Unit 7: Command Line
Table of Contents
Command Line
At some point in your life you have probably tried to use Command Prompt (Windows) or Terminal (Mac). It was probably very confusing and hard to use. That’s because they both use command line interfaces (CLIs), which are entirely text based, as opposed to graphic user interfaces (GUIs), which you’re used to (e.g. clicking things).
However, as a programmer, you will need to familiarize yourself with at least the basics of the command line in order to navigate file systems and use Git, which is a popular version control system we will learn about later (see Git).
So let’s get started!
Notes before we begin
Be Careful!
Be EXTREMELY careful when entering commands into the command line.
Because it is text based and can modify practically everything on your computer, it’s very easy to make a typo and then accidentally delete everything on your computer. (It probably won’t be that bad, but it happened to the animators of Toy Story 2, so just be careful.)
Bash
The 2 most popular CLIs are DOS (Windows) and bash (Mac, Linux). Each has its own commands. In this course, we will be learning basic bash.
If you are a knowledgeable Windows user who would like to contribute DOS content, please do! See Contributing.
Follow Along
We highly recommend that from now on when you read this section and come across an example, you try using that command on your own command line. Note that examples should be followed in order if you want it to work properly.
To follow along, you’ll need to download or clone the omega9656/learn-code-unit-7 repository. See the Usage section in the README file.
Here is a diagram of the file hierarchy on that repository:
|--src
| |--com
| |--omegarobotics
| |--books.txt
| |--opmode.txt
| |--songs.txt
| |--tvshows.txt
|
|--README.md
File Paths
On Windows, file paths use back slashes (\). On Mac, they use forward slashes (/). If you use \ on a Mac it will still understand what you mean though.
Examples
In the examples we show you here, anything that starts with # is a comment. The $ symbol indicates the command prompt. You aren’t supposed to type this in, it should just be there. If the line does not start with $, that means it is output displayed on the terminal.
Terms You Should Know
| Term | Definition |
|---|---|
~ | Home directory |
| directory | Folder |
| root directory | Parent directory of a project |
| path | Directories that lead to a file (for example, src/com/omegarobotics) |
| absolute path | Path from the root to a file |
| relative path | Path from the current directory to a file |
| (file) extension | Thing that goes after the file name which tells you what the file type is (for example, .pdf) |
src | “source”, usually where all the code is in a project |
| flag | An option that modifies a command (for example, -m). You can also chain flags (for example, -a -m is equivalent to -am or -ma). |
Commands You Should Know
cd
This command allows you to change directories.
Usage
$ cd <path>
Examples
Start in the root directory of the Learn Code Unit 7 repository (we’ll call this the U7 repo from now on).
$ cd src
$ cd com
$ cd omegarobotics
You should now be in src/com/omegarobotics.
Alternatively, you can give a relative path to go to that directory all in one line.
$ cd src/com/omegarobotics
To go “up” a directory, use:
$ cd ..
You can use .. in a relative path. Start in src/com/omegarobotics. For example:
$ cd ../../..
Note that . refers to the current directory.
You should now be in the root directory of the U7 repo.
To go to the home directory, use:
$ cd ~
To go to the last directory you were in, use:
$ cd -
ls
This command allows you to list all of the files and directories in the current directory.
Usage
$ ls
Options
The -a flag lists all files, including hidden files (which start with .).
$ ls -a
The -l flag lists all files in detailed, vertical format.
$ ls -l
Examples
cd into src/com/omegarobotics.
$ ls
books.txt opmode.txt songs.txt tvshows.txt
-a flag
$ ls -a
. .. books.txt opmode.txt songs.txt tvshows.txt
-l flag
$ ls -l
total 32
-rw-r--r-- 1 janedoe staff 225 Aug 4 22:26 books.txt
-rw-r--r-- 1 janedoe staff 424 Aug 4 22:25 opmode.txt
-rw-r--r-- 1 janedoe staff 210 Aug 4 22:23 songs.txt
-rw-r--r-- 1 janedoe staff 100 Aug 4 22:21 tvshows.txt
Note: You don’t really need to worry about what all of that text means. Basically, it’s just more details about the file, such as read/write permissions, users, user groups, date/times, etc.
clear
clear clears the terminal of previous commands and output.
Usage
$ clear
pwd
pwd prints the working (current) directory.
Usage
$ pwd
Example
Start in the root directory of the U7 repo.
$ cd src/com/omegarobotics
$ pwd
/Users/janedoe/Desktop/learn-code-unit-7/src/com/omegarobotics
mkdir
mkdir makes a new directory in the current directory you are in.
Usage
$ mkdir <name>
Example
Start in the root directory of the U7 repo.
$ cd src/com
$ mkdir hello
$ ls
hello omegarobotics
touch
touch is used to create a new file in the current directory.
Usage
$ touch <file>
Examples
Start in the root directory of the U7 repo.
$ cd src/com/hello
$ touch hello.txt
$ touch world.txt
$ ls
hello.txt world.txt
rm
rm is used to remove files or directories.
Usage
$ rm <path>
Options
The -r flag (for recursive) allows you to remove a directory which contains files.
$ rm -r <path>
Examples
Start in the root directory of the U7 repo.
$ cd src/com/hello
$ rm world.txt
$ ls
hello.txt
Start in the root directory of the U7 repo.
$ cd src/com
$ rm -r hello
$ ls
omegarobotics
rmdir
rmdir removes the given directory, assuming it is empty. (For non-empty directories, use rm -r <path>).
Usage
$ rmdir <path>
Examples
Start in the root directory of the U7 repo.
$ mkdir empty
$ ls
README.md empty src
$ rmdir empty
$ ls
README.md src
help
help prints some commands and their usages.
Usage
$ help
Examples
$ help
GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin19)
These shell commands are defined internally. Type `help' to see this list.
Type `help name' to find out more about the function `name'.
Use `info bash' to find out more about the shell in general.
Use `man -k' or `info' to find out more about commands not in this list.
A star (*) next to a name means that the command is disabled.
JOB_SPEC [&] (( expression ))
. filename [arguments] :
[ arg... ] [[ expression ]]
alias [-p] [name[=value] ... ] bg [job_spec ...]
bind [-lpvsPVS] [-m keymap] [-f fi break [n]
builtin [shell-builtin [arg ...]] caller [EXPR]
case WORD in [PATTERN [| PATTERN]. cd [-L|-P] [dir]
command [-pVv] command [arg ...] compgen [-abcdefgjksuv] [-o option
complete [-abcdefgjksuv] [-pr] [-o continue [n]
declare [-afFirtx] [-p] [name[=val dirs [-clpv] [+N] [-N]
disown [-h] [-ar] [jobspec ...] echo [-neE] [arg ...]
enable [-pnds] [-a] [-f filename] eval [arg ...]
exec [-cl] [-a name] file [redirec exit [n]
export [-nf] [name[=value] ...] or false
fc [-e ename] [-nlr] [first] [last fg [job_spec]
for NAME [in WORDS ... ;] do COMMA for (( exp1; exp2; exp3 )); do COM
function NAME { COMMANDS ; } or NA getopts optstring name [arg]
hash [-lr] [-p pathname] [-dt] [na help [-s] [pattern ...]
history [-c] [-d offset] [n] or hi if COMMANDS; then COMMANDS; [ elif
jobs [-lnprs] [jobspec ...] or job kill [-s sigspec | -n signum | -si
let arg [arg ...] local name[=value] ...
logout popd [+N | -N] [-n]
printf [-v var] format [arguments] pushd [dir | +N | -N] [-n]
pwd [-LP] read [-ers] [-u fd] [-t timeout] [
readonly [-af] [name[=value] ...] return [n]
select NAME [in WORDS ... ;] do CO set [--abefhkmnptuvxBCHP] [-o opti
shift [n] shopt [-pqsu] [-o long-option] opt
source filename [arguments] suspend [-f]
test [expr] time [-p] PIPELINE
times trap [-lp] [arg signal_spec ...]
true type [-afptP] name [name ...]
typeset [-afFirtx] [-p] name[=valu ulimit [-SHacdfilmnpqstuvx] [limit
umask [-p] [-S] [mode] unalias [-a] name [name ...]
unset [-f] [-v] [name ...] until COMMANDS; do COMMANDS; done
variables - Some variable names an wait [n]
while COMMANDS; do COMMANDS; done { COMMANDS ; }
cat
cat displays the contents of a file.
Usage
$ cat <file>
Examples
Start in the root directory of the U7 repo.
$ cd src/com/omegarobotics
$ cat books.txt
Six of Crows by Leigh Bardugo
Crooked Kingdom by Leigh Bardugo
Legend by Marie Lu
To Kill a Mockingbird by Harper Lee
The Great Gatsby by F. Scott Fitzgerald
The Lightning Thief by Rick Riordan
Fahrenheit 451 by Ray Bradbury
echo
echo echoes any text you type in. It is usually used to add text to a file.
Usage
$ echo <text>
Examples
Using echo just for fun
$ echo Hello World!
Hello World!
Using echo to add text to a file
Start in the root directory of the U7 repo.
$ cd src/com/omegarobotics
$ cat songs.txt
She Used to Be Mine by Sara Bareilles
City of Stars by Ryan Gosling and Emma Stone
In My Blood by Shawn Mendes
Radioactive by Imagine Dragons
Speechless by Naomi Scott
Paris in the Rain by Lauv
Epiphany by BTS
$ echo "Echo by Jason Walker" >> songs.txt
$ cat songs.txt
She Used to Be Mine by Sara Bareilles
City of Stars by Ryan Gosling and Emma Stone
In My Blood by Shawn Mendes
Radioactive by Imagine Dragons
Speechless by Naomi Scott
Paris in the Rain by Lauv
Epiphany by BTS
Echo by Jason Walker
Note: To append to a file, use >>. To overwrite a file, use >.
Further Reading
In no way is our lesson on the command line comprehensive. If you would like to learn more about the command line, see the following reference materials and guides.