standardOutput执行终端命令，ls、pwd等命令均可正常运行，但执行cd命令时standardOutput无任何输出。我清楚cd命令本身无显示响应，因此标准输出不会写入数据，但我需要判断该命令是否执行成功，以此更新UI。以下是我的代码，我使用NSFileHandleDataAvailableNotification读取输出，这对ping这类命令很有帮助：

-(void)setupTask {
    @autoreleasepool {
        // Commands are read from standard input:
        NSFileHandle *input = [NSFileHandle fileHandleWithStandardInput];
        
        NSPipe *inPipe = [NSPipe new]; // pipe for shell input
        NSPipe *outPipe = [NSPipe new]; // pipe for shell output
        NSPipe *errorPipe = [NSPipe new];
        
        task = [NSTask new];
        [task setLaunchPath:@"/bin/sh"];
        [task setStandardInput:inPipe];
        [task setStandardOutput:outPipe];
        [task setStandardError:errorPipe];
        
        [task launch];
        
        // Wait for standard input ...
        [input waitForDataInBackgroundAndNotify];
        // ... and wait for shell output.
        [[outPipe fileHandleForReading] waitForDataInBackgroundAndNotify];
        [[errorPipe fileHandleForReading] waitForDataInBackgroundAndNotify];
        
        
        // Wait asynchronously for standard input.
        // The block is executed as soon as some data is available on standard input.
        [[NSNotificationCenter defaultCenter] addObserverForName:NSFileHandleDataAvailableNotification
                                                          object:input queue: [NSOperationQueue mainQueue]
                                                      usingBlock:^(NSNotification *note)
         {
            NSData *inData = [input availableData];
            if ([inData length] == 0) {
                // EOF on standard input.
                [[inPipe fileHandleForWriting] closeFile];
            } else {
                // Read from standard input and write to shell input pipe.
                [[inPipe fileHandleForWriting] writeData:inData];
                
                // Continue waiting for standard input.
                [input waitForDataInBackgroundAndNotify];
            }
        }];
        
        // Wait asynchronously for shell output.
        // The block is executed as soon as some data is available on the shell output pipe.
        [[NSNotificationCenter defaultCenter] addObserverForName:NSFileHandleDataAvailableNotification
                                                          object:[outPipe fileHandleForReading] queue:[NSOperationQueue mainQueue]
                                                      usingBlock:^(NSNotification *note)
         {
            // Read from shell output
            NSData *outData = [[outPipe fileHandleForReading] availableData];
            if ([outData length] > 0) {
                NSString *outStr = [[NSString alloc] initWithData:outData encoding:NSUTF8StringEncoding];
                NSLog(@"Output: %@", outStr);
            }
            // Continue waiting for shell output.
            [[outPipe fileHandleForReading] waitForDataInBackgroundAndNotify];
        }];
        
        [[NSNotificationCenter defaultCenter] addObserverForName:NSFileHandleDataAvailableNotification
                                                          object:[errorPipe fileHandleForReading] queue:[NSOperationQueue mainQueue]
                                                      usingBlock:^(NSNotification *note)
         {
            // Read from shell output
            NSData *outData = [[errorPipe fileHandleForReading] availableData];
            if ([outData length] > 0) {
                NSString *outStr = [[NSString alloc] initWithData:outData encoding:NSUTF8StringEncoding];
                    NSLog(@"Error: %@", outStr);
            }
            // Continue waiting for shell output.
            [[errorPipe fileHandleForReading] waitForDataInBackgroundAndNotify];
        }];
    }
}

-(void)executeCommand:(NSString*)command {
    
    NSData* data = [command dataUsingEncoding:NSUTF8StringEncoding];
    NSError* error;
    [[[task standardInput] fileHandleForWriting] writeData:data error:&error];
    if (error != NULL) {
        NSLog(@"%@", [error localizedDescription]);
    }
    
    NSData* newLine = [@"\n" dataUsingEncoding:NSUTF8StringEncoding];
    [[[task standardInput] fileHandleForWriting] writeData:newLine error:&error];
    if (error != NULL) {
        NSLog(@"%@", [error localizedDescription]);
    }
}

-(void)runCommand {
    [self setupTask];
    [self executeCommand:@"cd Users"];
}

方法1：通过echo $?获取命令退出码 #

cd命令执行成功时退出码为0，失败则为非0值。可以在执行cd后紧接着执行echo $?，读取该输出即可判断cd是否成功：

[self executeCommand:@"cd Users && echo $?"];

在输出回调中，解析得到的数字，0表示成功，其他值表示失败。

方法2：监听错误输出（stderr） #

cd命令执行失败时（比如目录不存在），会将错误信息输出到stderr。你的代码已经监听了errorPipe，当cd失败时，error回调会收到类似cd: Users: No such file or directory的信息，通过判断是否有错误输出即可得知命令执行失败。

方法3：执行cd后主动获取当前工作目录 #

在执行cd命令后，执行pwd命令，读取输出的目录路径，对比是否是目标目录，以此验证cd是否成功：

[self executeCommand:@"cd Users && pwd"];

如果输出的路径包含目标目录名，则说明cd成功。

内容的提问来源于stack exchange，提问作者prabhu