Directories
pwd: print working directory
cd <directory>: change directory to <directory>
cd ..: change directory to parent one
ls: list directory contents
ls -la: list detailed directory contents, including hidden files
mkdir <directory>: create new directory named <directory>
Output
cat <file>: output the contents of <file>
less <file>: output the contents of <file> using the less command(which supports paginations etc.)
head <file>: output the first 10 lines of <file>
<cmd> <file>: impose <cmd> on <file>
clear: clear the command line window
Files
rm <file>: delete <file>
rm -r <directory>: clear contents of <directory>
rm -f <file>: force-delete <file>
rm -rf <directory>: force clear <directory>
mv <file-old> <file-new>: rename <file-old>
mv <file> <directory>: move file to <directory>
cp <file> <directory>: copy <file> to <directory>
cp -r <directory1> <directory2>: copy whole <directory1> to <directory2>(overwrit)
touch <file>: Update file or create file
Permissions
chmod 755 <file>: change permission of <file> to 755
chmod -R 600 <directory>: change permission of <directory> and its content to 600
chown <user>:<group> <file>: Change ownership of <file> to <user> and <group>
Search
find <dir> -name ‘<file>’: find all files named <file> inside <dir>(support regex)
grep ‘<text>’ <file>: output all occurrence of <text> inside <file>(add -i for case-insensitivity)
grep -rl ‘text’ <dir>: search for all files container <text> inside <dir>
Network
ping <host>: ping <host> and display status
whois <domain>: output whois information for <domain>
curl -O <url/to/file>: download <file>(via HTTP[S] or FTP)
ssh <username>@<host>: establish an SSH connection to <host> with user <username>
scp <file> <user>@<host>:/remote/path: Copy <file> to a remote <host>
Processes
ps ax: output currently running process
top: display live information about currently running processes
kill <pid>: Quit process with ID