Unix - Shell Substitution

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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:

EscapeDescription
\\backslash
\aalert (BEL)
\bbackspace
\c suppress trailing newline
\f form feed
\nnew line
\rcarriage 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:

FormDescription
${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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