Handling arithmetic expressions in shell scripting

Question

Kindly tell me that is it necessary to use "expr" keyword.

EG:-

 echo `expr a*b`

And where we can simply handle arithmetic expressions using simple arithmetic operators.

EG:-

echo a*b

Thanks in advance.

Answer 1

In a Posix shell you can evaluate expressions directly in the shell when they are enclosed in

$(( ... ))

So:

a=12
b=34
echo $(($a + $b))

And although this wasn't always the case, all Posix shells you are likely to encounter will also deal with:

echo $((a + b))

This all happened because, a long time ago, the shell did not do arithmetic, and so the external program expr was written. These days, expr is usually a builtin (in addition to still being in /bin) and there is the Posix $((...)) syntax available. If $((...)) had been around from day one there would be no expr.

The shell is not exactly a normal computer language, and not exactly a macro processor: it's a CLI. It doesn't do inline expressions; an unquoted * is a wildcard for filename matching, because a CLI needs to reference files more often than it needs to do arithmetic.

Answer 2

You can do arithmatic using $(())

echo $((2*3))

results in 6

Answer 3

The second form will almost surely never do what you want. In Bash, you have a built-in numeric expression handler, though:

A=4; B=6; echo $((A * B))