For more advanced and precise math consider using bc(1).

echo "3 * 2.19" | bc -l 
6.57

Internal Methods

Bash supports arithmetic expansion with $(( expression )). For example:

$ echo $(( 5 * 5 ))
25

External Methods

A number of utilities provide arithmetic, including bc and expr.

$ echo '5 * 5' | /usr/bin/bc
25

$ /usr/bin/expr 5 \* 5
25

The classical solution is:

 expr 5 \* 5

expr will only work with integer operands. Another nice option is:

 echo 5 5\*p | dc

dc can be made to work with non-integer operands.

A simple shell function (no sed needed) should do the trick of interpreting '5X5'

$ function calc { bc -l <<< ${@//[xX]/*}; };
$ calc 5X5
25
$ calc 5x5
25
$ calc '5*5'
25

I use this function which uses bc and thus supports floating point calculations:

c () { 
    local a
    (( $# > 0 )) && a="$@" || read -r -p "calc: " a
    bc -l <<< "$a"
}

Example:

$ c '5*5'
25
$ c 5/5
1.00000000000000000000
$ c 3.4/7.9
.43037974683544303797

Bash's arithmetic expansion doesn't support floats (but Korn shell and zsh do).

Example:

$ ksh -c 'echo "$((3.0 / 4))"'
0.75

I have a simple script I use for this:

me@mycomputer:~$ cat /usr/local/bin/c

#!/bin/sh

echo "$*" | sed 's/x/\*/g' | bc -l

It changes x to * since * is a special character in the shell. Use it as follows:

• c 5x5
• c 5-4.2 + 1
• c '(5 + 5) * 30' (you still have to use quotes if the expression contains any parentheses).

If you like python and have an option to install a package, you can use this utility that I made.

# install pythonp
python -m pip install pythonp

pythonp "5*5"
25

pythonp "1 / (1+math.exp(0.5))"
0.3775406687981454

# define a custom function and pass it to another higher-order function
pythonp "n=10;functools.reduce(lambda x,y:x*y, range(1,n+1))"     
3628800