UNIX / Linux Shell Substitution - Unix/Linux

What are UNIX / Linux Shell Substitution?

What is Substitution?

The shell performs substitution when it encounters an expression that contains one or more particular font.

Example

Here, the printing value of the variable is substituted by its value. Same time, "\n" is substituted by a new line –

You will collect the following result. Here the -e selection enables the interpretation of backslash escapes.

Following is the result without -e option –

Here are following escape sequences which can be used in echo command −

S.No. Escape & Description
1
\\
backslash
2
\a
alert (BEL)
3
\b
backspace
4
\c
suppress trailing newline
5
\f
form feed
6
\n
new line
7
\r
carriage return
8
\t
horizontal tab
9
\v
vertical tab

You can use the -E selection to disable the interpretation of the backslash escapes (default).

You can use the -n selection to disable the introduction of a new line.

Command Substitution

Command substitution is the instrument by which the shell performs a given set of commands and then substitutes their output in the place of the commands.

Syntax

The command substitution is perform when a command is given as –

When performing the command substitution make sure that you use the back quote, not the single quote character.

Example

Command substitution is normally used to consign the output of a command to a variable. Each of the following examples demonstrates the command substitution –

Upon execution, you will collect the following result –

Variable Substitution

Variable substitution enables the shell programmer to control the value of a variable based on its state.

Here is the following table for all the feasible substitutions −

S.No. Form & Description
1
${var}
Substitute the value ofvar.
2
${var:-word}
Ifvaris null or unset,wordis substituted forvar. The value ofvardoes not change.
3
${var:=word}
Ifvaris null or unset,varis set to the value ofword.
4
${var:?message}
Ifvaris null or unset,messageis printed to standard error. This checks that variables are set correctly.
5
${var:+word}
Ifvaris set,wordis substituted for var. The value ofvardoes not change.

Example

Following is the instance to show different state of the above substitution –

Upon implementation, you will accept the following result –

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