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# Unix - Shell Substitutions

# What is Substitution?

The shell performs substitution when it encounters an expression that contains one or more special characters.

## Example:

Following is the example, while printing value of the variable its substitued by its value. Same time "\n" is substituted by a new line:

    #!/bin/sh

    a=10
    echo -e "Value of a is $a \n"

This would produce following result. Here -e option enables interpretation of backslash escapes.

    Value of a is 10

Here is the result without -e option:

    Value of a is 10\n

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

Escape | Description
--- | ---
\\ | backslash
\a | alert (BEL)
\b | backspace
\c | suppress trailing newline
\f | form feed
\n | new line
\r | carriage return
\t | horizontal tab
\v | vertical tab

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

You can use -n option to disable insertion of new line.

# Command Substitution:

Command substitution is the mechanism 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 performed when a command is given as:

    `command`

When performing command substitution make sure that you are using the backquote, not the single quote character.

## Example:

Command substitution is generally used to assign the output of a command to a variable. Each of the following examples demonstrate command substitution:

    #!/bin/sh

    DATE=`date`
    echo "Date is $DATE"

    USERS=`who | wc -l`
    echo "Logged in user are $USERS"

    UP=`date ; uptime`
    echo "Uptime is $UP"

This will produce following result:

    Date is Thu Jul  2 03:59:57 MST 2009
    Logged in user are 1
    Uptime is Thu Jul  2 03:59:57 MST 2009
    03:59:57 up 20 days, 14:03,  1 user,  load avg: 0.13, 0.07, 0.15

# Variable Substitution:

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

Here is the following table for all the possible substitutions:

Form | Description
--- | ---
${var} | Substitue the value of var.
${var:-word} | If var is null or unset, word is substituted for var. The value of var does not change.
${var:=word} | If var is null or unset, var is set to the value of word.
${var:?message} | If var is null or unset, message is printed to standard error. This checks that variables are set correctly.
${var:+word} | If var is set, word is substituted for var. The value of var does not change.

## Example:

Following is the example to show various states of the above substitution:

    #!/bin/sh

    echo ${var:-"Variable is not set"}
    echo "1 - Value of var is ${var}"

    echo ${var:="Variable is not set"}
    echo "2 - Value of var is ${var}"

    unset var
    echo ${var:+"This is default value"}
    echo "3 - Value of var is $var"

    var="Prefix"
    echo ${var:+"This is default value"}
    echo "4 - Value of var is $var"

    echo ${var:?"Print this message"}
    echo "5 - Value of var is ${var}"

This would produce following result:

    Variable is not set
    1 - Value of var is
    Variable is not set
    2 - Value of var is Variable is not set

    3 - Value of var is
    This is default value
    4 - Value of var is Prefix
    Prefix
    5 - Value of var is Prefix

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