Command line expression solver?

bc & dc

bc and dc are the 2 calculators that I'll often use when needing access from a terminal.

Examples

$ bc
bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'. 

Then you can type your questions:

2
2
5+5
10

When you're done you can get out with a Ctrl+C.

Test drive

These calculators are pretty feature rich.

scaling

scale=5
193 * 1/3
64.33333

equations

principal=100
ir = 0.05
years = 5
futurevalue = principal * (1 + ir)^years

futurevalue
127.62800

your examples

8*6-4
44

8*(6-4)
16

calc

If you want something a little more interactive there's calc.

Example

$ calc
C-style arbitrary precision calculator (version 2.12.4.4)
Calc is open software. For license details type:  help copyright
[Type "exit" to exit, or "help" for help.]

; 10+10
20
; 8*6-4
    44
; 8*(6-4)
    16
; 

You can use the up/down arrows to go through past commands and it also has interactive help.

; help

Gives you this:

For more information while running calc, type  help  followed by one of the
following topics:

    topic               description
    -----               -----------
    intro               introduction to calc
    overview            overview of calc
    help                this file

    assoc               using associations
    builtin             builtin functions
    command             top level commands
    config              configuration parameters
    custom              information about the custom builtin interface
    define              how to define functions
    environment         how environment variables effect calc
    errorcodes          calc generated error codes
    expression          expression sequences
    file                using files
    history             command history
    interrupt           how interrupts are handled
    list                using lists
    mat                 using matrices
    ...

References

• calc's main reference page
• calc's man page

There are many answers to your question...

The simple ones you could do in the shell.

$ echo $((8*(6-4)))
16

As a dedicated program there is bc.

$ echo "8*(6-4)" | bc
16

Custom functions via scripting? Well, shell scripts and bc both have them, in a way. Depends on how far you want to take it.

Why not Python? It's easy to learn.

$ python
>>> from math import atan
>>> 8*(6-4)+atan(0)
16.0

In zsh:

$ autoload zcalc  # best in ~/.zshrc
$ zcalc
1> 8*(6-4)
16
2> $1*2
32