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Userland: Add an implementation of 'expr'

This implements all expressions except 'match', which errors out when executed.
Closes #1124?
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AnotherTest 2020-10-28 01:55:19 +03:30 committed by Andreas Kling
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commit a935a31ecf
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## Name
expr - evaluate expressions
## Synopsis
```**sh
$ expr <expression>
$ expr [--help]
```
## Description
expr evaluates and prints the result of an expression as described below to standard output.
An _expression_ may be any of the following:
- `expr1 | expr2`
`expr2` if `expr1` is falsy, `expr1` otherwise.
- `expr1 & expr2`
`expr1` if neither expression is falsy, `0` otherwise.
- `expr1 < expr2`
`1` if `expr1` is less than `expr2`, `0` otherwise.
- `expr1 <= expr2`
`1` if `expr1` is less than or equal to `expr2`, `0` otherwise.
- `expr1 = expr2`
`1` if `expr1` is equal to `expr2`, `0` otherwise.
- `expr1 = expr2`
`1` if `expr1` is not equal to `expr2`, `0` otherwise.
- `expr1 => expr2`
`1` if `expr1` is greater than or equal to `expr2`, `0` otherwise.
- `expr1 > expr2`
`1` if `expr1` is greater than `expr2`, `0` otherwise.
- `expr1 + expr2`
arithmetic integral sum of `expr1` and `expr2`.
- `expr1 - expr2`
arithmetic integral difference of `expr1` and `expr2`.
- `expr1 * expr2`
arithmetic integral product of `expr1` and `expr2`.
- `expr1 / expr2`
arithmetic integral quotient of `expr1` divided by `expr2`.
- `expr1 % expr2`
arithmetic integral quotient of `expr1` divided by `expr2`.
- `expr1 : expr2`
pattern match of `expr2` as a regular expression in `expr1` - currently not implemented.
- `match expr1 expr2`
same as `expr1 : expr2`.
- `substr expr1 expr2 expr3`
substring with length `expr3` of `expr1`, starting at `expr2`, indices starting at 1.
- `index expr1 expr2`
index of `expr2` in `expr1`, starting at 1. 0 if not found.
- `length expr1`
length of the string `expr1`
- `+ token`
interpret `token` as a string, regardless of whether it is a keyword or an operator.
- `( expr )`
value of `expr`
Note that many operators will need to be escaped or quoted if used from within a shell.
"falsy" means either the number 0, or the empty string.
## Options
* `--help`: Prints usage informations and exits.
## Examples
```sh
$ expr 1 + 2 * 3 # = 7
$ expr \( 1 + 2 \) = 3 # = 1
$ expr substr foobar 1 3 # foo
```
## See also