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Learn the Command Line

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pwd Print Working Directory

The shell command pwd displays the file path from the root directory to the current working directory.

$ pwd

mkdir Make Directory

The shell command mkdir is used to make a new directory in the filesystem according to its argument. If a file path is given, the new directory will be placed at the end. Otherwise, it will create a new directory in the current working directory.

$ mkdir new-directory
$ ls
old-directory new-directory

ls List

The shell command ls is used to list the contents of a directory. If no arguments are given, it will list the contents of the current working directory.

$ ls Desktop

cd Change Directory

The shell command cd is used to move throughout the filesystem of a computer. It accepts a variety of arguments:

  • Full file paths.
  • Names of children of the current directory.
  • .. the parent of the current directory.
$ cd some-directory
$ cd ..

Filesystem Structure

A computer’s filesystem organizes the data stored by a computer, so that it can be easily retrieved by the user.

Files are generally represented in a tree-like structure, in which any parent directory can have any number of children. The root directory is then found at the base of the tree.

touch Create New File

The shell command touch creates a new file in the current working directory with the name provided.

$ touch grocery-list.txt

The Command Line

The command line allows a user to navigate the filesystem and run built-in programs or custom scripts. In Unix, the command line interface is called Bash, and the shell prompt is the $.


Helper Commands

Helper commands for the command line include:

  • clear to clear the terminal
  • tab to autocomplete the line
  • and to cycle through previous commands

cp Copy

The shell command cp is used to copy files or directories.

The basic argument structure is cp source destination, where the source is the file/directory to copy to the destination file/directory.

$ cp file1 file1_copy
$ cp file1 file2 destination_folder

Command Options

Options can be used to modify the behavior of shell commands. Shell command options are commonly represented by a single letter preceded by a -. For example, -l, -a, and -d could all be options that follow a shell command.

mv Move

The shell command mv is used to move a file into a directory. Use mv with the source file as the first argument and the destination directory as the second argument.

$ mv index.html website/

rm Remove

The shell command rm is used to delete files and directories. The -r flag deletes a directory and all of its files and directories (rm -r).

$ rm -r bad_selfies

ls List Command Options

The shell command ls is used to list the contents in a directory. It can be combined with the following command options:

  • -a: lists all contents, including hidden files and directories.
  • -l: lists all contents, in long format.
  • -t: lists all contents, by the time they were last modified.
$ ls -a
$ ls -l
$ ls -t

Append Redirect shell command

The >> shell command is used to redirect the standard output of the command on the left and append (add) it to the end of the file on the right.

# This command will append "Hello World!" to greetings.txt
echo "Hello World!" >> greetings.txt

Pipe shell command

The | command is called a pipe. It is used to pipe, or transfer, the standard output from the command on its left into the standard input of the command on its right.

# First, echo "Hello World" will send Hello World to the standard output.
# Next, pipe | will transfer the standard output to the next command's standard input.
# Finally, wc -w will count the number of words from its standard input, which is 2.
echo "Hello World" | wc -w

Redirecting Output

The > symbol is used to redirect output by taking the output from the command on the left and passing as input to the file on the right.

echo "Hello" > hello.txt

cat Display

The shell command cat displays the contents of one or more files to the terminal.

$ cat poem.txt
$ cat poem.txt kitties.txt

The shell command grep is used to search files for lines that match a pattern and returns the results. Various options can be specified along with the grep command to specify the search.

In the provided example, the lines in the file names.txt which contain “sonny” will be returned.

grep 'sonny' names.txt

The shell grep command searches files for a particular pattern. The grep command with the -i option can be used to search files for lines that match a pattern, case insensitive, and returns the results.

grep -R shell command

The shell command grep has a -R option (grep -R) that searches all files in a directory, including its subdirectories, and outputs filenames and lines containing matched results.

Command Line Redirection

On a command line, redirection is the process of using the input/output of a file or command to use it as an input for another file. It is similar but different from pipes, as it allows reading/writing from files instead of only commands.

Redirection can be done by using the operators > and >>.

ls > directories_list.txt
ls >> directories_list.txt

Command line environment

The environment of the command line refers to the settings and preferences of the current user. It enables users to set greetings, alias commands, variables, and much more.

Shell Command env

For Unix-based systems like Mac OS and Linux (not Windows), the shell command env returns a list of environment variables for the current user.


The shell command alias is used to assign commonly used commands to shortcuts (or aliases). The assigned commonly used command should be wrapped in double quotes.

# The following command creates an alias `pd` for the command `pwd`
alias pd="pwd"

Environment Variables

Variables that can be used across terminal commands are called environment variables. They also hold information about the shell’s environment.

Source Bash Profile

All the commands in ~/.bash_profile are executed with the shell command source ~/.bash_profile. So when changes are made to ~/.bash_profile, run this command to activate the changes in the current session.

history Command

The history shell command is used to get a history of commands (also known as “events”) that were executed in the current session. The command also allows us to perform operations on this list of commands that have been executed, such as selecting or manipulating a command in the history.

Export command

The export command makes a given variable available to all child sessions initiated from the current session.

# This command will make the environment variable USER available
# to all child sessions with the value "Jane Doe".
export USER="Jane Doe"

HOME Environment Variable in Unix Systems

HOME is an environment variable present in command line environments. It is used to get the path to the current user’s home directory. This makes it easy for programs to access the home directory when needed.

# To show the path of the home directory use the following command:
echo $HOME