Change for the current process and the child-processes

Before explaining why you cannot permanently change directory using Perl, let's see what can you do.

The following example is Unix/Linux specific, but similar example could have been written for Windows as well.

examples/chdir.pl

use 5.010;
use strict;
use warnings;

use Cwd qw(getcwd);

say getcwd();
system 'pwd';


chdir "/home";
say getcwd();
system 'pwd';

In this example we use the getcwd function provided by the Cwd module that returns the Current Working Directory. That's normally what people say "the directory we are in".

In Unix/Linux you would use the pwd command to Print the Working Directory

So first we use the getcwd function to get the current working directory and we print it using the say function of Perl. Then we call system to execute the external program pwd to show what would an external program launched from our script see.

Then we use the built-in chdir function of Perl that works as cd on the command line and changes the Current Working Directory to whatever parameter you give to it.

After that both getcwd and the external pwd will report this new directory as the "working directory".

See the output.

$ pwd
/Users/gabor/work

$ perl perlmaven.com/exercises/chdir.pl
/Users/gabor/work
/Users/gabor/work
/home
/home

$ pwd
/Users/gabor/work

Right after the script exited we ran pwd on the command line to show that the working directory returned to what was the working directory before running the script.

Why cannot Perl change the directory permanently? And how come Bash can?

For this one needs to understand how processes work in Unix/Linux. When we launch a Perl script the operating system creates a new process that inherits everything from the original process. Including the current working directory. In order to protect the original process from anything bad the new process might do, each process can only impact its own environment and not the environment of the parent process. To the naked eye it looks as if the Operating System would reset everything, including the current working directory, after the process has finished.

So no process can make changes to the environment of the parent process.

Shell script to change directory permanently

The following Shell script can do it permanently:

examples/chdir.sh

pwd
cd /home
pwd

Let's see how does it work:

$ pwd
/Users/gabor/work

$ bash perlmaven.com/examples/chdir.sh
/Users/gabor/work
/home

$ pwd
/Users/gabor/work

Oops. This did not make a permanent change either. What happened to your promise? Wait a second.

We ran this shell script as an external program.

This is exactly the same case as we had with the Perl script above.

We created a new process, we changed something in it, but when the process ended the operating system reset everything to how the parent process had it.

Let's try something else:

Use the source!

If instead of running the Shell script as an external program we use the source command of our Shell then the command will be executed in the current process.

$ source perlmaven.com/examples/chdir.sh
/Users/gabor/work
/home

$ pwd
/home

Voila. we managed to permanently change the Current Working Directory.

That happened because we have not run an external program and thus the Operating System did not have to create a new process.