There is an online-documentation available: man cmd or apropos keyword

System Administration Commands (root only)

Command Meaning
shutdown brings the system down in a secure way
reboot brings the system down in a secure way and then up again
halt depending on parameter it stops the system, reboots or takes the power off
init brings the system to the specified runlevel, that means, reboots the system and executes the commands in the corresponding /etc/rcn.d/-directory - this may differ for example for Linux;
rc ... resource control
poweroff brings the system down and takes the power off
useradd administer a new user login on the system, adds an entry to /etc/passwd and /etc/shadow, that can be modified by vipw or usermod
userdel removes a user login from the system

File and Directory Management Commands

Command Meaning DOS equivalent
cat concatenate and display type
cd, chdir change working directory cd
chmod change file mode (access permission) attrib
chgrp change group ID of a file
chown change owner of a file
cp copy file(s) copy
file determine file type
find find files
head display first lines of file (file heading)
ln [-s] make (symbolic) links (BSD)
ls list directory content dir
mkdir create (make) a directory md
more page output to screen or display file pagewise type file | more
mv move or rename a file or directory ren
pg page output to screen or display file pagewise like more
rm [-r] remove files (and directories recursively) del [deltree]
rmdir remove (empty) directories rd
tail display last lines of file (file tail)
tee I/O redirection: read stdin and write to stdout and file(s)

Attention: There is no undelete in UNIX! (use rm, mv and cp with -i and set noclobber!)

And there is no way to find the date or time a file was created, you only can find the date or time a file was last modified (by using ls -l)!

Text Processing Commands

Command Meaning
awk search text for patterns and perform operations (Authors: Aho, Weinberger and Kernighan)
diff list differences between two files
echo [-n] [\c] sends arguments back to stdout (without NEWLINE)
grep global regular expression printer: search a file for a pattern
mail send and receive e-mails
news write system news items to stdout
sed stream editor (noninteractive)
sort sort or merge files
spell find spelling errors
split split large files
uniq report repeated lines in a file (lies that are not unique)
vi visual: screen editor
wc word, character and line counter
write send (write) messages to other users who are logged on

System Status Commands

Command Meaning
at run command(s) at a later time
cmp compare two files
crontab modifies or displays the cron table
date give back the system date and time
df disk free capacity
du disk usage
env display the current environment variables
exit terminate (exit) current shell (= logout in login shell)
finger general information about users
mesg [yn] deny or accept messages
nice run a command at a different priority
passwd changes the current password
ps display process status (e.g. ps auxww | grep pattern)
pwd print working directory
stty set (or display) terminal characteristics (terminal type)
tset terminal set program
tty display special device corresponding to current terminal
which, whereis gives pathname corresponding to command
who, w who is logged onto UNIX

Software Development Commands

Command Meaning
ar archive and library maintainer
cc C compiler
fc, f77 FORTAN (77) compiler
make maintains, updates, and regenerates groups of programs
nm lists defined names in object files
size size of an object

Common Environment Variables

CDPATH (cdpath) any specifies shorthand names for commonly accessed directories
ENV K specifies the pathname of the secondary startup file (also known as the environment file). The default is .kshrc (Korn shell)
HISTSIZE K specifies the number of commands to maintain in the history file
HOME (home) any your home directory
MAIL (mail) any name of the file in which the system stores your mail
MAILCHECK B value specifying how often system checks mail
MAILPATH B specifies a directory; if mail arrives in that directory, the mail program notifies user
PATH (path) any specifies the search path for executable files
PSl (prompt) any value of the shell prompt
PS2 B,K value of the secondary shell prompt.The C shell does not let you modify the secondary prompt
PWD (cwd) any value of the current working directory
SHELL (shell) any pathname of the shell (used when certain utilities provide a shell escape)
TERM any specifies your type of terminal
TMOUT K specifies the number of seconds before the shell times out and logs out the user in the absence of keyboard input
TZ (time) any sets your time zone
VISUAL K specifies the editor used for interactive command line editing


  $ cd
  $ pwd
  $ ls
  F90 bin help.txt info pub util work
  $ ls -a
  .  bin help.txt pub work
  .. F90 info     util
  $ ls -l
  drwxr----- 3 peter edvz 3340 Apr 22  2005 F90
  drwxr--r-- 2 peter edvz 1024 Apr 23  2005 bin
  -rw-rw-r-- 1 root  edvz 425  Apr 22  2005 help.txt
  -rw-rw---- 1 peter edvz 1256 Apr 22  2004 info
  drwxrwxrwx 2 peter edvz 1024 Apr 22  2005 pub
  drwx------ 2 peter edvz 1024 Apr 22  2005 util
  drwxr----- 2 peter edvz 1024 Sep  8 10:13 work 
  $ chmod 770 util
  $ ls -l util
  drwxrwx--- 2 peter edvz 1024 Apr 22  2005 util
  $ mv help.txt READ.ME
  $ ls
  F90  READ.ME bin info pub util work
  $ chmod g-w,o-r READ.ME
  $ ls -l READ.ME
  -rw-r----- 1 peter edvz 425 Apr 22  2005 READ.ME

As you can see, there are 2 modes for chmod to change the file mode: