jvmops/bash-cheatsheet.sh

## bash-cheatsheet.sh
#####################################################
# Author: sdwsk
# Date: 2015/12/19
#####################################################

# 0.1 Directory structure.

/dev/                 # devices
/etc/                 # system wide configuration files
/etc/rsyslog.conf     # log config
/proc/                # files used by a kernel
/bin/                 # binaries - used by everyone
/sbin/                # system binaries - used by system admin
/tmp/                 # temporary files
/usr/                 # programs and libraries installed and used by users (not required by system)
                      # if you install programs from source they will prob go here to bin, sbin, etc
/usr/local/           # if you install programs from yum/apt they will prob go here
/opt/                 # as well as /usr/local/ contain files not belonging to the OS but ops is a directory where
                      # to install unbundled packages, each in its own subdir
/var/                 # files whose content varies while the system is running (logs, etc)


# 0.2 Shortcuts.

CTRL+_  # undo

CTRL+A  # move to beginning of line
CTRL+E  # moves to end of line

CTRL+B  # moves backward one character
CTRL+F  # moves forward one character
CTRL+T  # transposes two characters

CTRL+K  # deletes (kill) forward to end of line
CTRL+W  # kills the word behind the cursor

CTRL+L  # clears screen and redisplay the line

CTRL+R  # searches backward
CTRL+S  # searches forward
CTRL+P  # previous line in command history
CTRL+N  # next line in command history
CTRL+O  # same as RETURN, then displays next line in history file

CTRL+Y  # retrieves (yank) last item killed
CTRL+Z  # stops the current command, resume with fg in the foreground or bg in the background

DELETE  # deletes one character backward
!!      # repeats the last command
exit    # logs out of current session

SHIFT + F4   # (in file explorer) opens terminal where pwd = folder

# 1. Bash Basics.


export              # displays all environment variables
echo $SHELL         # displays the shell you're using
echo $BASH_VERSION  # displays bash version
whereis bash        # finds out where bash is on your system
cd -                # takes you to the previous working directory
cd ~<username>      # takes you to the user's home directory


# 1.1. File Commands.

# ls                          # lists your files
  -t                          # sort files by the modification time
  -s                          # sort files by size
  -sr                         # -r reverses the sort (ascending -> descending)
  -F                          # adds trailing / to the folder name
ls a??                        # maches files that starts with a, and are followed by two characters
ls a*                         # maches files that starts with a, and are followed by any number of characters

mkdir -p ./some/path/; mv yourfile.txt $_
                              # moves file to newly created directory
cp -a <filename> <target>     # copy file and preserve permissions
   -r                         # recursive copy
   -u                         # copy only files that either do not exist in target or are newer
file <filename>               # info abount a file
ln -s <filename> <link>       # creates symbolic link to file
less <filename>               # shows the first part of a file (move with space/b, for find type "/<phrase>" and type q to quit)
                              # more is an older version of less utility (2000 vs 27000 lines of code more/less)
lsblk                         # list out information about all block devices (hard drives, flash drives)
sudo fdisk -l                 # ^ same as above but more info
emacs <filename>              # lets you create and edit a file
diff <filename1> <filename2>  # compares files, and shows where they differ
wc <filename>                 # tells you how many lines, words and characters there are in a file
chmod -options <filename>     # lets you change the read, write, and execute permissions on your files
gzip <filename>               # compresses files
gunzip <filename>             # uncompresses files compressed by gzip
gzcat <filename>              # lets you look at gzipped file without actually having to gunzip it
lpr <filename>                # print the file
lpq                           # check out the printer queue
lprm <jobnumber>              # remove something from the printer queue
genscript                     # converts plain text files into postscript for printing and gives you some options for formatting
dvips <filename>              # print .dvi files (i.e. files produced by LaTeX)
grep <pattern> <filenames>    # looks for the string in the files
grep -r <pattern> <dir>       # search recursively for pattern in directory


# 1.2. Directory Commands.


mkdir <dirname>  # makes a new directory
cd               # changes to home
cd <dirname>     # changes directory
pwd              # tells you where you currently are


# 1.3. SSH, System Info & Network Commands.


ssh user@host            # connects to host as user
ssh -p <port> user@host  # connects to host on specified port as user
ssh-copy-id user@host    # adds your ssh key to host for user to enable a keyed or passwordless login

whoami                   # returns your username
passwd                   # lets you change your password
quota -v                 # shows what your disk quota is
date                     # shows the current date and time
cal                      # shows the month's calendar
uptime                   # shows current uptime
w                        # displays whois online
finger <user>            # displays information about user
uname -a                 # shows kernel information
man <command>            # shows the manual for specified command
df                       # shows disk usage
du <filename>            # shows the disk usage of the files and directories in filename (du -s give only a total)
last <yourUsername>      # lists your last logins
ps -u yourusername       # lists your processes
kill <PID>               # kills (ends) the processes with the ID you gave
killall <processname>    # kill all processes with the name
top                      # displays your currently active processes
bg                       # lists stopped or background jobs ; resume a stopped job in the background
fg                       # brings the most recent job in the foreground
fg <job>                 # brings job to the foreground

ping <host>              # pings host and outputs results
whois <domain>           # gets whois information for domain
dig <domain>             # gets DNS information for domain
dig -x <host>            # reverses lookup host
wget <file>              # downloads file


# 2. Basic Shell Programming.


# 2.1. Variables.


varname=value                # defines a variable
varname=value command        # defines a variable to be in the environment of a particular subprocess
echo $varname                # checks a variable's value
echo $$                      # prints process ID of the current shell
echo $!                      # prints process ID of the most recently invoked background job
echo $?                      # displays the exit status of the last command
export VARNAME=value         # defines an environment variable (will be available in subprocesses)

array[0] = val               # several ways to define an array
array[1] = val
array[2] = val
array=([2]=val [0]=val [1]=val)
array(val val val)

${array[i]}                  # displays array's value for this index. If no index is supplied, array element 0 is assumed
${#array[i]}                 # to find out the length of any element in the array
${#array[@]}                 # to find out how many values there are in the array

declare -a                   # the variables are treaded as arrays
declare -f                   # uses funtion names only
declare -F                   # displays function names without definitions
declare -i                   # the variables are treaded as integers
declare -r                   # makes the variables read-only
declare -x                   # marks the variables for export via the environment

${varname:-word}             # if varname exists and isn't null, return its value; otherwise return word
${varname:=word}             # if varname exists and isn't null, return its value; otherwise set it word and then return its value
${varname:?message}          # if varname exists and isn't null, return its value; otherwise print varname, followed by message and abort the current command or script
${varname:+word}             # if varname exists and isn't null, return word; otherwise return null
${varname:offset:length}     # performs substring expansion. It returns the substring of $varname starting at offset and up to length characters

${variable#pattern}          # if the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest
${variable##pattern}         # if the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest
${variable%pattern}          # if the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest
${variable%%pattern}         # if the pattern matches the end of the variable's value, delete the longest part that matches and return the rest
${variable/pattern/string}   # the longest match to pattern in variable is replaced by string. Only the first match is replaced
${variable//pattern/string}  # the longest match to pattern in variable is replaced by string. All matches are replaced

${#varname}                  # returns the length of the value of the variable as a character string

*(patternlist)               # matches zero or more occurences of the given patterns
+(patternlist)               # matches one or more occurences of the given patterns
?(patternlist)               # matches zero or one occurence of the given patterns
@(patternlist)               # matches exactly one of the given patterns
!(patternlist)               # matches anything except one of the given patterns

$(UNIX command)              # command substitution: runs the command and returns standard output


# 2.2. Functions.
# The function refers to passed arguments by position (as if they were positional parameters), that is, $1, $2, and so forth.
# $@ is equal to "$1" "$2"... "$N", where N is the number of positional parameters. $# holds the number of positional parameters.


functname() {
  shell commands
}

unset -f functname  # deletes a function definition
declare -f          # displays all defined functions in your login session


# 2.3. Flow Control.


statement1 && statement2  # and operator
statement1 || statement2  # or operator

-a                        # and operator inside a test conditional expression
-o                        # or operator inside a test conditional expression

str1=str2                 # str1 matches str2
str1!=str2                # str1 does not match str2
str1<str2                 # str1 is less than str2
str1>str2                 # str1 is greater than str2
-n str1                   # str1 is not null (has length greater than 0)
-z str1                   # str1 is null (has length 0)

-a file                   # file exists
-d file                   # file exists and is a directory
-e file                   # file exists; same -a
-f file                   # file exists and is a regular file (i.e., not a directory or other special type of file)
-r file                   # you have read permission
-r file                   # file exists and is not empty
-w file                   # your have write permission
-x file                   # you have execute permission on file, or directory search permission if it is a directory
-N file                   # file was modified since it was last read
-O file                   # you own file
-G file                   # file's group ID matches yours (or one of yours, if you are in multiple groups)
file1 -nt file2           # file1 is newer than file2
file1 -ot file2           # file1 is older than file2

-lt                       # less than
-le                       # less than or equal
-eq                       # equal
-ge                       # greater than or equal
-gt                       # greater than
-ne                       # not equal

if condition
then
  statements
[elif condition
  then statements...]
[else
  statements]
fi

for x := 1 to 10 do
begin
  statements
end

for name [in list]
do
  statements that can use $name
done

for (( initialisation ; ending condition ; update ))
do
  statements...
done

case expression in
  pattern1 )
    statements ;;
  pattern2 )
    statements ;;
  ...
esac

select name [in list]
do
  statements that can use $name
done

while condition; do
  statements
done

until condition; do
  statements
done


# 3. Command-Line Processing Cycle.


# The default order for command lookup is functions, followed by built-ins, with scripts and executables last.
# There are three built-ins that you can use to override this order: `command`, `builtin` and `enable`.

command  # removes alias and function lookup. Only built-ins and commands found in the search path are executed
builtin  # looks up only built-in commands, ignoring functions and commands found in PATH
enable   # enables and disables shell built-ins

eval     # takes arguments and run them through the command-line processing steps all over again


# 4. Input/Output Redirectors.


cmd1|cmd2  # pipe; takes standard output of cmd1 as standard input to cmd2
> file     # directs standard output to file
< file     # takes standard input from file
>> file    # directs standard output to file; append to file if it already exists
>|file     # forces standard output to file even if noclobber is set
n>|file    # forces output to file from file descriptor n even if noclobber is set
<> file    # uses file as both standard input and standard output
n<>file    # uses file as both input and output for file descriptor n
<<label    # here-document
n>file     # directs file descriptor n to file
n<file     # takes file descriptor n from file
n>>file    # directs file description n to file; append to file if it already exists
n>&        # duplicates standard output to file descriptor n
n<&        # duplicates standard input from file descriptor n
n>&m       # file descriptor n is made to be a copy of the output file descriptor
n<&m       # file descriptor n is made to be a copy of the input file descriptor
&>file     # directs standard output and standard error to file
<&-        # closes the standard input
>&-        # closes the standard output
n>&-       # closes the ouput from file descriptor n
n<&-       # closes the input from file descripor n


# 5. Process Handling.


# To suspend a job, type CTRL+Z while it is running. You can also suspend a job with CTRL+Y.
# This is slightly different from CTRL+Z in that the process is only stopped when it attempts to read input from terminal.
# Of course, to interupt a job, type CTRL+C.

myCommand &  # runs job in the background and prompts back the shell

jobs         # lists all jobs (use with -l to see associated PID)

fg           # brings a background job into the foreground
fg %+        # brings most recently invoked background job
fg %-        # brings second most recently invoked background job
fg %N        # brings job number N
fg %string   # brings job whose command begins with string
fg %?string  # brings job whose command contains string

kill -l      # returns a list of all signals on the system, by name and number
kill PID     # terminates process with specified PID

ps           # prints a line of information about the current running login shell and any processes running under it
ps -a        # selects all processes with a tty except session leaders

trap cmd sig1 sig2  # executes a command when a signal is received by the script
trap "" sig1 sig2   # ignores that signals
trap - sig1 sig2    # resets the action taken when the signal is received to the default

disown <PID|JID>    # removes the process from the list of jobs

wait                # waits until all background jobs have finished


# 6. Tips and Tricks.


# set an alias
cd; nano .bash_profile
> alias gentlenode='ssh admin@gentlenode.com -p 3404'  # add your alias in .bash_profile

# to quickly go to a specific directory
cd; nano .bashrc
> shopt -s cdable_vars
> export websites="/Users/mac/Documents/websites"

source .bashrc
cd websites


# 7. Debugging Shell Programs.


bash -n scriptname  # don't run commands; check for syntax errors only
set -o noexec       # alternative (set option in script)

bash -v scriptname  # echo commands before running them
set -o verbose      # alternative (set option in script)

bash -x scriptname  # echo commands after command-line processing
set -o xtrace       # alternative (set option in script)

trap 'echo $varname' EXIT  # useful when you want to print out the values of variables at the point that your script exits

function errtrap {
  es=$?
  echo "ERROR line $1: Command exited with status $es."
}

trap 'errtrap $LINENO' ERR  # is run whenever a command in the surrounding script or function exists with non-zero status

function dbgtrap {
  echo "badvar is $badvar"
}

trap dbgtrap DEBUG  # causes the trap code to be executed before every statement in a function or script
# ...section of code in which the problem occurs...
trap - DEBUG  # turn off the DEBUG trap

function returntrap {
  echo "A return occured"
}

trap returntrap RETURN  # is executed each time a shell function or a script executed with the . or source commands finishes executing


## sdwsk_cheatsheet.md

      
    Raw
  

              sdwsk_cheatsheet.md
            
          
    Linux

https://linuxjourney.com/
http://patrakov.blogspot.com/2011/01/writing-systemd-service-files.html
http://vimgifs.com/?utm_content=buffera01fc&utm_medium=social&utm_source=facebook.com&utm_campaign=buffer

Aliases

To create aliases:
alias [alias]="[command and options]"
alias ll="ls -l"

To remove aliases:
unalias ll

Adding repo to apt

sudo add-apt-repository ppa:colingille/freshlight
sudo apt-get update
sudo apt-get install winusb

rsync

syncing examples
-df <- list mounted devices
sudo rsync -ah --progress --delete --compress-level=0 --inplace /home/ /media/backups/home

rsync -aAXn --info=progress2 --exclude={"/dev/*","/proc/*","/sys/*","/tmp/*","/run/*","/mnt/*","/media/*","/lost+found"} /* /path/to/backup/folder

Read command's folder location

http://stackoverflow.com/a/5251365
	#####################################################
	# Author: sdwsk
	# Date: 2015/12/19
	#####################################################

	# 0.1 Directory structure.

	/dev/ # devices
	/etc/ # system wide configuration files
	/etc/rsyslog.conf # log config
	/proc/ # files used by a kernel
	/bin/ # binaries - used by everyone
	/sbin/ # system binaries - used by system admin
	/tmp/ # temporary files
	/usr/ # programs and libraries installed and used by users (not required by system)
	# if you install programs from source they will prob go here to bin, sbin, etc
	/usr/local/ # if you install programs from yum/apt they will prob go here
	/opt/ # as well as /usr/local/ contain files not belonging to the OS but ops is a directory where
	# to install unbundled packages, each in its own subdir
	/var/ # files whose content varies while the system is running (logs, etc)


	# 0.2 Shortcuts.

	CTRL+_ # undo

	CTRL+A # move to beginning of line
	CTRL+E # moves to end of line

	CTRL+B # moves backward one character
	CTRL+F # moves forward one character
	CTRL+T # transposes two characters

	CTRL+K # deletes (kill) forward to end of line
	CTRL+W # kills the word behind the cursor

	CTRL+L # clears screen and redisplay the line

	CTRL+R # searches backward
	CTRL+S # searches forward
	CTRL+P # previous line in command history
	CTRL+N # next line in command history
	CTRL+O # same as RETURN, then displays next line in history file

	CTRL+Y # retrieves (yank) last item killed
	CTRL+Z # stops the current command, resume with fg in the foreground or bg in the background

	DELETE # deletes one character backward
	!! # repeats the last command
	exit # logs out of current session

	SHIFT + F4 # (in file explorer) opens terminal where pwd = folder

	# 1. Bash Basics.


	export # displays all environment variables
	echo $SHELL # displays the shell you're using
	echo $BASH_VERSION # displays bash version
	whereis bash # finds out where bash is on your system
	cd - # takes you to the previous working directory
	cd ~<username> # takes you to the user's home directory


	# 1.1. File Commands.

	# ls # lists your files
	-t # sort files by the modification time
	-s # sort files by size
	-sr # -r reverses the sort (ascending -> descending)
	-F # adds trailing / to the folder name
	ls a?? # maches files that starts with a, and are followed by two characters
	ls a* # maches files that starts with a, and are followed by any number of characters

	mkdir -p ./some/path/; mv yourfile.txt $_
	# moves file to newly created directory
	cp -a <filename> <target> # copy file and preserve permissions
	-r # recursive copy
	-u # copy only files that either do not exist in target or are newer
	file <filename> # info abount a file
	ln -s <filename> <link> # creates symbolic link to file
	less <filename> # shows the first part of a file (move with space/b, for find type "/<phrase>" and type q to quit)
	# more is an older version of less utility (2000 vs 27000 lines of code more/less)
	lsblk # list out information about all block devices (hard drives, flash drives)
	sudo fdisk -l # ^ same as above but more info
	emacs <filename> # lets you create and edit a file
	diff <filename1> <filename2> # compares files, and shows where they differ
	wc <filename> # tells you how many lines, words and characters there are in a file
	chmod -options <filename> # lets you change the read, write, and execute permissions on your files
	gzip <filename> # compresses files
	gunzip <filename> # uncompresses files compressed by gzip
	gzcat <filename> # lets you look at gzipped file without actually having to gunzip it
	lpr <filename> # print the file
	lpq # check out the printer queue
	lprm <jobnumber> # remove something from the printer queue
	genscript # converts plain text files into postscript for printing and gives you some options for formatting
	dvips <filename> # print .dvi files (i.e. files produced by LaTeX)
	grep <pattern> <filenames> # looks for the string in the files
	grep -r <pattern> <dir> # search recursively for pattern in directory


	# 1.2. Directory Commands.


	mkdir <dirname> # makes a new directory
	cd # changes to home
	cd <dirname> # changes directory
	pwd # tells you where you currently are


	# 1.3. SSH, System Info & Network Commands.


	ssh user@host # connects to host as user
	ssh -p <port> user@host # connects to host on specified port as user
	ssh-copy-id user@host # adds your ssh key to host for user to enable a keyed or passwordless login

	whoami # returns your username
	passwd # lets you change your password
	quota -v # shows what your disk quota is
	date # shows the current date and time
	cal # shows the month's calendar
	uptime # shows current uptime
	w # displays whois online
	finger <user> # displays information about user
	uname -a # shows kernel information
	man <command> # shows the manual for specified command
	df # shows disk usage
	du <filename> # shows the disk usage of the files and directories in filename (du -s give only a total)
	last <yourUsername> # lists your last logins
	ps -u yourusername # lists your processes
	kill <PID> # kills (ends) the processes with the ID you gave
	killall <processname> # kill all processes with the name
	top # displays your currently active processes
	bg # lists stopped or background jobs ; resume a stopped job in the background
	fg # brings the most recent job in the foreground
	fg <job> # brings job to the foreground

	ping <host> # pings host and outputs results
	whois <domain> # gets whois information for domain
	dig <domain> # gets DNS information for domain
	dig -x <host> # reverses lookup host
	wget <file> # downloads file


	# 2. Basic Shell Programming.


	# 2.1. Variables.


	varname=value # defines a variable
	varname=value command # defines a variable to be in the environment of a particular subprocess
	echo $varname # checks a variable's value
	echo $$ # prints process ID of the current shell
	echo $! # prints process ID of the most recently invoked background job
	echo $? # displays the exit status of the last command
	export VARNAME=value # defines an environment variable (will be available in subprocesses)

	array[0] = val # several ways to define an array
	array[1] = val
	array[2] = val
	array=([2]=val [0]=val [1]=val)
	array(val val val)

	${array[i]} # displays array's value for this index. If no index is supplied, array element 0 is assumed
	${#array[i]} # to find out the length of any element in the array
	${#array[@]} # to find out how many values there are in the array

	declare -a # the variables are treaded as arrays
	declare -f # uses funtion names only
	declare -F # displays function names without definitions
	declare -i # the variables are treaded as integers
	declare -r # makes the variables read-only
	declare -x # marks the variables for export via the environment

	${varname:-word} # if varname exists and isn't null, return its value; otherwise return word
	${varname:=word} # if varname exists and isn't null, return its value; otherwise set it word and then return its value
	${varname:?message} # if varname exists and isn't null, return its value; otherwise print varname, followed by message and abort the current command or script
	${varname:+word} # if varname exists and isn't null, return word; otherwise return null
	${varname:offset:length} # performs substring expansion. It returns the substring of $varname starting at offset and up to length characters

	${variable#pattern} # if the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest
	${variable##pattern} # if the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest
	${variable%pattern} # if the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest
	${variable%%pattern} # if the pattern matches the end of the variable's value, delete the longest part that matches and return the rest
	${variable/pattern/string} # the longest match to pattern in variable is replaced by string. Only the first match is replaced
	${variable//pattern/string} # the longest match to pattern in variable is replaced by string. All matches are replaced

	${#varname} # returns the length of the value of the variable as a character string

	*(patternlist) # matches zero or more occurences of the given patterns
	+(patternlist) # matches one or more occurences of the given patterns
	?(patternlist) # matches zero or one occurence of the given patterns
	@(patternlist) # matches exactly one of the given patterns
	!(patternlist) # matches anything except one of the given patterns

	$(UNIX command) # command substitution: runs the command and returns standard output


	# 2.2. Functions.
	# The function refers to passed arguments by position (as if they were positional parameters), that is, $1, $2, and so forth.
	# $@ is equal to "$1" "$2"... "$N", where N is the number of positional parameters. $# holds the number of positional parameters.


	functname() {
	shell commands
	}

	unset -f functname # deletes a function definition
	declare -f # displays all defined functions in your login session


	# 2.3. Flow Control.


	statement1 && statement2 # and operator
	statement1 \|\| statement2 # or operator

	-a # and operator inside a test conditional expression
	-o # or operator inside a test conditional expression

	str1=str2 # str1 matches str2
	str1!=str2 # str1 does not match str2
	str1<str2 # str1 is less than str2
	str1>str2 # str1 is greater than str2
	-n str1 # str1 is not null (has length greater than 0)
	-z str1 # str1 is null (has length 0)

	-a file # file exists
	-d file # file exists and is a directory
	-e file # file exists; same -a
	-f file # file exists and is a regular file (i.e., not a directory or other special type of file)
	-r file # you have read permission
	-r file # file exists and is not empty
	-w file # your have write permission
	-x file # you have execute permission on file, or directory search permission if it is a directory
	-N file # file was modified since it was last read
	-O file # you own file
	-G file # file's group ID matches yours (or one of yours, if you are in multiple groups)
	file1 -nt file2 # file1 is newer than file2
	file1 -ot file2 # file1 is older than file2

	-lt # less than
	-le # less than or equal
	-eq # equal
	-ge # greater than or equal
	-gt # greater than
	-ne # not equal

	if condition
	then
	statements
	[elif condition
	then statements...]
	[else
	statements]
	fi

	for x := 1 to 10 do
	begin
	statements
	end

	for name [in list]
	do
	statements that can use $name
	done

	for (( initialisation ; ending condition ; update ))
	do
	statements...
	done

	case expression in
	pattern1 )
	statements ;;
	pattern2 )
	statements ;;
	...
	esac

	select name [in list]
	do
	statements that can use $name
	done

	while condition; do
	statements
	done

	until condition; do
	statements
	done


	# 3. Command-Line Processing Cycle.


	# The default order for command lookup is functions, followed by built-ins, with scripts and executables last.
	# There are three built-ins that you can use to override this order: `command`, `builtin` and `enable`.

	command # removes alias and function lookup. Only built-ins and commands found in the search path are executed
	builtin # looks up only built-in commands, ignoring functions and commands found in PATH
	enable # enables and disables shell built-ins

	eval # takes arguments and run them through the command-line processing steps all over again


	# 4. Input/Output Redirectors.


	cmd1\|cmd2 # pipe; takes standard output of cmd1 as standard input to cmd2
	> file # directs standard output to file
	< file # takes standard input from file
	>> file # directs standard output to file; append to file if it already exists
	>\|file # forces standard output to file even if noclobber is set
	n>\|file # forces output to file from file descriptor n even if noclobber is set
	<> file # uses file as both standard input and standard output
	n<>file # uses file as both input and output for file descriptor n
	<<label # here-document
	n>file # directs file descriptor n to file
	n<file # takes file descriptor n from file
	n>>file # directs file description n to file; append to file if it already exists
	n>& # duplicates standard output to file descriptor n
	n<& # duplicates standard input from file descriptor n
	n>&m # file descriptor n is made to be a copy of the output file descriptor
	n<&m # file descriptor n is made to be a copy of the input file descriptor
	&>file # directs standard output and standard error to file
	<&- # closes the standard input
	>&- # closes the standard output
	n>&- # closes the ouput from file descriptor n
	n<&- # closes the input from file descripor n


	# 5. Process Handling.


	# To suspend a job, type CTRL+Z while it is running. You can also suspend a job with CTRL+Y.
	# This is slightly different from CTRL+Z in that the process is only stopped when it attempts to read input from terminal.
	# Of course, to interupt a job, type CTRL+C.

	myCommand & # runs job in the background and prompts back the shell

	jobs # lists all jobs (use with -l to see associated PID)

	fg # brings a background job into the foreground
	fg %+ # brings most recently invoked background job
	fg %- # brings second most recently invoked background job
	fg %N # brings job number N
	fg %string # brings job whose command begins with string
	fg %?string # brings job whose command contains string

	kill -l # returns a list of all signals on the system, by name and number
	kill PID # terminates process with specified PID

	ps # prints a line of information about the current running login shell and any processes running under it
	ps -a # selects all processes with a tty except session leaders

	trap cmd sig1 sig2 # executes a command when a signal is received by the script
	trap "" sig1 sig2 # ignores that signals
	trap - sig1 sig2 # resets the action taken when the signal is received to the default

	disown <PID\|JID> # removes the process from the list of jobs

	wait # waits until all background jobs have finished


	# 6. Tips and Tricks.


	# set an alias
	cd; nano .bash_profile
	> alias gentlenode='ssh admin@gentlenode.com -p 3404' # add your alias in .bash_profile

	# to quickly go to a specific directory
	cd; nano .bashrc
	> shopt -s cdable_vars
	> export websites="/Users/mac/Documents/websites"

	source .bashrc
	cd websites


	# 7. Debugging Shell Programs.


	bash -n scriptname # don't run commands; check for syntax errors only
	set -o noexec # alternative (set option in script)

	bash -v scriptname # echo commands before running them
	set -o verbose # alternative (set option in script)

	bash -x scriptname # echo commands after command-line processing
	set -o xtrace # alternative (set option in script)

	trap 'echo $varname' EXIT # useful when you want to print out the values of variables at the point that your script exits

	function errtrap {
	es=$?
	echo "ERROR line $1: Command exited with status $es."
	}

	trap 'errtrap $LINENO' ERR # is run whenever a command in the surrounding script or function exists with non-zero status

	function dbgtrap {
	echo "badvar is $badvar"
	}

	trap dbgtrap DEBUG # causes the trap code to be executed before every statement in a function or script
	# ...section of code in which the problem occurs...
	trap - DEBUG # turn off the DEBUG trap

	function returntrap {
	echo "A return occured"
	}

	trap returntrap RETURN # is executed each time a shell function or a script executed with the . or source commands finishes executing