оценивать аргументы как выражение (evaluate arguments as an expression)
Использование в приложениях (Application usage)
The expr utility has a rather difficult syntax:
* Many of the operators are also shell control operators or
reserved words, so they have to be escaped on the command
line.
* Each part of the expression is composed of separate
arguments, so liberal usage of <blank> characters is
required. For example:
┌─────────────────┬───────────────────────┐
│ Invalid │ Valid │
├─────────────────┼───────────────────────┤
│expr 1+2 │ expr 1 + 2 │
│expr "1 + 2" │ expr 1 + 2 │
│expr 1 + (2 * 3) │ expr 1 + \( 2 \* 3 \) │
└─────────────────┴───────────────────────┘
In many cases, the arithmetic and string features provided as
part of the shell command language are easier to use than their
equivalents in expr. Newly written scripts should avoid expr in
favor of the new features within the shell; see Section 2.5,
Parameters and Variables and Section 2.6.4, Arithmetic Expansion.
After argument processing by the shell, expr is not required to
be able to tell the difference between an operator and an operand
except by the value. If "$a" is '=', the command:
expr "$a" = '='
looks like:
expr = = =
as the arguments are passed to expr (and they all may be taken as
the '=' operator). The following works reliably:
expr "X$a" = X=
Also note that this volume of POSIX.1‐2017 permits
implementations to extend utilities. The expr utility permits the
integer arguments to be preceded with a unary minus. This means
that an integer argument could look like an option. Therefore,
the conforming application must employ the "--" construct of
Guideline 10 of the Base Definitions volume of POSIX.1‐2017,
Section 12.2, Utility Syntax Guidelines to protect its operands
if there is any chance the first operand might be a negative
integer (or any string with a leading minus).
For testing string equality the test utility is preferred over
expr, as it is usually implemented as a shell built-in. However,
the functionality is not quite the same because the expr = and !=
operators check whether strings collate equally, whereas test
checks whether they are identical. Therefore, they can produce
different results in locales where the collation sequence does
not have a total ordering of all characters (see the Base
Definitions volume of POSIX.1‐2017, Section 7.3.2, LC_COLLATE).
