[Chapter 5] 5.4 Expressions

5.4 Expressions

Expressions are used in @ (the C shell math operator), if , and while statements to perform arithmetic, string comparisons, file testing, etc. exit and set can also specify expressions. Expressions are formed by combining variables and constants with operators that resemble those in the C programming language. Operator precedence is the same as in C. It is easiest to just remember the following precedence rules:

* / %
+ -
Group all other expressions inside () s; parentheses are required if the expression contains < , < , & , or |

5.4.1 Operators

Operators can be one of the following types.

5.4.1.1 Assignment operators

Operator	Description
`=`	Assign value.
`+= -=`	Reassign after addition/subtraction.
`*= /= %=`	Reassign after multiplication/division/remainder.
`&= ^= \|=`	Reassign after bitwise AND/XOR/OR.
`++`	Increment.
`--`	Decrement.

5.4.1.2 Arithmetic operators

Operator	Description
`* / %`	Multiplication; integer division; modulus (remainder).
`+ -`	Addition; subtraction.

5.4.1.3 Bitwise and logical operators

Operator	Description
`~`	Binary inversion (one's complement).
`!`	Logical negation.
`<< >>`	Bitwise left shift; bitwise right shift.
`&`	Bitwise AND.
`^`	Bitwise exclusive OR.
`\|`	Bitwise OR.
`&&`	Logical AND (short-circuit).
`\|\|`	Logical OR (short-circuit).
`{` command `}`	Return 1 if command is successful; 0 otherwise. Note that this is the opposite of command 's normal return code. The `$status` variable may be more practical.

5.4.1.4 Comparison operators

Operator	Description
`== !=`	Equality; inequality.
`<= >=`	Less than or equal to; greater than or equal to.
`< >`	Less than; greater than.
`=~`	String on left matches a filename pattern containing `*` , `?` , or `[...]` .
`!~`	String on left does not match a filename pattern containing `*` , `?` , or `[...]` .

5.4.1.5 File inquiry operators

Command substitution and filename expansion are performed on file before the test is performed.

Operator	Description
`-d` file	The file is a directory.
`-e` file	The file exists.
`-f` file	The file is a plain file.
`-o` file	The user owns the file.
`-r` file	The user has read permission.
`-w` file	The user has write permission.
`-x` file	The user has execute permission.
`-z` file	The file has zero size.
`!`	Reverse the sense of any inquiry above.

5.4.2 Examples

The following examples show @ commands and assume n = 4.

Expression	Value of $x
`@ x = ($n > 10 \|\| $n < 5)`	1
`@ x = ($n >= 0 && $n < 3)`	0
`@ x = ($n << 2)`	16
`@ x = ($n >> 2)`	1
`@ x = $n % 2`	0
`@ x = $n % 3`	1

The following examples show the first line of if or while statements.

Expression	Meaning
`while ($#argv != 0)`	While there are arguments ...
`if ($today[1] == "Fri")`	If the first word is "Fri"...
`if ($file !~ *.[zZ])`	If the file doesn't end with `.z` or `.Z` ...
`if ($argv[1] =~ chap?)`	If the first argument is `chap` followed by a single character ...
`if (-f $argv[1])`	If the first argument is a plain file ...
`if (! -d $tmpdir)`	If `$tmpdir` is not a directory ...