Commands and Variables

Commands

apt-get

apt-get install <package>
apt-get install aptitude
aptitude install <package>
aptitude -f install

dpkg-reconfigure

dpkg-reconfigure tzdata
dpkg-reconfigure locales

System management

Date and time

date
date -s 2012-12-25 12:34:56
date -u -s 2012-12-25 04:34:56
hwclock --systohc --utc
hwclock --hctosys
ntpq -p
ntpdate

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Footnotes

[1]	<http://www.howtoforge.com/useful_linux_commands> presents a list of useful Linux commands.

[2]	Bash commands

[3]	A list of Windows commands for comparison.

Operating system commands

Command name	Command function/description
uname	provides information about the computer/system.
wait	wait until process completes and returns a termination status
wget	retrieves web pages and/or files.

File system commands

Command name	Command function/description
ls	LiSts directory information about files
rm	ReMoves/deletes file/folder directory entries
mv	MoVe and/or rename file/folder directory name entries
touch	Update/create file directory timestamp entries
cp	CoPy file contents to new locations
cd	Change the current working Directory
mkdir	MaKes a new DIRectory folder if it doesn’t exist yet
blkid	Lists BLocK device IDentifiers available to the system
mount	Mounts a block device in the directory tree
umount	Unmounts a previously mounted block device

Text editing services

Command name	Command function/description
grep	Global file [Regular Expression] text search and Print
sed	command-based line-oriented System EDitor
awk	command-based AWKward text-oriented editor

Shell scripting commands

Command name	Command function/description
echo
printf
shopt	set SHell OPTions
read	reads a line from standard input; gets user input

Variables

Global environment variables

Environment Variables are those which are predeclared and can be called from any program, script, or terminal.

Variable	Variable description of use.
DISPLAY	The id of the working primary display
HOME	Returns the present working directory
LANGUAGE	Returns the present working directory
LINENO	The line number of the statement being processed
LOGNAME	name of the user logged into the terminal
PATH	The search path for system commands
PWD	Returns the present working directory
SHELL	path to the active shell interpreter
TERM	terminfo database terminal definition
USER	name of the current user (different from LOGNAME due to su)

Tip

Type env at the terminal to list global variables.

Local environment variables

Below is a table of some of the variables exported by the BASH terminal.

Variable	Variable description of use.
BASH	path to BASH command interpreter
COLUMNS	Character columns defined in a terminal line
EUID	The ID number of the current user
IFS	string of characters to be treated as white space
LINES	rows (lines) of characters in a terminal line
MACHTYPE	string describing system type in GNU form CPU-COMPANY-SYSTEM
OSTYPE	string describing the operating system
PS1	The primary prompt string, defaults to “‘s-v$’”
UID	The login ID number of the current login

See also

Bash Beginners Guide

Terminal color settings

Commands echo -e and printf will process embedded escape sequences, in the form \e[xx;yy;zzm, to produce formatted or colored text. For example,

echo -e "\e[1;42;95m Some text \e[0m" ;# print Bold;FG Light Magenta;BG Green

printf is not documented further here, but color commands and codes are.

ANSI/VT100 color codes

Code	Color	|	FG	BG	Color	|	FG	BG	Color
0	Normal	|	30	40	Black	|	90	100	Dark Gray
1	Bold	|	31	41	Red	|	91	101	Light Red
2	Dim	|	32	42	Green	|	92	102	Light Green
		|	33	43	Brown/Orange	|	93	103	Light Yellow
4	Underlined	|	34	44	Blue	|	94	104	Light Blue
5	Blink	|	35	45	Magenta	|	95	105	Light Magenta
		|	36	46	Cyan	|	96	106	Light Cyan
7	Inverted	|	37	47	Light Gray	|	97	107	White
8	Hidden	|				|
		|	39	49	Default	|

Terminfo color commands

Instead of using embedded VT100 codes, the tput command can set terminal colors using a variety of arguments.

tput command arguments

Command	Description
tput setab (color code)	Set background color
tput setaf (color code)	Set foreground color
tput bold	Set bold mode
tput dim	Set half-bright mode
tput smul	Set underline mode
tput rmul	Exit underline mode
tput rev	Reverse mode
tput smso	Set standout mode
tput rmso	Exit standout mode
tput sgr0	Reset all attributes

setaf/setab xterm values

Code	Color	RGB values
0	black	0, 0, 0
1	red	max,0,0
2	green	0,max,0
3	yellow	max,max,0
4	blue	0,0,max
5	magenta	max,0,max
6	cyan	0,max,max
7	white	max,max,max

The tput command uses a terminfo terminal definition, of which the default xterm definition serves the 8 colors listed above. For more colors, you can load xterm-256color. Commands demonstrating this color set follow:

export TERM='xterm-256color'
tput colors
for i in {0..255}
do
  tput setab $i
  echo -n "  $i  "
done
tput setab 0
echo
export TERM='xterm'

or, this could be executed at the terminal as a single command:

export TERM=’xterm-256color’; tput colors; for i in {0..255}; do tput setab $i; echo -n ” $i ”; done; tput setab 0; echo; export TERM=’xterm’

See also

For more information, see the terminfo manpage.

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Variable Scope

Variables may have different scopes in bash shell.

• Sub-shell local variables
• Declared Local Variables
• Exported Environment Session Variables
• Global Environment Variables

Declared Variables

Example Declared Variable ::

THISVARIABLE=”A string”

The value can be changed by re-declaring it.

THISVARIABLE="A new string"

Declared Variables are those which are assigned within a script. The value of a variable and its data type can be changed at any time by re-assigning it.

Global Environment Variables

Global Environment Variables are declared and exported when a shell session is instantiated. A few useful Environment Variables are listed following:

Environment Variables	Variable description of use.
$BASH	full filename used to invoke bash
$EUID	effective user ID (number) of the current user
$GROUPS	array of groups the current user is a member of
$HOSTNAME	the name of the current host
$IFS	characters treated as white space
$LINENO	sequential number of executing line in a script
$MACHTYPE	GNU cpu-company-system machine type description
$OSTYPE	string describing the operating system
$PATH	search path for commands
$PPID	process id of the shell’s parent
$PWD	when called, it returns the current directory
$UID	user ID of the current user

Exported Variables

Type these commands into the terminal:

export NUMBER=12
echo "echo $NUMBER" > test-script.sh
bash ./test-script.sh

The value 12 should be returned, as that is the exported value of NUMBER.

Now, type:

echo "NUMBER=4" > test-script.sh
echo 'echo $NUMBER' >> test-script.sh
bash test-script.sh
echo $NUMBER

Notice that NUMBER is now equal to 4 within *``test-script.sh``, but it retains the value ``12`` at the terminal. If a shell script reassigns an exported variable, the variable is overloaded with a new local version *within the script, but the original assignment is retained in the shell session.

Exported variables must be assigned and then exported. Once exported, any script or command in the current shell session may reference the variable, but the value of the exported variable is cleared when the terminal is closed.