Different distros have more or less commands than others. Then it also depends on what applications are installed. There is no one anywhere that can give you a list of ALL commands and Functions... :P

Here is some to get you started:

Start using man pages.

a
accept
access
aclocal
aconnect
acpi
acpi_available
acpid
addr2line
addresses
agetty
alsactl
amidi
amixer
anacron
aplay
aplaymidi
apm
apmd
apropos
apt
ar
arch
arecord
arecordmidi
arp
as
at
atd
atq
atrm
audiosend
aumix
autoconf
autoheader
automake
autoreconf
autoscan
autoupdate

b
badblocks
banner
basename
bash
batch
bc
biff
bison
bzcmp
bzdiff
bzgrep
bzip2
bzless
bzmore

c
c++
c++filt
cal
cardctl
cardmgr
cat
cc
cdda2wav
cdparanoia
cdrdao
cdrecord
cfdisk
chage
chattr
chfn
chgrp
chkconfig
chmod
chown
chpasswd
chroot
chrt
chsh
chvt
cksum
clear
cmp
col
colcrt
colrm
column
comm
compress
cp
cpio
cpp
crond
crontab
csplit
ctags
cupsd
cut
cvs

d
date
dd
deallocvt
debugfs
depmod
devdump
df
diff
diff3
dig
dir
dircolors
dirname
disable
dlpsh
dmesg
dnsdomainname
dnssec-keygen
dnssec-makekeyset
dnssec-signkey


dnssec-signzone
doexec
domainname
dosfsck
du
dump
dumpe2fs
dumpkeys

e
e2fsck
e2image
e2label
echo
edquota
egrep
eject
elvtune
emacs
enable
env
envsubst
esd
esd-config
esdcat
esdctl
esddsp
esdmon
esdplay
esdrec
esdsample
etags
ex
expand
expr

f
factor
false
fc-cache
fc-list
fdformat
fdisk
fetchmail
fgconsole
fgrep
file
find
finger
fingerd
flex
fmt
fold
formail
free
fsck
ftp
ftpd
fuser

g
g++
gawk
gcc
gdb
getent
getkeycodes
gpasswd
gpg
gpgsplit
gpgv
gpm
gprof
grep
groff
groffer
groupadd
groupdel
groupmod
groups
grpck
grpconv
gs
gunzip
gzexe
gzip

h
halt
hdparm
head
hexdump
host
hostid
hostname
htdigest
hwclock

i
iconv
id
ifconfig
imapd
inetd
info
init
insmod
install
ipcrm
ipcs
iptables
iptables-restore
iptables-save
isodump
isoinfo
isosize
isovfy
ispell

j
join


k
kbd_mode
kbdrate
kernelversion
kill
killall
killall5
klogd
kudzu

l
last
lastb
lastlog
ld
ldconfig
ldd
less
lesskey
lftp
lftpget
link
ln
loadkeys
locale
locate
lockfile
logger
login
logname
logrotate
look
losetup
lpadmin
lpinfo
lpmove
lpq
lpr
lprm
lpstat
ls
lsattr
lspci
lsmod
lsusb

m
m4
mail
mailq
mailstats
mailto
make
makedbm
makemap
man
manpath
mattrib
mbadblocks
mcat
mcd
mcopy
md5sum
mdel, mdeltree
mdir
mdu
merge
mesg
metamail
metasend
mformat
mimencode
minfo
mkdir
mkdosfs
mke2fs
mkfifo
mkfs
mkfs.ext3
mkisofs
mklost+found
mkmanifest
mknod
mkraid
mkswap
mktemp
mlabel
mmd
mmount
mmove
modinfo
modprobe
more
mount
mountd
mpartition
mpg123
mpg321
mrd
mren
mshowfat
mt
mtools
mtoolstest
mtype
mv
mzip

n
named
namei
nameif
netstat
newaliases
newgrp
newusers
nfsd
nfsstat
nice
nm
nohup
nslookup
nsupdate

o
objcopy
objdump
od
openvt

p
passwd
paste
patch
pathchk
perl
pidof
ping
pinky
pmap
portmap
poweroff
pppd
pr
praliases
printenv
printf
ps
ptx
pwck
pwconv
pwd
python

q
quota
quotacheck
quotaon
quotaoff
quotastats

r
raidstart
ramsize
ranlib
rarpd
rcp
rdate
rdev
rdist
rdistd
readcd
readelf
readlink
reboot
reject
rename
renice
repquota
reset
resize2fs
restore
rev
rexec
rexecd
richtext
rlogin
rlogind
rm
rmail
rmdir
rmmod
rndc
rootflags
route
routed
rpcgen
rpcinfo
rpm
rsh
rshd
rsync
runlevel
rup
ruptime
rusers
rusersd
rwall
rwho
rwhod

s
sane-find-scanner
scanadf
scanimage
scp
screen
script
sdiff
sed
sendmail
sensors
seq
setfdprm
setkeycodes
setleds
setmetamode
setquota
setsid
setterm
sftp
sh
sha1sum
showkey
showmount
shred
shutdown
size
skill
slabtop
slattach
sleep
slocate
snice
sort
split
ssh
ssh-add
ssh-agent
ssh-keygen
ssh-keyscan
sshd
stat
statd
strace
strfile


strings
strip
stty
su
sudo
sum
swapoff
swapon
sync
sysctl
sysklogd
syslogd

t
tac
tail
tailf
talk
talkd
tar
taskset
tcpd
tcpdump
tcpslice
tee
telinit
telnet
telnetd
test
tftp
tftpd
time
tload
tmpwatch
top
touch
tr
tracepath
traceroute
troff
true
tset
tsort
tty
tune2fs
tunelp

u
ul
umount
uname
uncompress
unexpand
unicode_start
unicode_stop
uniq
uptime
useradd
userdel
usermod
users
usleep
uudecode
uuencode
uuidgen

v
vdir
vi
vidmode
vim
vmstat
volname

w
w
wall
warnquota
watch
wc
wget
whatis
whereis
which
who
whoami
whois
write

x
xargs
xinetd

y
yacc
yes
ypbind
ypcat
ypinit
ypmatch
yppasswd
yppasswdd
yppoll
yppush
ypserv
ypset
yptest
ypwhich
ypxfr

z
zcat
zcmp
zdiff
zdump
zforce
zgrep
zic
zless
zmore
znew