Things You Should Know About the Command Line

The command line is really powerful and there are a quite a few things made easier by using it.

When I started learning things, I did a lot of copy-and-paste. It got the job done (usually…), but I rarely understood what I was doing or why it worked. So I want to outline a handful of basic commands, and some tips and tricks.

In the code snippets below, $ indicates a command I’m entering into the terminal and the rest is output (known as standard output, standard out, or stdout) from the terminal. For example

$ echo "Hello"  
Hello

Basic Commands

Here are a few basic commands. There are certainly others that can be called “basic.” Perhaps I’ll do a Part II if it seems called for.

cd – Change Directory

This command will change the current directory, either an absolute or relative path will work.

$ cd somefolder
$ cd ~/Desktop/
$ cd /var/www/html/mysite.com/

And go up a directory

$ cd ..

pwd – Present Working Directory

Simply tells you where you are currently. If you’re working with symlinked directories, you may find the -P flag useful, pwd -P.

$ pwd
/Users/KLampert/Desktop

ls – List

List files in the current directory. By default, you’ll get a list, in columns, of all ‘regular’ files and directories. You won’t see hidden file/folders, or see any info about them.

$ ls
index.php            wp-config-sample.php wp-login.php
license.txt          wp-config.php        wp-mail.php
readme.html          wp-content           wp-settings.php
wp-activate.php      wp-cron.php          wp-signup.php
wp-admin             wp-includes          wp-trackback.php
wp-blog-header.php   wp-links-opml.php    xmlrpc.php
wp-comments-post.php wp-load.php

More options:

ls -a list all. Includes hidden files
ls -l list in long format. Shows filesize, ownership, permissions and more.
ls -lh used with -l. Makes filesizes human-readable

Altogether as ls -alh

mv – Move a File

Move a file or directory from one place to another. Of course you can move a file from one directory to another, but it’s also how you’d rename a file – a lot of people don’t think of renaming as function of moving, but that’s all it is.

Rename
$ mv source-file.txt destination.txt
Relocate
$ mv source-file.txt destination/source-file.txt

cp – Copy a File

Copy a file or directory. Like mv, the source/original file is first, and the new file second.

$ cp wp-config.php wp-config.php.backup

For directories, you’ll need to use the -r flag to recursively copy the contents.

$ cp -r wp-content wp-content-backup

cat – Concatenate and Print Files

You can pass multiple files into this command, and it will concatenate and print them out. But you can still pass just one file, which makes it really handy to look at a file without opening an editor.

cat wp-config.php

man – Manual

Get the manual page for a command. This really handy for remembering the order of parameters and what flags are available. Also, sometimes it looks funny.

$ man logger

Once you’ve opened a man page, there are a few commands you should know. q to leave, f to move forward one page, b to move backward, and h to learn more about those shortcuts. And yes, you can also look up the manual page for itself.

$ man man

Combining Commands

There’s a lot of value in being able to do two things at once. But even if combining commands is more than what you need now, it’s nice to know what it looks like, so you can better understand some of those lengthy one-liners in tutorials.

; The Simplest Way

Run one command, then another.

$ pwd; ls -lah

&& AND-AND

There’s probably a fancy name for this, I don’t know it. Similarly to the above, but the second part only runs if the first part evaluates to true.

$ true && echo "Hello"
Hello

Since true is true, “Hello” will be printed

$ false && echo "Hello"
$

But since false is not true (go figure), nothing will be printed. Of course, in the real world you wouldn’t use true and false you’d have something useful.

| Pipe, Redirect Standard Output to Command

Send the output of the first part to the command in the second part.

$ ls | grep wp
Here I list the files, but before that list is printed out, it is passed to grep which filters that list down to only items containing “wp”

> Redirect Standard Output to File

Similar to the Pipe, but instead of passing the output to a command, we’ll send it directly to a file. This works well with the cat command mentioned earlier.

$ cat onefile.txt twofile.txt > onetwofile.txt

$ pwd > current-directory.txt

This will create the file if it does not exist, and will overwrite any contents that may exist.

>> Redirect Standard Output to File (Append)

Just like > but will add to the end.

$ ls >> current-directory.txt

So if you run the last two commands together, you’ll get a file called current-directory.txt that has the path at top followed by a list of files.

< Redirect Standard Input

This works especially well with commands like sort. Suppose you have a big unsorted list of things that you need to, well, sort.

$ sort < things.txt

That will sort it, and print it back out. But probably you want to save that list, so we could use what we just learned and redirect standard output.

$ sort < things.txt > things-sorted.txt

Or, we can use what we learned earlier and read the man pages.

$ sort < things.txt -o things-sorted.txt

The advantage of the latter is that if we wanted, we could update the original file, rather than creating a second file.

$ sort < things.txt -o things.txt

Use It!

Really, as with many things, the best way to get better and more comfortable with it is to get out there and use it. That’s why I didn’t put the remove command in here 🙂