cal - list a calendar page.

cat file[s] - concatenates contents of files to standard out. Options to show end of line and non-visible characters such as tabs.

date - show current (or specified) date in a chosen format.

df - disk free, overall disk usage of partition (filesystem).

du [directory] - total disk usage of files in targets directory tree.

fg - bring background jobs into foreground.

finger [user] - lists users logged on, where they've logged in from, when they logged in, what their terminal is, their plans (if posted), and if they have unread email. If specific user targeted, lists most of the above info, even if user is not currently logged in.

head - list 1st 10 lines of file. Options to vary line count. Useful for checking doc section on code.

id [user] - lists specified user's id and any groups he/she belongs to.

jobs - list background jobs in current shell.

od - octal dump shows octal value of each character in file. Options to show value in hex, decimal, character, or mixed.

ps - list processes. Several options to change scope of list.

pstree - process tree

tail - lists last 10 lines of file. Options to vary lines shown. Useful for checking log files that are constantly growing.

time - time a process. Lists time in user's environment, in the systems environment, and the actual time to complete.

top - list processes consuming most resources on system.

whereis - show path to specified command.

whoami - just user's id.

who am i - who applied specificly to user.

who - show who is logged on and from where.

w [user] - lists users' login info and what computing resources they are consuming.

less - file content browser with ability to move easily back and forth in file and to search for specific text.

diff - compares two similar files and lists difference.

comm - compares two similar files and creates a three column list of similarities and differences.

pr - formats the contents of a text file for printing, can control number of lines per page, add page numbering, headers, arrange on columns, etc. Output sent to standard out.

grep - global regular expression parser - search utility with a 'language' that is used to describe target text using symbolic terms.

aspell -c file - interactive spell checker.

sort - sorts file or standard input and sends to standard output. Options to vary exactly how sort is performed.

stty - set tty - configures current login shell to use/recognize features of terminal equipment being used by user.

scp - secure copy - used to securely copy files from one system to another. Requires user to be known on remote system.

find start-loc search-criteria action - very powerful search command. Looks for files anywhere in the directory structure under start-location using specified search criteria and taking specified action when a match is found.

ar - archive, takes a specified collection of files and combines them into a signgle file, often called a library. Often used to create code libraries. Does not handle directory structures.

tar - tape archive, archives into a single file all files found under a specified starting point in the filesystem. Does recognize directories and will record path info as part of the archive. Extensively used for backups and for move who directory structures to other systems.