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.

An option which doesn't require a subshell and is built in to bash

(pushd SOME_PATH && run_stuff; popd)

Demo:

$ pwd
/home/abhijit
$ pushd /tmp # directory changed
$ pwd
/tmp
$ popd
$ pwd
/home/abhijit