Simple admin commands

Here are some simple sysadmin commands:

• To create accounts for users (the nice way), the command is adduser <user> which will create an entry in the etc/passwd file and add a home directory for that user (and probably some other stuff, too...)
• userdel -r <user> delete accounts for users (the nice way) (-r means delete the home directory as well)
• passwd <user> change a user's password
  
• uptime tell how long a system has been running
• du -h <file/directory> display a human-readable estimation of how much disk space something is taking
• df -h display a human-readable report of file system disk space usage
  
• ps report a snapshot of the current processes
• ps aux (username, process ID, %CPU, %MEM, virtual memory used (pages), physical memory used, TTY, STAT (process state), date of start, time of start, command)
• pstree display a tree of processes
• arp display IP->MAC translation tables
• acpi display battery status
• shutdown -h now shut down, power down
• uname print system information (kernel version, processor, etc)
• whereis locate the binary, source, and manual page files for a command
• locate find file names on the whole system FAST (if it's not finding a new file, run updatedb to update locate's index and then run locate again)
  
• last -a show a listing of users that have logged into the system and from where
• screen screen manager (very useful for doing heavy duty stuff on remote systems... see tip #7 of this post for details)
• free -m display free and total memory in MB (RAM and swap)
• fuser -v <file> show the user, PID, access type and command accessing of an open file (v for verbose)
• lsof <file> for even more information than fuser
  

Important files/places:

• /proc directory that contains a lot of hardware and system information