Variable Substitution
The shell allows us to manipulate the value of a variable based upon its initialization status.
| Sr No. | Expression | Significance |
|---|---|---|
| 1 | ${myVariable} | substitute the value of myVariable. |
| 2 | ${myVariable:-value} | If myVariable is not-set (or null) then the value is substituted for myVariable. |
| 3 | ${myVariable:=value} | If myVariable is not-set (or null), then it is set to value. |
| 4 | ${myVariable:? message} | If myVariable is not-set (or null) then the message is printed as standard error. |
| 5 | ${myVariable:+value} | If myVariable is set then the value is substituted for myVariable. |
Example:
These expressions are demonstrated in the below shell script.
#!/bin/sh
# If myVariable is unset or null
# then assign 12 to it
echo ${myVariable:- 11}
echo "1. The value of myVariable is ${myVariable}"
# If myVariable is unset or null
# then assign "w3wiki" to it
echo ${myVariable:="w3wiki"}
echo "2. Value of myVariable is ${myVariable}"
# unset myVariable
unset myVariable
# If myVariable is set then substitute
# the value
echo ${myVariable:+"w3wiki"}
echo "3. Value of myVariable is $myVariable"
myVariable="w3wiki"
# If myVariable is set then substitute
# the value
echo ${myVariable:+"Bhuwanesh"}
echo "4. Value of myVariable is $myVariable"
# If myVaraible is not-set or null then
# print the message
echo ${myVariable:?"message"}
echo "5. Value of myVariable is ${myVariable}"
unset myVariable
# If myVaraible is not-set or null then
# print the message
echo ${myVariable:?"message"}
echo "6. Value of myVariable is ${myVariable}"
Output:
Shell Scripting – Substitution
There are certain expressions that convey special meanings. In other words, they are not what they look like. A shell carries out substitution whenever it encounters such expressions. Hence, substitution is defined as a mechanism carried out by a shell in which it substitutes the value of an expression with its actual value.
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