To find a program not on this list (and there are hundreds), try the man program with the -k option followed by a keyword, for example,
man -k tape
to find all on-line manual entries that refer to programs that work with tapes.

Terminal control characters for C-shell (csh)

^h, backspace	erase previously typed character
^u	erase entire line of input so far typed
^d	end-of-input for programs reading from terminal
^s	suspend writing to terminal (freezes terminal)
^q	continue writing to terminal
^z	suspend currently running job; restart with bg or fg
^c	kill currently running program and allow clean-up before exiting
^\	emergency kill of currently running program with no chance of cleanup

Also see a list of special characters that should not be used in filenames.

Login and authentication

login	access computer; start interactive session
logout	disconnect terminal session
passwd	change local login password; you must set a strong password that is not easily guessed
kinit	obtain kerberos ticket for connections to other kerberized computers
kdestroy	destroy kerberos tickets (authorizations)

Information

date	show date and time
history	list of previously executed commands
man	show online documentation by program name
info	online documentation for GNU programs
w, who	who is on the system and what they are doing
whoami	who is logged onto this terminal
top	show system stats and top CPU using processes
uptime	show one line summary of system status

File management

cat	combine files
cp	copy files
ls	list files in a directory and their attributes
mv	change file name or directory location
rm	remove files
ln	create another link (name) to a file
chmod	set file permissions
crypt	encode/decode a file with a private key
gzip, gunzip	compress/decompress a file
find	find files that match specific criteria

Display contents of files

cat	copy files to display device
more	show text file on display terminal with paging control
head	show first few lines of a file(s)
tail	show last few lines of a file; or reverse line order
vi	full-featured screen editor for modifying text files
pico	simple screen editor for modifying text files
grep	display lines that match a pattern
lpr	send file to printer
pr	format file with page headers, multiple columns, etc.
diff	compare two files and show differences
cmp	compare two binary files and report if different
comm	compare two files; show common or unique lines
od	display binary files as eqivalent octal/hex codes
strings	show printable text embedded in binary files
file	examine file(s) and guess type: text, data, program, etc.
wc	count characters, words, and lines in a file

Directories

cd	change to new directory
mkdir	create new directory
rmdir	remove empty directory (you must remove files first)
mv	change name of directory
pwd	show current directory

Disks

df	summarize free space on disk filesystems
du	show disk space used by files or directories

Special character handling for C-shell (See man csh)

*	match any characters in a file name
~user	shorthand for home directory of user
$name	substitute value of variable name
\	turn off special meaning of character that follows
‘	in pairs, quote string with special chars, except !
“	in pairs, quote string with special chars, except !, $
`	in pairs, substitute output from enclosed command

Controlling program execution for C-shell (See man csh)

&	run job in background
^c	kill job in foreground
^z	suspend job in foreground
fg	restart suspended job in foreground
bg	run suspended job in background
;	delimit commands on same line
()	group commands on same line
!	re-run earlier commands from history list
jobs	list current jobs
ps	show process information
kill	kill background job or previous process
nice	run program at lower priority
at	run program at a later time
crontab	run program at specified intervals
limit	see or set resource limits for programs
alias	create alias name for program (normally used in .login file)
sh, csh	execute command file

Controlling program input/output for C-shell (See man csh)

|	pipe output to input
>	redirect output to a storage file
<	redirect input from a storage file
>>	append redirected output to a storage file
tee	copy input to both file and next program in pipe
script	make file record of all terminal activity

Email and communication

msgs	read system bulletin board messages
mailx	send/read email; can be run by other programs to send exisiting files via email
uuencode uudecode	encode/decode a binary file for transmission via email
rn	read USENET news groups

Editors and formatting utilities

sed	programmable text editor for data streams
vi	full-featured editor for character terminals
emacs	GNU emacs editor for character terminals
xemacs	GNU emacs editor for X Window terminals
pico	very simple text editor
fmt	fill and break lines to make all same length
fold	break long lines to specified length

X Window client programs (output to X terminal or server)

xterm	provide login shell window
xauth	manipulate authorization files
xload	show system load
xman	full screen online manual viewer
xemacs	GNU emacs editor
gv	interface to contol gs to display PostScript or PDF files on screen
xdvi	display DVI files on X Window (screen preview)
gnuplot	interactive data plotting on screen

TeX typesetting system

tex	process TeX files to DVI (device independent) output
latex	process LaTeX files to DVI
dvips	print DVI files on Postscript laser printer
xdvi	display DVI files on X Window (screen preview)
latex2html	translate LaTeX files to HTML (for web pages)

Printing (BSD based)

lpr	send file to print queue
lpq	examine status of files in print queue
lprm	remove a file from print queue
enscript	convert text files to PostScript format for printing

Interpreted languages and data manipulation utilities

sed	programmable text editor for data streams
awk	pattern scanning and processing language
perl	Practical Extraction and Report Language
sort	sort or merge lines in a file(s) by specified fields
tr	translate characters
cut	cut out columns from a file
paste	paste columns into a file
dd	copy data between devices; reblock; convert EBCDIC

Graphics and mapping

gnuplot	interactive data plotting; outputs to PostScript or X Window
gs	“ghostscript” converter displays PostScript files on X Window displays or other devices

Networking/communications

ssh	remote login/command execution; encrypted
scp	remote non-interactive file copy; encrypted
sftp	remote interactive file copy; encrypted
telnet	remote network login; plain text password – not recommended
ftp	network file transfer program; plain text password – not recommended
host	find IP address for given host name, or vice versa
lynx	web browser for character based (text-only) terminals
gzip, gunzip	compress/decompress a file
tar	combine multiple files/dirs into single archive
uuencode uudecode	encode/decode a binary file for transmission via email

Compilers, interpreters and programming tools

csh	command language interpreter (C-shell scripts)
ksh	command language interpreter (Korn-shell scripts)
sh	command language interpreter (Borne-shell scripts)
f77	Fortran 77 compiler
f2c	convert fortran source code to C source code
gcc	GNU C compiler
g++	GNU C++ compiler
dbx	command-line symbolic debugger for compiled C or Fortran
make	recompile programs from modified source
cflow	generate C flow graph

Programming libraries (see man library_name)

lapack	Fortran 77 routines for numerical linear algebra (supersedes LINPACK and EISPACK)
X	routines to interface with X window system (no man page — get the X Toolkit book)
dbm	database routines
xdr	library routines for external data representation
netcdf	routines for machine independent data representation

Tape manipulation and archiving

mt	manipulate tape drive and position tape
dd	unformatted tape read and write; file conversion
tar	archive disk files on tape or disk
ltf	read/write ANSI standard label tapes