Linux_logo_errorbits.comHere is a list that contains one of the most used Linux commands, and a short explanation for them:
– list – lists files and directories, can be used with a lot of options like: -a, -R, -l, -g, -r;

ln – create symbolic link to files and folders;

pwd – print working directory – shows the path of the current directory;

ifconfig – interface configuration – used to view and change the configuration of the network;

cd – change directory – used to change the current working directory;

chmod – change file permissions;

chown – change file owner and group;

df – disk free – must be used with -h option(the values will be displayed in KB/MB/GB);

du – disk usage – must be used with -h option, -s option will sum the subfolders and files capacity;

grep – global regular expression print – processes text line by line and returns the value that matches the request;

cat / tac – catenate – is the simplest way to display the contents of a file, directly in command line interface / tac reverses the order of the contents;

zcat – display a compressed file after decompressing;

zless – display a compressed file, page-by-page, you can go back by pressing “b” key;

zmore – display a compressed file, page-by-page;

cut – extract the specified section from each line of text in a file;

dd – copy blocks of data from a file to another,  is used to copy data from devices;

diff – compare two text files and finds the differences;

expand – convert all tabs to spaces;

tail – outputs the last part of the contents of a file;

tee – used to dublicate input and to route it to multiple outputs at once;

touch – used to update the access and modification time of a file, if the file does not exist it is created with the specified name;

mkdir – make directory – used to create directories;

mv – rename and move the file from a location to another;

rm – remove – used to remove/delete files or directories;

rmdir – remove directories;

cp – copy – used to make copies of files and directories;

compress – compress files/folders;

uncompress – decompress files/folders compressed with compress;

gzip – compress files using GNUZip;

tar – create an archive of files in one or more directories;

cpio – copy files from/to an archive;

less – is a simple command line file viewer;

head –  used to display the first part of files;

file – used to determine the file’s type;

fold – warp each line of text to fit a specified width;

lpr – print files;

more – display contents of a text file, page-by-page;

nl – number all non-blank lines in a text file and print the lines to standard output;

paste – concatenate corresponding lines from some files;

patch – update a text file using the differences between the original and revised copy of a file;

sed – copy a file to standard output while applying specified editing commands;

sort – sort lines in a text file;

split – break a file into smaller ones with the specified size;

tr – substitute a group of characters for another throughout a file;

uniq – eliminate duplicate lines from a file;

passwd – change the password;

id – display the user and group ID for a specified username;

su – start a new shell as another user;

sudo – start a new shell as another user;

fdisk – partition a harddrive;

fsck – check and repair a file-system;

mkfs – create a new file-system;

mknod – create a device file;

mkswap – create a swap space for Linux in a file or a harddrive partition;

mount – mount a device on a directory in the file-system;

unmount – unmount a device from the system;

swapoff – deactivate a swap space;

swapon – activate a swap space;

sync – write buffered , saved in memory data, to files;

tty – display the device name for the current terminal;

cal – display the calendar for the specified month/year;

date – display/set date and time;

wc – word count – counts the words in a file / -m counts characters / -l counts lines;

ln – link – creates links between files;

find – searches the file system for the specified file;

locate / slocate – it searches in a database, it’s faster than “find” but not that accurate;

whereis – find files based in the typical directories where executable/binary files are located;

which – find files in the directories listed in PATH environment variables;

yum – used to install/remove packages, update/upgrade system (CentOS);

apt or apt-get – used to install/remove packages, update/upgrade system (Ubuntu);

whoami – shows the current user username;

who – shows the list of usernames;

uname – displays the operating system type, -a switch can be used for more details like distribution/version/kernel/release;

less /proc/cpuinfo – shows CPU hardware information;

cat /proc/meminfo – displays used and free memory information;

apropos – find online manual for a specified keyword;

info – show online help information for a specified command;

man – show online help information for a specified command;

alias – define an abbreviation for a long command;

type – show the type and location of a command;

unalias – delete an abbreviation defined using alias;

bg – run an interrupted process in background;

fg – run a process in foreground;

free – display the amount of free/used memory in file-system;

halt – shutdown Linux and halt computer;

reboot – reboot computer;

shutdown – shutdown computer;

kill – send a signal to a process, usually to close it;

ldd – display the shared libraries needed by a program to run;

nice – run a process with a lower priority;

ps – display a list of current running processes;

pstree – show parent-child process relationships;

top – display a list of processes that uses the most processor and memory resources;

clear – clear the terminal;

netstat – show list of active connections;

printenv – display the current environment variables;

chsh – change the shell;

groups – print the list of groups that include a specified user;

dig/nslookup – show details about a domain;

[updating with new commands…]

Useful links:

Linux Directory Map / Structure

Introduction to Linux

Using Help in Linux

Download and install Oracle VirtualBox

How to install CentOS on Oracle VirtualBox