Linux Command Line Basics

This is a very fast introduction to the Linux Command Line. To get more help on these commands, either type the command with no files to operate on or type the command --help. To go deeper into this subject, check out the resources page. It’s worth mentioning that everything here applies to the Mac terminal (and everything Unix-based) as well.

1.1 Display Files in a Directory

ls displays the contents of a directory. To see other information like file ownership and permissions and hidden files, type

ls -al

1.2 Change Directory

cd DIRECTORY

2 File Operations

2.1 Copy

cp SOURCE DESTINATION

2.2 Move (==Rename)

Syntax:

mv SOURCE DESTINATION

This command is equivelant to cut and paste, as well as file rename.

2.3 Delete

Caution: Make sure you know what what you are doing here, this command is ireversible!

rm FILE

For directories, use option -r

rm -r DIRECTORY

2.4 Wildcards

For example, to remove all files with .pdf extention:

rm *.pdf

Here is more info on wildcards.

3 File Permissions and Ownership

As mentioned before, ls -al displays the file permissions and ownership.

chown command changes the ownerships. More details
chmod command changes read/write/execute permissions. More details

Many of the commands above won’t execute without the proper user privilege. To force a command to execute, use sudo (you will be asked for a password)

sudo COMMAND

4 File Transfer

I covered ssh, scp,and rsync before.

5 Redirection

Before we go into redirection, let’s define two commands that print output to the screen: echo and cat.

Echo

echo "this comment" # prints "this comment" to stdout (the screen)

Concatenate

Suppose the file file.txt contains the following:

aziz is awesome
this is a beautiful blog

The cat command stands for concatenate, which just types the contents of a file to stdout.

vi (or other text editors)

vi is a ubiquitous text-editor that’s usually preinstalled on most unix-like system. You can use that to create and edit files from the command line.

Back to redirection, suppose you want to redirect this output to a file or another command, you can use |, >,<,>>,<<. See some examples below.

Examples

# these are the some ways you can use redirection

# piping
cat file.txt|grep aziz # displays the line that has the word 'aziz' using grep. This sign | is called a pipe, because redirects the output of the first command to the second.

# creating file
cat file.txt > file2.txt # (use the single > with caution) writes the contents of file.txt to file2.txt

# adding to a file
cat file.txt >> file2.txt # appends the output from the first command to file.txt
# Remember: don't cat a file and redirect it to itself.

echo "this useful comment" >> file2.txt # appends "this useful comment" to file2.txt

cat << EOF # you can think of this as a multi-line echo command

SED

The grep command above performs a search through text. To do a lot more functions like this, e.g. find and replace, see this website.