01-command-line.md

Tutorial: Command line

By Dan Phiffer

This tutorial describes some basic functions of the macOS command line. If you are not using macOS, you may need to adjust some of the details. Linux behaves very similarly to macOS, but Windows less so. If you are already comfortable using the command line, you may want to check out the Further reading section at the end of this tutorial.

When things go wrong

It's important to remember that errors are inevitable in this process. It's more important that you notice when the computer is telling you something went wrong than avoiding errors altogether. I am often suspicious when something works perfectly the first time. Recognizing what an error message is telling you is a skill you will develop with experience.

If you ever want to stop a command as it's running, use ctrl-C (that's the actual control key, not command).

Hello, Terminal

• In the Finder, go to Applications → Utilities and double-click on Terminal.app.

A window will open where you can type things. Some commands are substitutes for things you can do with a graphical user interface (GUI), but other commands have no equivalent. The command-line is where software development happens. You type a command, press enter, and the computer responds with a result.

Where am I?

• Type pwd (then press enter).

$ pwd
/Users/danphiffer

(Instead of danphiffer you should see your own username.)

The $ sign is a common notation for the prompt, an invitation to type in a command. The prompt may not always look like a $ (for example, it's common to notate admin-level prompts as #), but that's the symbol that gets used in lots of documentation as the default prompt.

The pwd command responds with your "working directory," which is where your commands are executed. By default your terminal session starts out in your home directory (in macOS, that's located in Users → username).

What is here?

• Type ls.

The ls command lists the files and folders in the current working directory.

$ ls
Desktop         Downloads       Movies          Pictures
Documents       Library         Music           Public

Moving around

• Type cd Desktop to move to the Desktop.
• Double-check you have moved by typing pwd.
• Type ls -l to see what files are here.

$ cd Desktop
$ pwd
/Users/danphiffer/Desktop
$ ls -l
total 123008
-rw-r--r--  1 danphiffer  staff  61965791 Jun 17  2016 paulsboutique.mp3

The cd command changes our working directory. When we type cd Desktop the Desktop part is called an argument.

The -l argument for the ls command makes the listing include more details, including file permissions, file size (in bytes) and last modified information.

Ways of moving

To move into a subfolder:

$ cd folder/subfolder

To move into the parent folder:

$ cd ..

To move back to your last location:

$ cd -

To move to your home directory (no argument):

$ cd

You can also use the ~ symbol as a shortcut for your home directory:

$ cd ~/Desktop

Making folders

Let's create a folder that we can use to save our work.

$ mkdir -p dsj/01

The command mkdir is like what it sounds like, it makes a directory. The -p argument is an option to create any intermediate subdirectories as necessary, so both dsj and 01 get created.

Paths can be tricky

The paths we use for files can be kind of tricky, and there are lots of shortcuts you can use to refer to files located in different places.

• . is the current working directory
• .. is the directory above the current directory (the one that contains it)
• ~ is a shortcut to your home directory
• when a path starts with / it is called an absolute path, otherwise it is relative to the current working directory

These are all ways of writing paths to the same folder (relative to the Desktop):

• dsj/01
• ./dsj/01
• ../Desktop/dsj/01
• ~/Desktop/dsj/01
• /Users/danphiffer/Desktop/dsj/01 (the absolute path)

Sneaky hidden files

Next, change directory into the new 01 folder and list the contents with ls -la.

$ cd dsj/01
$ ls -la
total 0
drwxr-xr-x  2 danphiffer  staff  64 Aug 19 17:16 .
drwxr-xr-x  3 danphiffer  staff  96 Aug 19 17:16 ..

When we include the -la argument, it shows all of the files including the current folder (.) and its parent (..).

Let's make a new folder .secret that will get hidden by default. You can try listing with regular ls and then again with ls -a.

$ mkdir .secret
$ ls
$ ls -a
.       ..      .secret

Any file or folder that starts with . gets omitted by ls (unless you provide -a) and gets hidden by default in the Finder (you can configure it to show all files).

Because you typed -a instead of -la the results were listed in the default summary format.

Apple-y things

The open command is the equivalent to double-clicking on things in macOS.

• Type open . to open the current working directory (dsj/01, on your Desktop).

You should see an empty folder (.secret is hidden because its name starts with .).

• Now type open .secret to open up the hidden folder.

Hidden folders are not really all that private so you shouldn't count on them to hide anything important. But you may discover interesting configuration dot files in your home directory you didn't know were there.

You can work with the macOS clipboard using pbcopy and pbpaste.

• Type pbcopy < file.txt to copy the contents of file.txt to your clipboard.
• Type pbpaste > file.txt to copy the current clipboard contents into file.txt.

Shortcuts

There are keyboard shortcuts that can save you lots of time when you're working in the terminal.

• Without pressing enter, type: open .s and then press the tab key (it should autocomplete the rest of .secret).

The terminal recognized that you want to open something that starts with .s and then fills in the rest.

• Press the up/down arrow keys to browse earlier commands.
• Enter history to look through all of the commands you've typed.
• Use ctrl-R to search previous commands

Create a file

Next, we will create a new file test.txt in the 01 folder.

• Type nano test.txt to create a new file called test.txt.

The text editor will let you write and save a text document, kind of like a pared-down version of Microsoft Word.

When you are ready to save your changes, press ctrl-O (that's ctrl not cmd, and the letter "o" not a "zero") to write out the file to disk. You will be prompted for the file location, and it's fine to accept the default by pressing enter.

When you are ready to close nano, press ctrl-X.

Read the file

You can view the contents of your file with the cat command. 😸

• Type cat test.txt to see the contents of the file.

You might be asking, "but what if you want to read the contents of a very long file?". You can check just the beginning or just the end of the file, or you can search for matches in the middle.

• head mobydick.txt can be used to get the beginning part of a file.
• tail donquixote.txt shows the end of a file.
• tail -f output.log will continuously show the end of a log file as it gets written to.
• more frankenstein.txt will let you browse through the contents of a long file in small increments.
  • Press the enter key to move down or use the up/down arrow keys.
  • Press the spacebsar to page down.
  • Press the Q key to quit.
• grep morlock timemachine.txt will show all the lines in timemachine.txt that contain the word morlock.

Update the file

You can continue editing the file with nano test.txt.

Delete the file

Remove the file:

• rm test.txt to delete your test file.

To delete a folder and all of its contents you need to provide -rf arguments:

• rm -rf folder

If you get stuck

• You can make a new tab in the terminal by pressing cmd-T.
• Force a running command to cancel by pressing ctrl-C. (In some cases you need to press the Q key to quit.)
• Suspend a command to the background by pressing ctrl-Z (and then resume it by entering fg, bringing it back to the foreground).
• You can always ask for help! Send me email: danphiffer@bennington.edu

Further reading

• UNIX text processing tools by Allison Parrish
• Terminal Cheatsheet for Mac by Onno Schwanen
• In the Beginning... Was the Command Line by Neal Stephenson