How do I run a program with a different working directory from current, from Linux shell?

Call the program like this:

(cd /c; /a/helloworld)

The parentheses cause a sub-shell to be spawned. This sub-shell then changes its working directory to /c, then executes helloworld from /a. After the program exits, the sub-shell terminates, returning you to your prompt of the parent shell, in the directory you started from.

Error handling: To avoid running the program without having changed the directory, e.g. when having misspelled /c, make the execution of helloworld conditional:

(cd /c && /a/helloworld)

Reducing memory usage: To avoid having the subshell waste memory while hello world executes, call helloworld via exec:

(cd /c && exec /a/helloworld)

[Thanks to Josh and Juliano for giving tips on improving this answer!]

Similar to David Schmitt's answer, plus Josh's suggestion, but doesn't leave a shell process running:

(cd /c && exec /a/helloworld)

This way is more similar to how you usually run commands on the shell. To see the practical difference, you have to run ps ef from another shell with each solution.

sh -c 'cd /c && ../a/helloworld'

I always think UNIX tools should be written as filters, read input from stdin and write output to stdout. If possible you could change your helloworld binary to write the contents of the text file to stdout rather than a specific file. That way you can use the shell to write your file anywhere.

$ cd ~/b

$ ~/a/helloworld > ~/c/helloworld.txt

One way to do that is to create a wrapper shell script.

The shell script would change the current directory to /c, then run /a/helloworld. Once the shell script exits, the current directory reverts back to /b.

Here's a bash shell script example:

#!/bin/bash
cd /c
/a/helloworld

If you always want it to go to /C, use an absolute path when you write the file.

If you want to perform this inside your program then I would do something like:

#include <unistd.h>
int main()
{
  if(chdir("/c") < 0 )  
  {
     printf("Failed\n");
     return -1 ;
  }

  // rest of your program...

}