Computer Science

Command Line

You're interacting with your server using a Terminal application, which accepts text commands that you type into it. This is sometimes referred to as a command-line interface (CLI).

The Terminal application itself gives you access to a shell program that runs on the server. This program—bash most commonly, or zsh on recent Macs—allows you to give commands to the computer via that command-line interface.

Let's go over some of the commands that are available to you in the command line. The more you work in the terminal, the more you'll have occasion to use these. You'll know that your Terminal, running a shell, accepting input via the command line, is ready for input when it gives you a prompt:

• $ indicates that you're a user running the bash shell
• % indicates you're running the zsh shell
• # indicates that you're running commands as root (not recommended at the beginning)

Useful shell commands

• pwd
  What it does - Prints the working (current) directory
  Example of use, with output
  
  $ pwd /Users/rwhite/Documents
  Other info - Not used much once you get more experienced, but always useful (especially as you're learning) to identify where in the filesystem you're currently located
• ls
  What it does - lists the contents of a directory
  Examples of use
  
  $ ls # lists the contents of the current directory $ ls .. # lists the contents of the directory one level up $ ls -a # lists all the contents of the current directory, including "hidden" files # (the ones that start with a .) $ ls -l # lists the contents of the current directory in long format $ ls ~ # lists the contents of the home directory $ ls -R # lists the contents of the current directory recursively # (including the contents of directories below this one)
  Other info - Not used much once you get more experienced, but always useful (especially as you're learning) to identify where in the filesystem you're currently located
• cd
  What it does - Changes your location in the filesystem to a different Directory
  Examples of use
  
  $ cd .. # changes to the directory above the working directory $ cd / # changes to the root directory $ cd ~ # changes to the user's home directory $ cd ./subdirectory/subdirectory # changes to a directory below the current one $ cd ../../directory # changes to a directory in a different part of the filesystem
  Other info - Not used much once you get more experienced, but always useful (especially as you're learning) to identify where in the filesystem you're currently located
• cat
  What it does - Displays the contents of a file
  Example of use, with output
  
  $ cat hello_world.py #!/usr/bin/env python3 print("Hello, world!")
  Other info - cat is useful for quickly displaying the contents of a small file. If you want to search through a larger file, you're probably better off using less (see below). You can also use nano, although you run the risk of accidentally changing the file then.
• mkdir
  What it does - Makes a new, empty, directory
  Example of use
  

  $ mkdir new-project

  Other info - If you've got a new project you're about to start work on, a typically sequence is using mkdir to create the project directory, and then cd to change into that project directory to begin working.
• sudo
  What it does - "SuperUser DO" elevates privileges for accomplishing some tasks
  Example of use, with output
  
  $ sudo apt update Hit:1 http://ports.ubuntu.com/ubuntu-ports focal InRelease Get:2 http://ports.ubuntu.com/ubuntu-ports focal-updates InRelease [114 kB] Get:3 http://ports.ubuntu.com/ubuntu-ports focal-backports InRelease [101 kB] . . $ sudo shutdown now Connection to 192.168.7.207 closed by remote host. Connection to 192.168.7.207 closed. $
  Other info - Not all users on a system have the ability to use the sudo command. Obligatory XKCD reference.
• cp
  What it does - Makes a copy of a source file to a destination file
  Examples of use
  
  $ cp /etc/netplan/50-cloud-init.yaml /etc/netplan/50-cloud-init-original.yaml cp: cannot create regular file '/etc/netplan/50-cloud-init-original.yaml': Permission denied $ sudo cp /etc/netplan/50-cloud-init.yaml /etc/netplan/50-cloud-init-original.yaml
  Other info - In the example here, before modifying the file we've made a copy of the original, to be reverted to in the event we make a mistake in editing that file. The first we failed because we needed to have elevated privileges. Using sudo gave us those privileges.
• mv
  What it does - Moves a file from one directory to another and/or renames the file
  Examples of use
  
  $ mv hello_world.py ~/archive/ # Moves the file from the current directory into the archive directory $ mv hello_world.py goodbye_world.py # Moves the file from an old filename to a new filename
  Other info - A common beginner mistake is to issue an mv command like mv hello_world.py ~/archive. If the directory archive exists, the file will be moved into it as desired, but if that directory doesn't already exist, the hello_world.py file will be moved to the home directory (~) and renamed as archive, which is probably not what we intended. By adding an extra forward-slash at the end of "archive", we are indicating that we want the file to go into the directory archive. If that directory doesn't exist, we'll get an error message and we can fix our mistake by creating that directory before continuing with the mv operation.
• rm
  What it does - Removes files and directories
  Exampled of use
  
  $ rm hello_world.py # removes the indicated file $ rm *.py # removes all of the files with extension .py $ rm -r * # removes all of the files in the current directory, # recursively removing files in the directories below as well
  Other info - The rm command should only be used with extreme caution; there is no recovery once you've deleted a file or directory.
• less
  What it does - Displays a file one terminal page at a time
  Example of use
  

  $ less filename

  Other info - Once the file is opened you can navigate it with a series of simple commands:
  • q - quit (close file)
  • f - moves forward one page
  • b - moves back one page
  • gg - moves to beginning of file
  • G - moves to end of file
  • / - enter expression to search for, hit [Enter] to complete
  • n - find next occurrence of search expression
  • N - find previous occurrence of search expression
  There are a large number of other options, but these will get you started.
• nano
  What it does - Uses the nano editor to edit a file
  Example of use, with output
  

  $ nano hello_world.py

  Other info - There are more powerful text editors, but nano is the perfect command-line editor to get you started working in the Terminal. See Intro to nano for more info.
• man
  What it does - Displays the manual for a given command
  Examples of use
  
  $ man less # Gives help on using the less command $ man sudo # Gives help on using the sudo command
  Other info - man pages use the same navigation system that less uses.
• find
  What it does - finds files in the filesystem
  Examples of use
  
  $ find . -type f -iname "hello" # Begins looking in the current directory (.), # looking for a file (f) called 'hello' $ find . -type f -iname "*hello*" # Looks for a file with 'hello' # anywhere in the name. The asterisks are an example of 'globbing', # where the '*' acts as a kind of wildcard for any expression $ find . -type d -iname "*Doc*" # Looks for a directory with 'Doc' # anywhere in the name $ find ~ -type f -size +100M # Looks starting in the home directory # for any file that is larger than 100M in size $ find ~ -type f -mtime -7 # Looks starting in the home directory # for any file that was changed less than 7 days ago $ find ~ -type f -mtime +30 # Looks starting in the home directory # for any file that was changed more than 30 days ago
  Other info - The find command is extraordinarily powerful, and can be used in lots of different ways.
• grep
  What it does - searches files looking for patterns (regular expressions). "grep" stands for "global regular expression print"
  Examples of use
  
  $ grep -ri password project # Searches recursively, insensitive to case, # for the expression "password" in the "project" directory $ grep -ri "rw.*@crashwhite.*" project # Searches in the project directory for # any expression that starts with 'rw" and # has '@crashwhite' connected to it
  Other info - Note that the ".*" used in these regular expressions is differnt from the "globbing" wildcard we used with the find command. Regular expressions are one of the most important and powerful pattern matching tools in computer science--once you get to know them a little better, you'll love them!