tr a A < filename.txt = replaces a with A
tr -d a < filename.txt = deletes all a characters
uname = operating system name
uname -a = architecture type
uname -r = Linux version
uname -o = operating system name
uname -m = system architecture
uname -n = host name
uname -p = processor architecture
uname -s = os name
uname -v = Linux update time
uname –hardware-platform
uuidgen = strange key
ulimit -a = limit on a variety of system resources
unmount /mnt/cdrom = unmounts cd rom
uptime = how long the computer has been up and running
unix2dos abc.txt xyz.txt = converts file into dos format
/usr/share/background/images = database of background images
/var/log/lastlog = login database, open with vi etc
/var/log/wtmp = history of all logins and attempts
vmstat = virtual memory usage
vmstat 5 = updates virtual memory every 5 seconds
vmstat -k = virtual memory in 1000-byte kilobytes
vmstat -K = virtual memory in 1024-byte kilobytes
vmstat -m = virtual memory in 1000-kilobyte megabytes
vmstat -M = virtual memory in 1024-kilobyte megabytes
vmstat -m | less = kernel memory slab
vmstat -S M -s | less = displays statictics in megabytes
watch ‘cat /proc/loadavg’ = check command output every 2 seconds, press Ctl+C to stop…
watch -n 10 ‘ls -l’ = checks command ouput every 10 seconds
watch ‘ls -l download.jpg’ = checks file size as it is being downloaded
wc -w filename = no. of words in a file
wc -l filename = no of lines
whatis php = info about php modules
whatis php-pdo = info about php-pdo module
whereis php = locations of php binary, source and man pages
whereis -m php = location/path of the php manual
whereis -m php | awk ‘{print $2}’ = 2nd part of the command output
which php = full path of executables/shell commands
who | sort = names of users logged in
who | wc -l = no of users
whoami = who is logged in
who = who is logged in