The very very useful ones

man

• man <command name>
• The main command returns a helpful help page that gives you a brief description of what exactly a command does and how to use it.
• In case that man doesn't work, make sure that you have mandocs or man installed.
• The manual entries can also be accessed via a browser.

--help or -h flag

• Sometimes a command is too niche to warrant a page in the manual. In such cases you can use the help flags. There is no set standard but they're usually one of the two
• <command> --help or <command> -h
• If one doesn't work, try the other.

sudo and su

• su opens the current shell as root while sudo runs a specified command as root.
• In Linux root priviliges are very similiar to Adminstrator priviliges in Windows
• Be very careful when running anything as root. It can break your system.--
• It also requires the current user to know the root password as well be a part of the sudoers list

The Essentials

cd

• cd is used to traverse the filesystem from the terminal
• cd <path> will move the terminal to the defined path
• cd .. will move the terminal to the parent directory if it exists
• cd / will move the terminal to the root directory
• cd ~ or just cd will move the terminal to the home directory

ls

• ls shows all the visible files and folders in directory.
• ls -a shows all the hidden and visible files and folders in the directory.
• ls -A shows almost all the hidden and visible files and folders in the directory(It excludes the . which loops back to the current directory and the .. which points to the parent directory)
• ls -l displays all the visible files and folders and lists them in tabular form with some extra information(like size, author, date modified, etc)

touch

• touch <filename> creates an empty folder with the name specified

mkdir

• mkdir <foldername> creates a folder with the specified name in the current location of the terminal.
• mkdir -p <path_to_folder> creates the folder in the path specified as well as all the missing folders in the path to the folder.

mv

• mv <source_path> <destination_path> moves a file from the source_path to the destination_path
• mv <path>/old_name <path>/new_name will rename a file named old_name in the location <path> to new_name

cp

• cp <source_path> <destination_path> copies a file from the source_path to the destination_path
• cp -r is used to copy folders by recursively copying their contents

rm

• rm <file1> <file2> <file3> deletes the files mentioned
• The Recycle Bin or Trash does not exist when it comes to deleting things from the terminal so be careful with this command.
• rm -f <file> force deletes the file and overrides any warnings.
• rm -r <folder> recursively deletes the contents of a folder and finally deletes the folder too.
• UNDER NO CIRCUMSTANCE SHOULD YOU RUN ANY OF THE FOLLOWING UNLESS YOU ARE ABSOLUTELY SURE ABOUT WHAT YOU ARE DOING
  • sudo rm -rf / this will delete everything in your root directory
  • rm -rf ~ this will delete everything in your home directory
• Running rm -rf with elevated priviliges in a dangerous location will most probably break your OS.
• Exercise caution.

grep

• grep is used to search the content of a specified file or durectory for a given string or a regex
• grep '<search_term>' <file> searches for the search_term in the <file>
• grep -i '<search_term>' <file> will search for the search_term in a case insensitive way.
• grep -r '<search_term>' <folder_path> will search for the search_term recursively within the specified directory.

cat

• cat allows the user to execute basic text modification from the terminal
• It is not a full blown editor like Vim or Emacs but it can read and append to files
• cat <file> will display the contents of the file
• cat >> <file> will allow you to enter some text into the terminal. The entered text is then appeneded to the end of the file.

The Extras

pwd

• pwd returns the path to the active directory

top

• top displays the processes running in real time. It also displays resource utilization and other information regarding the process.

pkill

• pkill <pattern> kills the first processes with the string <pattern> in their name

pgrep

• pgrep <pattern> returns the PID of all processes with the string <pattern> in their name

kill

• kill <PID> terminates the process with the PID specified

Useful for Bash Scripting

echo

• echo "Hello, World!" will print Hello, World! on the terminal.

wc

• wc <file> will print the number of words in a file
• wc -l <file> will print the number of lines in a file

Piping data

• We can pass the output of one command to the input of another by using the | operator.
• grep -r '<search_term>' <folder_name> | wc -l will return the number of entries that match the search

SSH

ssh

• ssh is program used to log into and access a machine. It was designed as a more secure replacement to telnet.
• The commands that you type into your ssh shell terminal are excecuted on the host instead of the login shell
• ssh user_name@host(IP/Domain_name) will ssh into the specified host in the user user_name

scp

• scp is Secure Copy, it is mostly used to copy files over ssh.
• Its usage is very similar to cp, one key difference is that we need to mention the user and the hostname of the devices in the address.
• scp -r us<er1@host1:home/Documents/ user2@host2:home/target_path will copy the contents of the Documents folder in user1's home directory in host1 to the home directory of user2 in host2host.

System Calls

fgets

• fgets() is a system call in the stdio.h package.
• fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or a newline. If a newline is read, it is stored into the buffer. A terminating null byte (aq\0aq) is stored after the last character in the buffer.

Example:

#include <stdio.h>
int main() //Program that uses fget() to accept a string from the user and print it
{
    char buf[10];
    fgets(buf, 10, stdin);
    printf("%s\n", buf);
    return 0;
}