substitutions

summary

This subchapter looks at substitutions in UNIX (and Linux).

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substitutions

This subchapter looks at substitutions in UNIX (and Linux).

• command substitutions
• arithmetic substitutions

command substitution

Command substitution is used to assign the output of a command to a variable.

Place the command in sideways ticks (`) around the command. Do not confuse these with regular single quotation marks (').

You can place a simple command, a pipeline, or a command list inside the tick marks.

An example of a simple command:

$ DATE=`date`

An example of a pipeline:

$ CONSOLEUSER=`who | grep console`

An example of a command list:

$ FILEUSAGE=`date ; df`

You can use command substitution to create parameters for other commands.

In the following example, the user name is used for grep word search of the file named names:

$ grep `id -un` names

You can use $( ) as a replacement for ` `. If you nest backticks inside of each other, you need to escape the internal backticks. You do not need to escape nested $( ).

$ DATEVAR=$(date)
$ echo $DATEVAR
$Mon Aug 26 19:35:47 PDT 2013
$

If there is only one level of nesting, some people use the convention of using $() for the outer expression adn backicks (``) fr the inner expression.

arithmetic substitution

You can use arithmetic substitution for quick integer artihmetic.

Place an integer arithmetic expression inside double parenthesis and place a dollar mark before the parenthesized expression (wow, that’s a lot of words), $(( expression )).

$(( 5 + 3 ))

You may use integer constants or integer variables:

$(( 5 + $z ))

You may post-increment, post-decrement, pre-increment, or pre-decrement variables:

$(( --x + z++ ))

You may use negative integers:

$(( -1 * -2 ))

You may use logical (!) and bitwise (~) negation:

$(( !1 * ~2 ))

The order of precedence (and complete list of possible operations):

operator	meaning
VAR++ VAR--	variable post-increment and pre-increment
++VAR --VAR	variable pre-increment and pre-decrement
- +	unary minus and plus
! ~	logical negation and bitwise negation
**	exponentiation
* / %	multiplication, division, and modulo
+ -	addition and subtraction
<< >>	left bitwise shift and right bitwise shift
<= >= < >	comparison operators
== !=	equality and inequality
&	bitwise AND
^	bitwise exclusive OR
|	bitwise OR
&&	logical AND
||	logical OR
expression ? expression : expression	C-style conditional evaluation
= *= /= %= += -= <<= >>= &= ^= !=	assignments
,	separator between expressions

Applying the precedence rules:

$(( ((3 + 5*2) -8) /2 ))

The result for the above expression is 2. It is not 2.5 because this is integer arithmetic. It is not 4 because the multiplcation has a higher precedence than addition.

You can have a raw expression, which is useful if it includes an assignment operator:

$ n=1
$ echo $n
1
$ (( n += 3 ))
$ echo $n
4
$

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