Week 5 Part 2

Question 1

how to create a shell variable?

Answer 1

myvar=”value string”

variables cannot start with a number and cannot have a space.

Question 2

how to make a variable available to other shells?

Answer 2

export myvar=”value string”
or
myvar =”value string”
export myvar

Question 3

How to use variable values?

Answer 3

echo $myvar
echo ${myvar}
echo “${myvar}_something”

Question 4

how to remove a variable?

Answer 4

unset myvar
myvar =
(if we leave blank a variable after equal sign, we unset it)

Question 5

how to test if a variable is set or not?

Answer 5

[[ -v myvar ]];
echo $?

Return codes:
0: success (variable myvar is set)
1: failure (variable myvar is not set)

[[ -z ${myvar + x} ]];
works opposite to ‘-v’ command above. If var is available, it will set the value

Question 5

How to print variable value when set or some other value if not set?

Answer 5

echo ${myvar:-“default”}
if myvar is set then displays its value. If not set, then displays default value

echo ${myvar:=”default”}
If myvar is set then displays myvar else sets myvar to defalut value and displays its new value

Question 6

How to reset value if variable is set?

Answer 6

echo ${myvar:+”default”}
If myvar is set then set the default value and display the new value else display null.

Question 7

How to find the list of variables stored in the memory?

Answer 7

echo ${!H*}
List of names of shell variables that start with H

Question 8

how to find the length of the string value of the variable?

Answer 8

echo ${#myvar}

displays length of the string value myvar. If myvar is not set it displays 0

Question 9

how to get a slice of the string value?

Answer 9

echo ${myvar:5:4}

displays 4 charcters starting from 5th position

Question 10

How to remove matching pattern

Answer 10

echo ${myvar#pattern} - match once and remove

echo ${myvar##pattern} - match max possible times and remove

Question 11

how to keep matching pattern within a variable?

Answer 11

echo ${myvar%pattern} -once
${myvar%%pattern} - multiple times

Question 12

how to match and replace a pattern ?

Answer 12

echo ${myvar/pattern/string} - match once and replace with string.

echo ${myvar//pattern/string} - match max possible &replace with string

Question 13

how to replace matching pattern by location/

Answer 13

echo ${myvar/#pattern/string}
-match at beginning and replace with string

echo${myvar/%pattern/string}
-match at the end and replace with string

Question 14

how to change case of the variable?

Answer 14

echo ${myvar,} - change first char to lower case

echo ${myvar,,} - change all chars to lower case

echo ${myvar ^} - change first char to upper case

echo ${myvar ^^} -change all chars to upper case.

Question 15

how to restrict value types of variables?

Answer 15

( + removes restrictions
- gives restrictions)

declare -i myvar (integers)

declare -l myvar (only lower case chars)

declare -u myvar(only upper case chars)

declare -r myvar (read only variable) (+r is not possible)

Question 16

how are indexed arrays created in bash?

Answer 16

declare -a arr (declare arr as an indexed array)

$arr[0]=”value” (set element 0 to value)

echo ${arr[0]} (display value of 0th element of array)

echo${#arr[@]} (displays no of elements in the array)

echo${!arr[@]} (display all indices used)

echo${arr[@]}(display values of all elements of the array)

unset ‘arr[2]’ (delete element 2 in the array)

arr+=(“value”) (append an element with a value to the end of the array

Question 17

how are associative arrays (hash) declared?

Answer 17

declare -A hash
$hash[“a”]=”value”
echo ${hash[“a”]}
echo ${#hash[@]}
echo ${!hash[@]}
echo ${hash[@]}
unset ‘hash[“a”]’