Linux Command Basics
Introduction
The Linux Commands section introduces commands that I use a lot when dealing with text. They are categorized according to their use cases. This document introduces only the basics.
Absolute and Relative Paths
. # current directory
./ # Current directory, same as .
../ # One level up, i.e., the parent directory of the current directory
../../ # Two levels up, i.e., the parent of the parent directory
$PATH Variable
The $PATH variable tells your Linux shell where to look for executable files.
When you run a command, you are actually telling the system (Linux or Linux-like OSs) to run a program. Even simple commands, like ls, mkdir, rm, and others are just small programs that usually live inside a directory on your computer called /usr/bin.
Common directories: usr/local/bin, /usr/local/sbin, and /usr/sbin
However, when you type a command into your Linux shell, it doesn't look in every directory for the program. Instead, it only looks to the ones you specify. These directories are a part of an environment variable called $PATH, which your shell checks in order to know where to look.
echo $PATH
echo $PATH is used to show your $PATH.
echo $PATH
export PATH
export PATH is used to add a directory to $PATH. Each directory is separated with a colon (:).
export PATH=$PATH:<other-path> # by appending one
export PATH=<other-path>:$PATH # by adding one to the beginning
Let's say you wrote a shell script called hello.sh and have it located in /place/with/the/file. This script provides some useful function to all of the files in your current directory, that you'd like to be able to execute no matter what directory you're in.
In this case, you can set your $PATH using the command below:
export PATH=$PATH:/place/with/the/file
You should now be able to execute the script anywhere on your system by just typing in its name, without having to include the full path as you type it.
Setting $PATH Permanently
By design, if you restart your computer or create a new terminal instance, the path you added will be gone. Luckily, you can set $PATH so that it always includes your added path.
The procedure varies depending on the shell you use. You can first determine it by running the command below, which returns the shell you use:
echo $0
For Bash, you simply need to add the line above, export PATH=$PATH:/place/with/the/file, to the appropriate file that will be read when your shell launches. They might be ~/.bash_profile, ~/.bashrc, or ~/.profile. Their difference is primarily when they are read by the shell. In most cases, ~/.bashrc would be a good choice.
For other shells (like mine, zsh), you can put the export statement to something like ~/.zshrc.
which
which is used to locate a program file in the user's path.
# query the path of a command line tool
which <command-line-tool>
Shortcuts
The following shortcuts can be useful in the terminal.
| Shortcut | Description |
|---|---|
| Tab | Auto-completes a command. |
| Ctrl + L | Clears the terminal screen. |
| Ctrl + C | Stops a running command immediately. |
| Ctrl + D | Logs you out of the current terminal. |
| Ctrl + A | Moves the cursor to the line's beginning. |
| Ctrl + E | Moves the cursor to the line's end. |
| Ctrl + U | Erases content from the cursor to the line's beginning. |
| Ctrl + K | Erases content from the cursor to the line's end. |
| Ctrl + W | Erases the word preceding to the cursor. |
| Ctrl + Y | Pastes the erased text coming from the shortcuts above. |
| Ctrl + P | Displays the previous command. |
| Ctrl + N | Displays the next command. |
| Shift + PageUp | Scrolls up. |
| Shift + PageDown | Scrolls down. |
| ↑ & ↓ | Shows the history of commands executed. |