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cheatsheet
Example
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#!/bin/bash
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​
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NAME="Payam"
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echo "Hello $NAME!"
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​
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exit 0
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Variables:
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varname=value                # defines a variable
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varname=value command        # defines a variable to be in the environment of a particular subprocess
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echo $varname                # checks a variable's value
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read <varname>               # reads a string from the input and assigns it to a variable 
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let <varname> = <equation>   # performs mathematical calculation using operators like +, -, *, /, %
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export VARNAME=value         # defines an environment variable (will be available in subprocesses)
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#Special shell variables
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echo $                      # prints process ID of the current shell
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echo $!                      # prints process ID of the most recently invoked background job
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echo $?                      # displays the exit status of the last command
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echo $0                      # display Filename of the shell script
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Quoting:
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\c             #Take character c literally.
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`cmd`          #Run cmd and replace it in the line of code with its output.
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"whatever"     #Take whatever literally, after first interpreting $, `...`, \
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'whatever'     #Take whatever absolutely literally.
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​
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#Example:
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match=`ls *.bak`        #Puts names of .bak files into shell variable match.
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echo \*                 #Echos * to screen, not all filename as in:  echo *
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echo '$1$2hello'        #Writes literally $1$2hello on screen.
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echo "$1$2hello"        #Writes value of parameters 1 and 2 and string hello.
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Redirection
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python hello.py > output.txt   # stdout to (file)
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python hello.py >> output.txt  # stdout to (file), append
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python hello.py 2> error.log   # stderr to (file)
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python hello.py 2>&1           # stderr to stdout
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python hello.py 2>/dev/null    # stderr to (null)
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python hello.py &>/dev/null    # stdout and stderr to (null)
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python hello.py < foo.txt      # feed foo.txt to stdin for python
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Brace expansion
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{A,B}	Same as A B
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{A,B}.js	Same as A.js B.js
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{1..5}	Same as 1 2 3 4 5
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Parameter expansions
Basics
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name="John"
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echo ${name}
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echo ${name/J/j}    #=> "john" (substitution)
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echo ${name:0:2}    #=> "Jo" (slicing)
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echo ${name::2}     #=> "Jo" (slicing)
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echo ${name::-1}    #=> "Joh" (slicing)
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echo ${name:(-1)}   #=> "n" (slicing from right)
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echo ${name:(-2):1} #=> "h" (slicing from right)
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echo ${food:-Cake}  #=> $food or "Cake"
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​
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length=2
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echo ${name:0:length}  #=> "Jo"
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STR="/path/to/foo.cpp"
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echo ${STR%.cpp}    # /path/to/foo
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echo ${STR%.cpp}.o  # /path/to/foo.o
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echo ${STR%/*}      # /path/to
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​
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echo ${STR##*.}     # cpp (extension)
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echo ${STR##*/}     # foo.cpp (basepath)
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​
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echo ${STR#*/}      # path/to/foo.cpp
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echo ${STR##*/}     # foo.cpp
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​
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echo ${STR/foo/bar} # /path/to/bar.cpp
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STR="Hello world"
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echo ${STR:6:5}   # "world"
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echo ${STR: -5:5}  # "world"
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SRC="/path/to/foo.cpp"
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BASE=${SRC##*/}   #=> "foo.cpp" (basepath)
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DIR=${SRC%$BASE}  #=> "/path/to/" (dirpath)
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Substitution
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${FOO%suffix}	Remove suffix
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${FOO#prefix}	Remove prefix
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${FOO%%suffix}	Remove long suffix
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${FOO##prefix}	Remove long prefix
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${FOO/from/to}	Replace first match
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${FOO//from/to}	Replace all
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${FOO/%from/to}	Replace suffix
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${FOO/#from/to}	Replace prefix
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Length
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${#FOO}	Length of $FOO
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Default Values
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${FOO:-val}	$FOO, or val if unset (or null)
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${FOO:=val}	Set $FOO to val if unset (or null)
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${FOO:+val}	val if $FOO is set (and not null)
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${FOO:?message}	Show error message and exit if $FOO is unset (or null)
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​
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#Omitting the : removes the (non)nullity checks, 
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#e.g. ${FOO-val} expands to val if unset otherwise $FOO.
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Comment
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# Single line comment
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: '
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This is a
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multi line
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comment
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'
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Substrings
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${FOO:0:3}	Substring (position, length)
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${FOO:(-3):3}	Substring from the right
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Manipulations
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STR="HELLO WORLD!"
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echo ${STR,}   #=> "hELLO WORLD!" (lowercase 1st letter)
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echo ${STR,,}  #=> "hello world!" (all lowercase)
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​
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STR="hello world!"
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echo ${STR^}   #=> "Hello world!" (uppercase 1st letter)
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echo ${STR^^}  #=> "HELLO WORLD!" (all uppercase)
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Conditionals:
Test Operators 
In Bash, the test command takes one of the following syntax forms:
test EXPRESSION
[ EXPRESSION ]
[[ EXPRESSION ]]
To make the script portable, prefer using the old test [ command which is available on all POSIX shells. The new upgraded version of the test command [[ (double brackets) is supported on most modern systems using Bash, Zsh, and Ksh as a default shell.  To negate the test expression, use the logical NOT (!) operator.
Checking Numbers
Note that a shell variable could contain a string that represents a number. If you want to check the numerical value use one of the following:
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[[ NUM -eq NUM ]]	Equal
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[[ NUM -ne NUM ]]	Not equal
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[[ NUM -lt NUM ]]	Less than
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[[ NUM -le NUM ]]	Less than or equal
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[[ NUM -gt NUM ]]	Greater than
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[[ NUM -ge NUM ]]	Greater than or equal
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Checking Strings
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[[ -z STRING ]]	Empty string
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[[ -n STRING ]]	Not empty string
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[[ STRING == STRING ]]	Equal
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[[ STRING != STRING ]]	Not Equal
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​
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Checking files
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[[ -e FILE ]]	Exists
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[[ -r FILE ]]	Readable
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[[ -h FILE ]]	Symlink
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[[ -d FILE ]]	Directory
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[[ -w FILE ]]	Writable
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[[ -s FILE ]]	Size is > 0 bytes
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[[ -f FILE ]]	File
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[[ -x FILE ]]	Executable
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[[ FILE1 -nt FILE2 ]]	1 is more recent than 2
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[[ FILE1 -ot FILE2 ]]	2 is more recent than 1
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[[ FILE1 -ef FILE2 ]]	Same files
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More conditions:
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[[ -o noclobber ]]	If OPTIONNAME is enabled
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[[ ! EXPR ]]	Not
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[[ X && Y ]]	And
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[[ X || Y ]]	Or
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if statement:
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#if Statement 
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​
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echo -n "Enter a number: "
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read VAR
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​
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if [[ $VAR -gt 10 ]]
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then
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  echo "The variable is greater than 10."
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fi
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#if..else Statement
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​
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echo -n "Enter a number: "
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read VAR
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​
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if [[ $VAR -gt 10 ]]
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then
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  echo "The variable is greater than 10."
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else
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  echo "The variable is equal or less than 10."
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fi
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#if..elif..else Statement
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​
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echo -n "Enter a number: "
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read VAR
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​
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if [[ $VAR -gt 10 ]]
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then
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  echo "The variable is greater than 10."
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elif [[ $VAR -eq 10 ]]
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then
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  echo "The variable is equal to 10."
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else
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  echo "The variable is less than 10."
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fi
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# Nested if Statements
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echo -n "Enter the first number: "
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read VAR1
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echo -n "Enter the second number: "
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read VAR2
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echo -n "Enter the third number: "
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read VAR3
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​
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if [[ $VAR1 -ge $VAR2 ]]
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then
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  if [[ $VAR1 -ge $VAR3 ]]
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  then
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    echo "$VAR1 is the largest number."
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  else
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    echo "$VAR3 is the largest number."
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  fi
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else
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  if [[ $VAR2 -ge $VAR3 ]]
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  then
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    echo "$VAR2 is the largest number."
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  else
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    echo "$VAR3 is the largest number."
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  fi
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fi
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Loops:
for:
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#basic for loop
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for i in 1 2 3 4 5
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do
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   echo "Welcome $i times"
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done
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#Basic for loop
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for i in /etc/rc.*; do
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  echo $i
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done
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#Ranges
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for i in {1..5}; do
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    echo "Welcome $i"
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done
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#C-Like for loop
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for ((i = 0 ; i < 100 ; i++)); do
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  echo $i
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done
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#with step size
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for i in {5..50..5}; do
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    echo "Welcome $i"
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done
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while:
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n=1
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​
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while [ $n -le 5 ]
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do
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	echo "Welcome $n times."
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	n=$(( n+1 ))	
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done
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#Using ((expression)) Format With The While Loop
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n=1
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while (( $n <= 5 ))
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do
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	echo "Welcome $n times."
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	n=$(( n+1 ))	
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done
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#for ever
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while true; do
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  ···
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done
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# Reading a test file:
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###example1/2:
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cat /etc/resolv.conf | while read line; do
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  echo $line
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done
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​
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###example2/2:
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file=/etc/resolv.conf
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while IFS= read -r line
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do
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	echo $line
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done < "$file"
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​
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### Reading A Text File With Separate Fields:
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file=/etc/resolv.conf
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# set field separator to a single white space 
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while IFS=' ' read -r f1 f2
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do
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	echo "field # 1 : $f1 ==> field #2 : $f2"
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done < "$file"
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Until:
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#!/bin/bash
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​
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counter=0
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​
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until [ $counter -gt 5 ]
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do
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  echo Counter: $counter
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  ((counter++))
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done
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Case:
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Case/switch
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case "$1" in
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  start | up)
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    vagrant up
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    ;;
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​
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  *)
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    echo "Usage: $0 {start|stop|ssh}"
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    ;;
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esac
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Functions:
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# Defining functions:
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myfunc() {
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    echo "hello $1"
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}
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# Same as above (alternate syntax)
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function myfunc() {
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    echo "hello $1"
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}
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myfunc "John"
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#Returning values:
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myfunc() {
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    local myresult='some value'
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    echo $myresult
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}
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result="$(myfunc)"
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#Raising errors:
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myfunc() {
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  return 1
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}
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if myfunc; then
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  echo "success"
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else
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  echo "failure"
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fi
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#Arguments:
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$#	Number of arguments
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$*	All arguments
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[email protected]	All arguments, starting from first
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$1	First argument
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$_	Last argument of the previous command
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Arrays
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Defining arrays
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Fruits=('Apple' 'Banana' 'Orange')
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Fruits[0]="Apple"
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Fruits[1]="Banana"
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Fruits[2]="Orange"
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Operations
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Fruits=("${Fruits[@]}" "Watermelon")    # Push
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Fruits+=('Watermelon')                  # Also Push
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Fruits=( ${Fruits[@]/Ap*/} )            # Remove by regex match
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unset Fruits[2]                         # Remove one item
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Fruits=("${Fruits[@]}")                 # Duplicate
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Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
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lines=(`cat "logfile"`)                 # Read from file
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Working with arrays
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echo ${Fruits[0]}           # Element #0
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echo ${Fruits[-1]}          # Last element
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echo ${Fruits[@]}           # All elements, space-separated
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echo ${#Fruits[@]}          # Number of elements
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echo ${#Fruits}             # String length of the 1st element
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echo ${#Fruits[3]}          # String length of the Nth element
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echo ${Fruits[@]:3:2}       # Range (from position 3, length 2)
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echo ${!Fruits[@]}          # Keys of all elements, space-separated
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Iteration
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for i in "${arrayName[@]}"; do
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  echo $i
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done
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Dictionaries:
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Defining
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declare -A sounds
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sounds[dog]="bark"
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sounds[cow]="moo"
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sounds[bird]="tweet"
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sounds[wolf]="howl"
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Working with dictionaries
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echo ${sounds[dog]} # Dog's sound
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echo ${sounds[@]}   # All values
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echo ${!sounds[@]}  # All keys
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echo ${#sounds[@]}  # Number of elements
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unset sounds[dog]   # Delete dog
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Iteration
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Iterate over values
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for val in "${sounds[@]}"; do
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  echo $val
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done
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Iterate over keys
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for key in "${!sounds[@]}"; do
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  echo $key
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done
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Debugging
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bash -n scriptname  # don't run commands; check for syntax errors only
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set -o noexec       # alternative (set option in script)
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​
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bash -v scriptname  # echo commands before running them
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set -o verbose      # alternative (set option in script)
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​
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bash -x scriptname  # echo commands after command-line processing
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set -o xtrace       # alternative (set option in script)
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Miscellaneous:
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#Numeric calculations
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$((a + 200))      # Add 200 to $a
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$(($RANDOM%200))  # Random number 0..199
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#Inspecting commands
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command -V cd
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#=> "cd is a function/alias/whatever"
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#Heredoc:
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cat <<END
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hello world
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END
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#printf:
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printf "Hello %s, I'm %s" Sven Olga
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#=> "Hello Sven, I'm Olga
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​
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printf "1 + 1 = %d" 2
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#=> "1 + 1 = 2"
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​
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printf "This is how you print a float: %f" 2
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#=> "This is how you print a float: 2.000000"
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#Reading input
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echo -n "Proceed? [y/n]: "
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read ans
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echo $ans
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​
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#Reading  Just one character:
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read -n 1 ans    
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#Getting options
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while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
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  -V | --version )
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    echo $version
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    exit
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    ;;
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  -s | --string )
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    shift; string=$1
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    ;;
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  -f | --flag )
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    flag=1
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    ;;
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esac; shift; done
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if [[ "$1" == '--' ]]; then shift; fi
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#Check for command’s result
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if ping -c 1 google.com; then
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  echo "It appears you have a working internet connection"
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fi
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​
Payam Borosan.Goodluck
Refrences
Last modified 1yr ago