Linux Security files and command

/etc/passwd

User account information.
/etc/shadow
Secure user account information.
/etc/group
Group account information.
/etc/gshadow
Secure group account information.
/etc/default/useradd
Default values for account creation.
/etc/skel/
Directory containing default files.
/etc/login.defs
Shadow password suite configuration.

Exit Values
The useradd command exits with the following values:
0 success
1 can't update password file
2 invalid command syntax
3 invalid argument to option
4 UID already in use (and no -o)
6 specified group doesn't exist
9 username already in use
10 can't update group file
12 can't create home directory
13 can't create mail spool

http://linux.die.net/man/8/useradd

http://www.pixelbeat.org/cmdline.html

Command Description

a2p Creates a Perl script from an awk script.

ac Prints statistics about users' connect time.

alias Create a name for another command or long command string.

ar Maintain portable archive or library.

arch Display the architecture of the current host.

arp Manipulate the system ARP cache.

as An assembler.

at Command scheduler.

awk Awk script processing program.

basename Deletes any specified prefix from a string.

bash Command Bourne interpreter

bc Calculator.

bdiff Compare large files.

bfs Editor for large files.

bg Continues a program running in the background.

biff Enable / disable incoming mail notifications.

break Break out of while, for, foreach, or until loop.

bs Battleship game.

bye Alias often used for the exit command.

cal Calendar

calendar Display appointments and reminders.

cancel Cancels a print job.

cat View and/or modify a file.

cc C compiler.

cd Change directory.

chdir Change directory.

checkeq Language processors to assist in describing equations.

checknr Check nroff and troff files for any errors.

chfn Modify your own information or if super user or root modify another users information.

chgrp Change a groups access to a file or directory.

chkey Change the secure RPC key pair.

chmod Change the permission of a file.

chown Change the ownership of a file.

chsh Change login shell.

cksum Display and calculate a CRC for files.

clear Clears screen.

cls Alias often used to clear a screen.

cmp Compare files.

col Reverse line-feeds filter.

comm Compare files and select or reject lines that are common.

compress Compress files on a computer.

continue Break out of while, for, foreach, or until loop.

copy Copy files.

cp Copy files.

cpio Creates archived CPIO files.

crontab Create and list files that you wish to run on a regular schedule.

csh Execute the C shell command interpreter

csplit Split files based on context.

ctags Create a tag file for use with ex and vi.

cu Calls or connects to another Unix system, terminal or non-Unix system.

curl Transfer a URL.

cut Cut out selected fields of each line of a file.

date Tells you the date and time in Unix.

dc An arbitrary precision arithmetic package.

df Display the available disk space for each mount.

deroff Removes nroff/troff, tbl, and eqn constructs.

dhclient Dynamic Host Configuration Protocol Client.

diff Displays two files and prints the lines that are different.

dig DNS lookup utility.

dircmp Lists the different files when comparing directories.

dirname Deliver portions of path names.

dmesg Print or control the kernel ring buffer.

dos2unix Converts text files between DOS and Unix formats.

dpost Translates files created by troff into PostScript.

du Tells you how much space a file occupies.

echo Displays text after echo to the terminal.

ed Line oriented file editor.

edit Text editor.

egrep Search a file for a pattern using full regular expressions.

elm Program command used to send and receive e-mail.

emacs Text editor.

enable Enables / Disables LP printers.

env Displays environment variables.

eqn Language processors to assist in describing equations.

ex Line-editor mode of the vi text editor.

exit Exit from a program, shell or log you out of a Unix network.

expand Expand copies of file s.

expr Evaluate arguments as an expression.

fc The FC utility lists or edits and re-executes, commands previously entered to an interactive sh.

fg Continues a stopped job by running it in the foreground

fgrep Search a file for a fixed-character string.

file Tells you if the object you are looking at is a file or if it is a directory.

find Finds one or more files assuming that you know their approximate filenames.

findsmb List info about machines that respond to SMB name queries on a subnet.

finger Lists information about the user.

fmt Simple text formatters.

fold Filter for folding lines.

for

Shell built-in functions to repeatedly execute action(s) for a selected number of times.

foreach Shell built-in functions to repeatedly execute action(s) for a selected number of times.

fromdos Converts text files between DOS and Unix formats.

fsck Check and repair a Linux file system.

ftp Enables ftp access to another terminal.

getfacl Display discretionary file information.

gprof The gprof utility produces an execution profile of a program.

grep Finds text within a file.

groupadd Creates a new group account.

groupdel Enables a super user or root to remove a group.

groupmod Enables a super user or root to modify a group.

gunzip Expand compressed files.

gview A programmers text editor.

gvim A programmers text editor.

gzip Compress files.

halt Stop the computer.

hash Remove internal hash table.

hashstat Display the hash stats.

head Displays the first ten lines of a file, unless otherwise stated.

help If computer has online help documentation installed this command will display it.

history Display the history of commands typed.

host DNS lookup utility.

hostid Prints the numeric identifier for the current host.

hostname Set or print name of current host system.

id Shows you the numeric user and group ID on BSD.

ifconfig Sets up network interfaces.

ifdown take a network interface down

ifup bring a network interface up

isalist Display the native instruction sets executable on this platform.

jobs List the jobs currently running in the background.

join Joins command forms together.

keylogin Decrypt the user's secret key.

kill Cancels a job.

ksh Korn shell command interpreter.

ld Link-editor for object files.

ldd List dynamic dependencies of executable files or shared objects.

less Opposite of the more command.

lex Generate programs for lexical tasks.

link Calls the link function to create a link to a file.

ln Creates a link to a file.

lo Allows you to exit from a program, shell or log you out of a Unix network.

locate List files in databases that match a pattern.

login Signs into a new system.

logname Returns users login name.

logout Logs out of a system.

lp Prints a file on System V systems.

lpadmin Configure the LP print service.

lpc Line printer control program.

lpq Lists the status of all the available printers.

lpr Submits print requests.

lprm Removes print requests from the print queue.

lpstat Lists status of the LP print services.

ls Lists the contents of a directory.

mach Display the processor type of the current host.

mail One of the ways that allows you to read/send E-Mail.

mailcompat Provide SunOS 4.x compatibility for the Solaris mailbox format.

mailx Mail interactive message processing system.

make Executes a list of shell commands associated with each target.

man Display a manual of a command.

mesg Control if non-root users can send text messages to you.

mii-tool View, manipulate media-independent interface status.

mkdir Create a directory.

mkfs Build a Linux file system, usually a hard disk partition.

more Displays text one screen at a time.

mount Disconnects a file systems and remote resources.

mt Magnetic tape control.

mv Renames a file or moves it from one directory to another directory.

nc TCP/IP swiss army knife.

neqn Language processors to assist in describing equations.

netstat Shows network status.

newalias Install new elm aliases for user and/or system.

newform Change the format of a text file.

newgrp Log into a new group.

nice Invokes a command with an altered scheduling priority.

niscat Display NIS+ tables and objects.

nischmod Change access rights on a NIS+ object.

nischown Change the owner of a NIS+ object.

nischttl Change the time to live value of a NIS+ object.

nisdefaults Display NIS+ default values.

nisgrep Utilities for searching NIS+ tables.

nismatch Utilities for searching NIS+ tables.

nispasswd Change NIS+ password information.

nistbladm NIS+ table administration command.

nmap Network exploration tool and security / port scanner.

nohup Runs a command even if the session is disconnected or the user logs out.

nroff Formats documents for display or line-printer.

nslookup Queries a name server for a host or domain lookup.

on Execute a command on a remote system, but with the local environment.

onintr Shell built-in functions to respond to (hardware) signals.

optisa Determine which variant instruction set is optimal to use.

pack Shrinks file into a compressed file.

pagesize Display the size of a page of memory in bytes, as returned by getpagesize.

passwd Allows you to change your password.

paste Merge corresponding or subsequent lines of files.

pax Read / write and writes lists of the members of archive files and copy directory hierarchies.

pcat Compresses file.

perl Perl is a programming language optimized for scanning arbitrary text files, extracting information from those text files.

pg Files perusal filters for CRTs.

pgrep Examine the active processes on the system and reports the process IDs of the processes

pico Simple and very easy to use text editor in the style of the Pine Composer.

pine Command line program for Internet News and Email.

ping Sends ICMP ECHO_REQUEST packets to network hosts.

pkill Examine the active processes on the system and reports the process IDs of the processes

poweroff Stop the computer.

pr Formats a file to make it look better when printed.

priocntl Display's or set scheduling parameters of specified process(es)

printf Write formatted output.

ps Reports the process status.

pvs Display the internal version information of dynamic objects within an ELF file.

pwd Print the current working directory.

quit Allows you to exit from a program, shell or log you out of a Unix network.

rcp Copies files from one computer to another computer.

reboot Stop the computer.

red Line oriented file editor.

rehash Recomputes the internal hash table of the contents of directories listed in the path.

remsh Runs a command on another computer.

repeat Shell built-in functions to repeatedly execute action(s) for a selected number of times.

rgview A programmers text editor.

rgvim A programmers text editor.

rlogin Establish a remote connection from your terminal to a remote machine.

rm Deletes a file without confirmation (by default).

rmail One of the ways that allows you to read/send E-Mail.

rmdir Deletes a directory.

rn Reads newsgroups.

route Show / manipulate the IP routing table.

rpcinfo Report RPC information.

rsh Runs a command on another computer.

rsync Faster, flexible replacement for rcp.

rview A programmers text editor.

rvim A programmers text editor.

s2p Convert a sed script into a Perl script.

sag Graphically displays the system activity data stored in a binary data file by a previous sar run.

sar Displays the activity for the CPU.

script Records everything printed on your screen.

sdiff Compares two files, side-by-side.

sed Allows you to use pre-recorded commands to make changes to text.

sendmail Sends mail over the Internet.

set Set the value of an environment variable.

setenv Set the value of an environment variable.

setfacl Modify the Access Control List (ACL) for a file or files.

settime Change file access and modification time.

sftp Secure file transfer program.

sh Runs or processes jobs through the Bourne shell.

shred Delete a file securely, first overwriting it to hide its contents.

shutdown Turn off the computer immediately or at a specified time.

sleep Waits a x amount of seconds.

slogin OpenSSH SSH client (remote login program).

smbclient An ftp-like client to access SMB/CIFS resources on servers.

sort Sorts the lines in a text file.

spell Looks through a text file and reports any words that it finds in the text file that are not in the dictionary.

split Split a file into pieces.

stat Display file or filesystem status.

stop Control process execution.

strip Discard symbols from object files.

stty Sets options for your terminal.

su Become super user or another user.

sysinfo Get and set system information strings.

sysklogd Linux system logging utilities.

tabs Set tabs on a terminal.

tail Delivers the last part of the file.

talk Talk with other logged in users.

tac Concatenate and print files in reverse.

tar Create tape archives and add or extract files.

tbl Preprocessor for formatting tables for nroff or troff.

tcopy Copy a magnetic tape.

tcpdump Dump traffic on a network.

tee Read from an input and write to a standard output or file.

telnet Uses the telnet protocol to connect to another remote computer.

time Used to time a simple command.

timex The timex command times a command; reports process data and system activity.

todos Converts text files between DOS and Unix formats.

top Display Linux tasks.

touch Change file access and modification time.

tput Initialize a terminal or query terminfo database.

tr Translate characters.

traceroute Print the route packets take to network host.

troff Typeset or format documents.

ul Reads the named filenames or terminal and does underlining.

umask Get or set the file mode creation mask.

unalias Remove an alias.

unhash Remove internal hash table.

uname Print name of current system.

uncompress Uncompressed compressed files.

uniq Report or filter out repeated lines in a file.

unmount Crates a file systems and remote resources.

unpack Expands a compressed file.

untar Create tape archives and add or extract files.

until Execute a set of actions while/until conditions are evaluated TRUE.

useradd Create a new user or updates default new user information.

userdel Remove a users account.

usermod Modify a users account.

vacation Reply to mail automatically.

vedit Screen-oriented (visual) display editor based on ex.

vgrind Grind nice program listings

vi Screen-oriented (visual) display editor based on ex.

vim A programmers text editor.

view A programmers text editor.

w Show who is logged on and what they are doing.

wait Await process completion.

wc Displays a count of lines, words, and characters in a file

whereis Locate a binary, source, and manual page files for a command.

while Repetitively execute a set of actions while/until conditions are evaluated TRUE.

which Locate a command.

who Displays who is on the system.

whois Internet user name directory service.

write Send a message to another user.

X Execute the X windows system.

xfd Display all the characters in an X font.

xlsfonts Server font list displayer for X.

xset User preference utility for X.

xterm Terminal emulator for X.

xrdb X server resource database utility.

yacc Short for yet another compiler-compiler, yacc is a compiler.

yes Repeatedly output a line with all specified STRING(s), or 'y'.

yppasswd Changes network password in the NIS database.

zcat Compress files.