An A-Z Index of common Mac OS X commands.

a
alias     Create an alias
alloc     List used and free memory
awk       Find and Replace text within file(s)
b
basename  Convert a full pathname to just a folder path
bash      Bourne-Again SHell (Linux)
bless     Set volume bootability and startup disk options.
break     Exit from a loop
c
cal       Display a calendar
case      Conditionally perform a command
cat       Display the contents of a file
cd        Change Directory
chflags   Change a file or folder's flags.
chgrp     Change group ownership
chmod     Change access permissions
chown     Change file owner and group
chroot    Run a command with a different root directory
cksum     Print CRC checksum and byte counts
clear     Clear terminal screen
cmp       Compare two files
comm      Compare two sorted files line by line
complete  Edit a command completion [word/pattern/list]
continue  Resume the next iteration of a loop
cp        Copy one or more files to another location
cron      Daemon to execute scheduled commands
crontab   Schedule a command to run at a later date/time
cut       Divide a file into several parts
d
date      Display or change the date & time
dc        Desk Calculator
dd        Data Dump - Convert and copy a file
defaults  The Mac OS X user defaults system.
df        Display free disk space
diff      Display the differences between two files
diff3     Show differences among three files
dig       DNS lookup
dirname   Convert a full pathname to just a path
dirs      Display list of remembered directories
diskutil  Disk utilities - Format, Verify, Repair
ditto     Copy files and folders
dot_clean Remove dot-underscore files
dscacheutil Query or flush the Directory Service/DNS cache
dscl      Directory Service command line utility
du        Estimate file space usage
e
echo      Display message on screen
ed        A line-oriented text editor (edlin)
enable    Stop or start printers and classes.
env       Set environment and run a utility
eval      Evaluate several commands/arguments
exec      Execute a command
exit      Exit the shell
expect    Programmed dialogue with interactive programs
          Also see AppleScript
expand    Convert tabs to spaces
expr      Evaluate expressions
f
false     Do nothing, unsuccessfully
fdisk     Partition table manipulator for Darwin UFS/HFS/DOS
find      Search for files that meet a desired criteria
fmt       Reformat paragraph text
fold      Wrap text to fit a specified width
for       Expand words, and execute commands
foreach   Loop, expand words, and execute commands
fsck      Filesystem consistency check and repair
fsaclctl  Filesystem enable/disable ACL support
fs_usage  Filesystem usage (process/pathname)
ftp       Internet file transfer program
g
GetFileInfo Get attributes of HFS+ files
getopt    Parse positional parameters
goto      Jump to label and continue execution
grep      Search file(s) for lines that match a given pattern
groups    Print group names a user is in
gzip      Compress or decompress files
h
head      Display the first lines of a file
hdiutil   Manipulate iso disk images
history   Command History
hostname  Print or set system name
i
id        Print user and group names/id's
if        Conditionally perform a command
info      Help info
install   Copy files and set attributes
j
jobs      List active jobs
join      Join lines on a common field
k
kextfind  List kernel extensions
kill      Stop a process from running
l
l         List files in long format (ls -l)
ll        List files in long format, showing invisible files (ls -la)
less      Display output one screen at a time
ln        Make links between files (hard links, symbolic links)
locate    Find files
logname   Print current login name
login     log into the computer
logout    Exit a login shell (bye)
lpr       Print files
lprm      Remove jobs from the print queue
lpstat    Printer status information
ls        List information about file(s)
lsbom     List a bill of materials file
lsof      List open files
m
man       Help manual
mkdir     Create new folder(s)
mkfifo    Make FIFOs (named pipes)
more      Display output one screen at a time
mount     Mount a file system
mv        Move or rename files or directories
n
net       Manage network resources
networksetup Network and System Preferences
nice      Set the priority of a command
nohup     Run a command immune to hangups
o
onintr    Control the action of a shell interrupt
open      Open a file/folder/URL/Application
osascript Execute AppleScript
p
passwd    Modify a user password
paste     Merge lines of files
pbcopy    Copy data to the clipboard
pbpaste   Paste data from the Clipboard
pico      Simple text editor
ping      Test a network connection
pkgutil   List installed packages
pmset     Power Management settings
popd      Restore the previous value of the current directory
pr        Convert text files for printing
printenv  Print environment variables
printf    Format and print data
ps        Process status
pushd     Save and then change the current directory
pwd       Print Working Directory
q
quota     Display disk usage and limits
r
rcp       Copy files between machines.
repeat    Execute a command multiple times
rm        Remove files
rmdir     Remove folder(s)
rpm       Remote Package Manager
rsync     Remote file copy - sync file tree
s
say       Convert text to audible speech
sched     Schedule a command to run at a later time.
screen    Multiplex terminal, run remote shells via ssh
screencapture Capture screen image to file or disk
sdiff     Merge two files interactively
security  Administer Keychains, keys, certificates and the Security framework
sed       Stream Editor
set       Set a shell variable = value
setenv    Set an environment variable = value
setfile   Set attributes of HFS+ files
shift     Shift positional parameters
shutdown  Shutdown or restart OS X
sleep     Delay for a specified time
softwareupdate System software update tool
sort      Sort text files
split     Split a file into fixed-size pieces
stop      Stop a job or process
su        Substitute user identity
sudo      Execute a command as another user
sum       Print a checksum for a file
switch    Conditionally perform a command
systemsetup Computer and display system settings
t
tail      Output the last part of files
tar       Tape ARchiver
tee       Redirect output to multiple files
test      Condition evaluation
textutil  Manipulate text files in various formats
time      Measure Program Resource Use
touch     Change file timestamps
traceroute Trace Route to Host
tr        Translate, squeeze, and/or delete characters
true      Do nothing, successfully
tty       Print filename of terminal on stdin
type      Describe a command
u
umask     Users file creation mask
umount    a device
unalias   Remove an alias
uname     Print system information
unexpand  Convert spaces to tabs
uniq      Uniquify files
units     Convert units from one scale to another
unset     Remove variable or function names
unsetenv  Remove environment variable
users     Print login names of users currently logged in
uuencode  Encode a binary file 
uudecode  Decode a file created by uuencode
v
vi        Text Editor
w
wc        Print byte, word, and line counts
where     Report all known instances of a command
which     Locate a program file in the user's path
while     Execute commands
who       Print all usernames currently logged on
whoami    Print the current user id and name (`id -un')
x,y
xargs     Execute utility - passing arguments
yes       Print a string until interrupted