Demystifying $PATH

Whenever we write a command in the shell to execute, it consults an environment variable called $PATH which lists directories the shell should look for to find this program. This is why we can use certain commands like echo and pwd and such from any directory we happen to be in.

❯ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

❯ which echo
/bin/echo

When we run the echo command, it searches through the ":" separated list of directories in $PATH for a file by that name.

The order of directories listed in $PATH does matters. In the above example, the shell tries to find the executable from left to right i.e it looks for echo first in /usr/local/sbin then in /usr/local/bin and so on.

We can find out which file is executed for a given program name using the which program.

Modifying PATH

Let's say you have created a program that prints if a number is a prime or not. The current location of this program is /Users/me/scripts/isPrime.

❯ pwd
/Users/me/scripts/isPrime

❯ isPrime 42
42 is not a prime number

❯ cd ..
❯ isPrime 42
No such file or directory

Here isPrime is not available as soon as we change directories. To make our program accessible to us wherever we are, without having to point at its location (its path), we will have to add the scripts folder in our $PATH.

To make a program available anywhere, we can either place that program in a directory that’s already in our $PATH, or we can add a new directory to our $PATH that contains the program.

Temporary assignment

Whenever we launch a terminal session, it inherits all the global variables - including $PATH. We can use the export command to set the variable for any sub-processes launched by the terminal.

❯ export PATH="$PATH:/Users/me/scripts"

❯ isPrime 42
42 is not a prime number

❯ cd ..
❯ isPrime 42
42 is not a prime number

Now our program works even if we change the location.

Permanent assignment

As noted above, that method only temporarily modifies our $PATH, which is sometimes useful. For permanent addition, we need to add this export command in the shell profile.

The location of these profiles depends upon the kind of shell we are using.

# find the shell in use
❯ echo $SHELL
/bin/zsh

❯ vi ~/.profile # for bash, ksh, bourne
❯ vi ~/.zprofile # for zsh

#add lines at the bottom of the file:
export PATH="$PATH:/Users/me/scripts"

❯ source ~/.zprofile

Since this profile file gets run each time we open a terminal window, we will have our modified $PATH every time!