3.7 KiB
| tags | ||
|---|---|---|
|
We know that $PATH is an environment variable. It is an important one because it keeps track of certain directories. Not just any directories but the directories where executables are found.
Whenever any command is run, the shell looks up the directories contained in the PATH for the target executable file and runs it. We can see this is the case by using the which command which traces the executable of bash commands. Take the echo program:
which echo
/home/trinity/.nvm/versions/node/v16.10.0/bin/npm
Or npm :
which npm
/home/trinity/.nvm/versions/node/v16.10.0/bin/npm
By default the path will always contain the following locations:
/usr/bin/usr/sbin/usr/local/bin/usr/local/sbin/bin/sbin
All the inbuilt terminal programs reside at these locations and most of them are at /usr/bin. This is why they run automatically without error. If you attempt to run a program that doesn’t reside at these locations then you will get an error along the lines of ‘program x is not found in PATH’.
Structure of the PATH
/home/trinity/.nvm/versions/node/v16.10.0/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/usr/lib/wsl/lib:/mnt/c/Python39/Scripts/:/mnt/c/Python39/:/mnt/c/Windows/system32:/mnt/c/Windows:/mnt/c/Windows/System32/Wbem:/mnt/c/Windows/System32/WindowsPowerShell/v1.0/:/mnt/c/Windows/System32/OpenSSH/:/mnt/c/Program Files/dotnet/:/mnt/c/Program Files/nodejs/:/mnt/c/ProgramData/chocolatey/bin:/mnt/c/Users/thomas.bishop/AppData/Local/Microsoft/WindowsApps:/mnt/c/Users/thomas.bishop/AppData/Local/Programs/Microsoft VS Code/bin:/mnt/c/Users/thomas.bishop/AppData/Local/Programs/Hyper/resources/bin:/mnt/c/Users/thomas.bishop/AppData/Roaming/npm
Adding to the PATH
Only the default directories load to the PATH on every session. How then can we add custom directories to the path without them being lost every time we start a new session? Remember that the user config .bashrc loads on init for every bash session. Therefore, if we set the custom path in this file, it will be created every time we start a session. This is why when you add a new program it often ask you to append a script to the bottom of your .bashrc .
For example, at the bottom of my .zshrc on my work computer I have:
export CHROME_BIN=/mnt/c/Program\\ Files\\ \\(x86\\)/Google/Chrome/Application/chrome.exe
This enables me to access the Chromium binaries from my terminal session (needed for running Angular tests) but it doesn’t add it to the path, it creates an environment variable on every session.
For demonstration, let’s add a user’s desktop directory to the PATH.
First we go to the .bashrc and add the export command. Remember that this is the command for creating a new environment variable:
export PATH="$PATH=:~/Desktop"
We force a reload of the .bashrc with the command:
source ~/.bashrc
Then we can check this directory has been added to the path with an echo
echo $PATH
...:~/Desktop
Relation between commands and programs
Whenever we issue a command in bash we are really running an executable program that is associated with the command. After all, this is why when we create our own bash scripts we must run chmod to make them executables.
When we issue ./file.sh we are running an executable program.
How come, however that when we use a program like cd or npm we don’t have to type ./cd.sh or ./npm.sh ?