Some common commands that I can't always remember, but may also be helpful to anyone running linux.

Commands

You can type help, but it seems to be generally useless. You gotta know the commands. 

• ls -al: like dir on DOS.
• man command - manual pages for command
• tail -n XX filename - the last XX lines of filename. main log is stored at /var/log
• head filename - first XX lines of filename
• ps -A - to see all processes by all users (-a for all by your user, without subcommand to get just your shell). use ps -aux to list all the running processes. ps pid where pid is the Process ID (found using the previous command, you get additional useless info).
• kill XXX - to kill a process. to get a process ID (XXX), look in /var/run or run ps -A. /etc/rc.d/init.d has startup and stop scripts (for RedHat). Or kill pid. Or kill -f pid.
•  

Problems?: See the /var/log/messages file. Use tail -n XX /var/log/messages to see the last XX lines of the logfile. I've also setup an alias command for logtail and loghead for 25 lines each.

Managing Users and Groups

• id username - shows info on the username including groups, etc.
• groups username - shows info on the groups belonging to. similar to id
• useradd, userdel, usermod - manages users
• groupadd, groupdel, groupmod - manages groups
• passwd - update a user's authentication token(s)
• last - shows the last X users logged into the server; can use last -f <filename> to see others previous. lastb shows the last bad login attempts (if setup).

VI/VIM Commands

• :help - open help file (Ctrl-] to surf; Ctrl-T or -O to go back); :q to close help file and return to editor
• :w - write
• :q - quit; :q! quit without saving
• :wq - write and quit
• u - undo
• Ctrl-R or :red - redo
• /pattern - highlight all matching pattern (regex)
• /pattern/x - jump to xth match
• / - move to next match
• ? - move to previous match
• :[range]s/pattern/string/[g][c] - replace pattern with string; g is global; c requires confirmation; a range of 1,$ will do whole file
• i - insert (before cursor)
• a - append (after cursor)
• x - delete character
• dd - delete line
• . - repeat command
• N yy - yank N lines into register
• :reg - shows registers

Find

Part of Findutils: Website | Documentation

Finding and fixing permissions for files/directories:

find . -type f -not -perm 644 | xargs chmod 644
find . -type d -not -perm 755 | xargs chmod 755
chmod 444 mainfile.php