NSTask code which outputs directly to a file as well.

NSTask+Tee

NSTask *task;
task = [[NSTask alloc] init];
[task setLaunchPath: @"/bin/bash"];
[task setArguments: @[ @"-c", @"echo Hello | tee -a ~/Desktop/output.txt" ]];
[task setCurrentDirectoryPath:@"~/Development/"];

NSPipe *pipe;
pipe = [NSPipe pipe];
[task setStandardOutput: pipe];
[task setStandardError: pipe];

NSFileHandle *file;
file = [pipe fileHandleForReading];

[task launch];

NSData *data;
data = [file readDataToEndOfFile];
NSString *response = [[NSString alloc] initWithData: data encoding: NSUTF8StringEncoding];

Is it possible to add variable to the command in setArguments. For ex.
Instead of [task setArguments: @[ @"-c", @"echo Hello | tee -a ~/Desktop/output.txt" ]]; we do path = ~/Desktop/output.txt; [task setArguments: @[ @"-c", @"echo Hello | tee -a path" ]];