You should know, or at least be aware of all these commands. You don’t need to memorize their usage, you can always read the man pages.

• tail - display the end of a file
• head - display the beggining of a file
• xargs - convert stdin to command line arguments
• nmap - network scanner
• watch (linux) - run a command periodically and watch the output
• watch (freebsd) - monitor a tty
• lsof - lists the open files
• strace (linux) - trace the system calls of a program
• truss (freebsd) - trace the system calls of a program
• calc - command line calculator program
• bc - another command line calculator program
• dc - command line RPN calculator program
• du - how much disk space is being used by something
• df - disk free, how much free space there is
• touch - creates an epty file or updates the timestamp on a file
• diff - show the difference between 2 files
• strings - show the strings in a file (useful for binaries)
• file - tells you what type a file is
• finger - find out information about users
• w / who - who is currently on the machine and what are they doing
• id - tells you who you are
• uname - tells you basic information about the system.
• whois - information about hosts
• crontab - execute commands at specified times
• last - last users to log into the system
• mail - very basic mailer, useful for redirection
• nice/renice - set the niceness (”priority”) level of a program
• nohup - leave a program running after logging off
• screen - virtual terminal for leaving programs running
• psnup - print multiple pages in one page
• time - time the execution of a command
• wc - word count, tells you the size in bytes, words and lines
• which / whereis - locates commands
• locate - locates files
• whatis - gives brief summary of a command
• wget / fetch - get files off the net
• talk - chat with other users on the system
• write / wall - send messages to users of the system
• zcat - cat gzipped files
• ln - link files
• sleep - wait a specified amount of time
• md5sum (linux) - calculate the md5 signature of a file
• md5 (freebsd) - calculate the md5 signature of a file
• enscript - convert text files to ps
• awk, sed - useful scripting programs
• cat, echo, more/less, find, grep - you need to know these, it’s not an option.

• netstat - network information
• ping - check if hosts are up
• tcpdump - look at the traffic going through the network
• nmap - scan hosts
• finger - see info about remote users
• whois - get info about hosts
• ifconfig - configure interfaces
• ifup/ifdown - script/program to enable interfaces

• ispell/aspell - spell checkers
• dict - dictionary tool

• How to run a command to a set of files (The while read command can handle names with spaces but you need to quote the variable after):

[unix]$ for i in `ls *mp3`; do cp $i $i.bak; done
[unix]$ ls *mp3 | while read i; do cp "$i" "$i with more spaces"; done

• How to rename part of a file:

[unix]$ echo filename | sed 's/file/another/'

• How to replace text in multiple files:

[unix]$ sed -e 's/oldtext/newtext/' -i file1 file2 file3 ...

• How to delete lines matching a pattern in some files:

[unix]$ sed -e '/pattern/ d' -i file1 file2 file3 ...

• How to change all letters to lowercase:

[unix]$ echo FilENamE | tr A-Z a-z

• A good way to change all filenames in a directory to lowercase:

This handles a colored ls (hence the quotes around ls), but you can use any file list output (e.g. find) and names with spaces (hence the while read and not the for).

[unix]$ "ls" | while read name; do mv "$name" "`echo $name | tr A-Z a-z`"; done

• How to convert man pages to text files:

[unix]$ man man | col -b

• How to list all the open ports and programs:

[linux]$ netstat -apne --inet
[freebsd]$ sockstat

• How to list all the open files and programs:

[linux]$ lsof
[freebsd]$ fstat

• How to find information about an ip (i.e. 10.0.0.0):

[unix]$ host 10.0.0.0
[unix]$ dig -x 10.0.0.0

• How to find information about an ip owner (i.e. 10.0.0.0):

[unix]$ whois -h whois.arin.net 10.0.0.0

• How to find out the mailserver for an ip:

[unix]$ dig mx some.domain.name.net

• How to generate a filename with yesterdays date (i.e. for logfiles/backups):

[linux]$ touch logfile-`date -dyesterday '+%Y-%m-%d'`
[freebsd]$ touch logfile-`date -v-1d '+%Y-%m-%d'`