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ArnobKumarSahaArnobKumarSaha
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paramter expansion
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‎basics/03-variable2.bash

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@@ -18,11 +18,14 @@ echo ${var^} # 1st letter to uppercase
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echo ${var^^} # all letters to uppercase
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echo ${#var} # length
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echo "*****"
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echo ${var::5} # first 5 characters
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echo ${var:4} # all excluding first 4 characters
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echo ${var: -3} # last 3 characters, space before minus is required.
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echo ${var:2:5} # staring from 2nd letter, cut a srring of length 5.
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echo ${var:2:5} # staring from 2nd letter, cut a string of length 5.
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echo ${var:2: -3} # strating from 2nd letter, not including last 3 letter
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echo ${var: -4:3} # takes last 4 letters, & then take first 3 of that
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echo ${var: -4: -3} # takes last 4 letters, & then skip last 3 of that
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echo "-------------------------"
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‎how-bash-process/parameter.bash

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#!/bin/bash
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# AUTHOR : Arnob kumar saha
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# Date created : 19-01-2022
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# Lase modified : 20-01-2022
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# DESCRIPTION : describes shell parameter expansion with examples
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# USAGE : ./parameter.bash
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: '
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There are total of GROUPs :
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1. ${parameter:-word}
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${parameter-word}
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2. ${parameter:=word}
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${parameter=word}
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3. ${parameter:?word}
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${parameter?word}
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4. ${parameter:+word}
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${parameter+word}
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'
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: '
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Parameters are three types :
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1) Set & not null
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2) set & null [[colon(:) only makes impact in this case]]
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3) unset (not declared)
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'
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: '
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#CASE 1 (-) : If unset, use value
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# Parameter is set and not null
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VAR_1=var1
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echo "VAR_1 is ${VAR_1:-abc}" # "var1"
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# Parameter is set but null
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VAR_2=
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echo "VAR_2 is ${VAR_2:-abc}" # "abc"
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echo "This will not hold the value $VAR_2." # ""
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# Parameter is not set
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echo "VAR_3 is ${VAR_3:-abc}" # "abc"
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# now you will see, :- makes impact for VAR_2 where - doesnot
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VAR_2=
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echo "VAR_2 is ${VAR_2-abc}" # ""
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'
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: '
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#CASE 2 (=) : If unset, set value
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# Parameter is set and not null
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VAR_1="var 1"
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echo "VAR_1 is ${VAR_1:=abc}."
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# Parameter is set but null
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VAR_2=
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echo "VAR_2 is ${VAR_2:=abc}."
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echo "The VAR_2 holds the value $VAR_2." # "abc"
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# Parameter is not set
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echo "VAR_3 is ${VAR_3:=abc}."
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VAR_2=
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echo "VAR_2 is ${VAR_2=abc}"
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'
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: '
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#CASE 3 (+) : If set, use value
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VAR_1=var1
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echo "VAR_1 is ${VAR_1:+new var}" # "new var"
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echo "VAR_1 is $VAR_1." # "var1"
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# null value, no substitution
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VAR_2=
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echo "VAR_2 is ${VAR_2:+new var}" # ""
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# unset, no substitution
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echo "VAR_3 is ${VAR_3:+new var}" # ""
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# null vaule, substitution
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VAR_2=
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echo "VAR_2 is ${VAR_2+abc}" # "abc"
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'
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: '
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#CASE 4 (?) : If set, use value. else give error-value in the standard error
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# Parameter is set and not null
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VAR_1="var 1"
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echo "VAR_1 is ${VAR_1:?no value}."
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# Parameter is set but null
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VAR_2=
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echo "VAR_2 is ${VAR_2:?no value}." # provides error./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
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echo "The VAR_2 holds the value $VAR_2."
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# Parameter is not set
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echo "VAR_3 is ${VAR_3:?no value}." # provides error
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echo "The VAR_3 holds the value $VAR_3."
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# Parameter is set but null
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VAR_2=
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echo "VAR_2 is ${VAR_2?no value}" # ""
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'
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# SetAndNotNull SetButNull Unset
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# ${parameter:-word} substitute parameter substitute word substitute word
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# ${parameter-word} substitute parameter substitute null substitute word
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# ${parameter:=word} substitute parameter assign word assign word
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# ${parameter=word} substitute parameter substitute null assign word
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# ${parameter:+word} substitute word substitute null substitute null
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# ${parameter+word} substitute word substitute word substitute null
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# ${parameter:?word} substitute parameter error, exit error, exit
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# ${parameter?word} substitute parameter substitute null error, exit
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# If the parameter is SetButNull, -,=,? takes the null value.
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# Whereas, :-.:=,;? takes the word value.
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# + is exactly the opposite.
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‎how-bash-process/parameter2.bash

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#!/bin/bash
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# AUTHOR : Arnob kumar saha
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# Date created : 19-01-2022
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# Lase modified : 20-01-2022
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# DESCRIPTION : describes shell parameter expansion with examples
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# USAGE : ./parameter2.bash
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# all the examples from basics/03-variable.bash are of parameter expansion.
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: '
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set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
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echo ${@} # list of all paramters, note that ${0} has not been echoed.
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# to do so, use this, echo ${@:0}
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echo ${0} # first positional parameter
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echo ${@:7} # all positional params starting from index 7
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echo ${@:7:2} # 2 param starting from index 7
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echo ${@:7:-2} #bash: -2: substring expression < 0
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echo ${@: -7:2} # -1 is the last positional paramter. -7 is 7th from the last, then take 2 items
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#b c
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echo ${@:0:2}
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# We could run these commands for array also .
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array=(1 2 3 4 5 6 7 8 9 10)
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echo ${array[@]: -7:2} $ prints 4 5
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echo ${array[5]}
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'
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VAR1="VAR1"
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VAR2="VAR2"
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NON_VAR="VAR3"
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echo "${!VAR*}"
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echo "${!VAR@}"
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echo "${!^*}"
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#name=(3 5 7)
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name=
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echo ${!name[@]}
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# if name is array , it prints 0 1 2
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# else : if name is set (null or not null), prints 0
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#if name is not declared, prints nothing

‎how-bash-process/readme.md

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### b) Intermidiate Expansion :
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In this step, different types expansions & substitutions occur. Namely, <br>
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i) Tilde Expansion : ~ converts to $HOME. For example, ~arnob/Documents converts to /home/arnob/Documents. <br>
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ii) Parameter expansion : Discussed in parameter.bash & paramter2.bash files. <br>
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iii) Arithmatic expansion : <br>
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iv) Command Substitution : <br>
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v) Process substitution :
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### c) Word splitting :
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### d) Globbing :

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