OK-Fox, Richard - Information Technology _ An Introduction for Today’s Digital World (2013, CRC Press) - libgen.lc

528   ◾    Appendix B
cp source target—copy file(s) listed as source to target. Target is typically a directory

although if source is a single file, target can be a new file name. Two options of
note are –i for interactive mode (prompt the user before overwriting a file that may
already exist) and –r for a recursive copy (if source is a directory, it not only copies
the contents of the directory, but all subdirectories).
echo string—output string to screen. The string can comprise literal values, variables (with
a $ before each variable name) and Linux commands. When placed in ‘’, output
all items literally. When command is placed in ` `, execute command and use its
output as part of the string.
exit—leave the current bash session. If another session exists in the same window, you will
resume it, otherwise the window will close.
fg number—where number is an integer number, it moves the job corresponding to num-
ber to the foreground.
find path options—the find command searches for files that fill some criteria, the path
is the starting point of the search, you might use your home directory (~) or you
might start at root (/). Keep in mind that if you run this program without being
root, some directories and files may not be accessible by you. The options that you
most likely will use are –name string to specify that you are searching for all files
with string in their name followed by –print to output the list of files found.
fsck—file system check, used to check the integrity of the file system.
jobs—list all active processes in the given terminal window, often used along with fg and
bg.
gcc—the GNUs C compiler used to compile programs. g++ is the GNU’s C++ compiler.
grep expression file(s)—match the regular expression to each line of the file, returning any
matches. egrep uses the extended regular expression set. Common options include
–e (same as using egrep), –c to output just the number of matches, –i to ignore the
case of the letters, and –n to output line numbers for each match.
gzip/gunzip filename—zips/unzips filename.
history—display the history list.
ifconfig—may require /sbin/ifconfig—display network interface information for computer
including IP address.
kill signal pid—kill the process of the given process ID using the given signal, signal is
often –9.
less filename—like cat, displays filename to the screen, but pauses at each screen. You can
also use the arrow keys to move up and down in the file. The command more is
similar but cannot step backward.
ln [–s] filename linkname—create a link (symbolic link if –s is used) from linkname to
filename.
ls filenames—list the filenames provided. ls by itself lists all items in the current directory.
ls permits wildcards for filename expansion such as ls *.txt. Common options for
ls are given:
ls –l—long listing
ls –a—include “hidden” files (filenames that start with a dot)

Appendix B: Linux and DOS Information   ◾    529
make—run the makefile script to compile software, options include make all, make install,

make tar.
man command—provide the manual page for the given command.
mkdir dirname—create the directory named dirname in the current directory.
mount physical logical—a command to mount a partition indicated by physical (the actual

name of the file system) to the file system at the location logical. The command
umount is used to unmounts a partition. mount –a mounts all partitions found in
the file /etc/fstab.
mv source destination—move the item source to destination. If destination is a filename,
then this is a rename operation; otherwise, the file is moved to the new directory.
nice value command—execute command with the niceness value of value. Can also adjust
a running process using nice –n value PID.
nslookup alias—query the local DNS for the IP address of the given alias. Can also be
called as nslookup alias server if you want to specify a different DNS.
passwd username—prompt the user to change username’s password. If username is omit-
ted, then change the current user’s password. Unless you are root, you will be
required to enter username’s password before changing it. Options include:
-l—lock the account so that only root can access it
-u—unlock the account
-d—delete the password for the account
-x—establish an expiration date for the password (a date by which the password
must be changed)
-w—establish the number of days in advance that the user will be warned to change
passwords
ps—process status information, that is, output the running processes. There are many
options available. The ps command by itself only prints processes running in the
current terminal window owned by the current user. Other options of note include:
a—print processes of all users of the current window
f—denote parent-child relationships using ASCII “art” output
u—user “user-oriented format” for output (see below)
x—print processes no matter which window or console it originated from
Using option u outputs processes by user name, PID, %CPU usage (the percentage
of time it has been running versus waiting), %memory usage (as a percentage of the
computer’s main memory capacity), total amount of memory usage, virtual memory
utilization, the console that started the process, the process’ execution status, the date
that the process started, the amount of CPU time it has used, and the process’ name.
pwd—print working directory.
Redirection—although this is not a Linux command, you can use redirection in your
Linux commands in order to alter the standard input or output of a command.
The redirection operators are
>—redirect output to the given file as in cat foo1.txt foot2.txt > foo3.txt.
>>—redirect output to append to the given file.
<—redirect input from keyboard to the given file.

530   ◾    Appendix B
<<—redirect input from file to keyboard, end input after given string is reached, as
in cat << quit > foo.txt, which will allow the user to enter items until the string
“quit” is entered.
|—a pipe, take the output of one Linux command and use it as input to another,
such as ls –al | egrep “rwx”, which takes the output of the ls –al command and
uses it as input to the egrep command.

rm item(s)—remove (delete) the item(s). Options include –r for a recursive delete (this
deletes subdirectories and their contents recursively), –f to delete without permis-
sion, and –i to delete interactively—that is, to ask the user before deleting each
item.

rmdir item—remove the given directory. This is only available if the directory is empty.
service servicename command—command to change the service servicename. The com-

mand is one of start, stop, status, and restart.
ssh host—open a secure shell communication with host using encryption. This has replaced

telnet, which opens communication with a host but without encryption.
su [username]—switch to username’s account. If username is omitted, switch to root. The

instruction requires that you provide the account’s password (unless you are cur-
rently root).
sudo username command—literally, this command executes command as username, that
is, it executes the specified command as if it was issued by the given username.
Commonly, this command is used without username so that the command exe-
cutes as if issued by root. An entry must be placed in the file /etc/sudoers that
specifies that the given user has access to the specified command. For instance,
if zappaf were permitted to use useradd, the following line would be added to the
suoders file:
zappaf localhost=/usr/sbin/useradd
Before the command executes, the user is required to enter their password.
tar filename [source]—the tape archive utility bundles one or more files and directories
into a single file. When the options –xf are used, the files/directories are extracted
from the bundle and copied into the current directory. When used with –cf, a
new tar file is created from the entry(ies) listed under source. For instance, tar –cf
foo.tar /myfiles would take all of the files in the subdirectory myfiles along with
the myfiles directory and place them into foo.tar. An added option, z, will gzip or
gunzip the files (gzip when used with –c and gunzip when used with –x). –c stands
for create, –x for extract.
useradd options username—create a new account with the name username. This instruc-
tion can only be executed by root. The instruction adds associated entries in /etc/
passwd, /etc/shadow, and /etc/group. The command has many options including:
-m—create a home directory
-d dir—use dir the in place of the default directory
-s shell—use shell in place of the default log in shell

Appendix B: Linux and DOS Information   ◾    531

-G groups—add user to the list of groups (the list is separated by commas but no
spaces)

-c comment—add comment to the /etc/passwd file, usually used to specify the
user’s full name

-p password—use the encrypted password for an initial password
-u uid—give the user the specified uid (user ID)
userdel username—delete username’s account. The option –f forces the deletion even if
the user is still logged in and –r removes the user’s home directory and e-mail files
along with the user account.
usermod options username—modifies username’s account based on the options specified.
These options are the same as in useradd.
vi—launch the vi text editor (also vim). Emacs launches the Emacs text editor.
wget URL—download the file specified by the URL. Similar to entering an http request in
a web browser except that the file is stored to disk.
who—lists all usernames of currently logged in users.
whoami—outputs the current user’s username. This is useful if you cannot remember who
you are if you have used su.
yum command package—the Yellowdog Updater, Modifier program is used to install,
remove, and update software packages. Command is usually one of install, update,
check-update, upgrade, remove, list, info, clean, or reinstall. The yum program
uses the rpm program.

Linux Files and Directories of Note
/etc/group—file that stores all group information.
/etc/init.d—a Linux directory where services are stored.
/etc/inittab—the initialization script first executed after the Linux operating system boots,

it is responsible for setting the runlevel.
/etc/passwd—file that stores all user account information (excluding user passwords).
/etc/rc#.d—a set of directories in Linux which stores symbolic links to services that should

be started or stopped at system initialization time. The # is the run-level, one of
0–6.
/etc/rc.sysinit—a script executed during system initialization that starts and stops services.
/etc/shadow—file that stores all user passwords in an encrypted form.
/etc/syslog.conf—the configuration file for the syslogd service that logs application soft-
ware and non-kernel operating system messages.

DOS Commands of Note
cd or chdir—change directories, used with a path as in cd ..\foo\bar. See also C:, D:, etc.
chkdsk—checks the specified disk for file system integrity.
cls—clear the screen.
copy—copy a file from one location to another in the file system. Wildcards are permissible.
C:—change to the C: partition. D:—change to the D: partition. This also applies for any

other partitions.

532   ◾    Appendix B
del (also erase)—delete the given file(s). Wildcards are permissible.
dir—list the contents of the current directory or the given directory if a path is supplied.
echo—output the given string and/or variables to the screen.
edit—launch the DOS text editor program.
exit—can be used to exit the DOS window if running DOS within windows.
format—to format a disk. WARNING: formatting a disk erases its contents.
help—gives general help on DOS. help command lists command’s help page.
md (also mkdir)—create a new directory.
mem—display memory usage information.
more—displays contents of a file, one screen at a time.
move—move a file to a new location. If the new location is the same directory, this per-

forms a rename. The ren instruction is a rename instruction.
path—display the contents of the path variable. Can also be used to set new paths in the

variable.
print—sends a specified file to a printer.
rd (also rmdir)—remove an empty directory.
undel—undelete a previously deleted file. This works on files that are “recoverable” only.

A deleted file may or may not be recoverable based on the amount of time that has
elapsed since the deletion.