The expr command in the shell

Expr can do the following:

Logical operations

Arg1 | Arg2 logic or operation, true return arg1, otherwise return arg2 (null or to determine the true and false parameters, there is a short-circuit function)

Arg1 & arg2 Logic and operation, true return arg1, otherwise return arg2 (null or to determine the parameter is true, short circuit function)

Relational operations

Arg1 = Arg2 (or arg1 = = arg2) returns equal to 1, unequal return 0

Arg1! = arg2 returns 1, equal returns 0

Arg1 > Arg2 is greater than return 1, otherwise returns 0

Arg1 >= arg2 is greater than or equal to return 1, otherwise returns 0

Arg1 < arg2 less than return 1, otherwise return 0

Arg1 <= arg2 is less than or equal to return 1, otherwise returns 0

It is important to note that in a relational operation, only all parameters are numeric and are numerically compared, otherwise, by dictionary order

Arithmetic operations

Arg1 + arg2

Arg1-arg2

Arg1 * arg2

Arg1/arg2

Arg1% arg2

It is important to note that * There are other meanings in the shell, so you need to escape using \*

String manipulation

String:regexp

1) REGEXP represents a regular expression, and the expression implies ^, that is, REGXP is ^regexp;

2) If there is no \ (and \) in the regexp, then the number of matched characters is returned, or the total mismatch returns 0, and if \ (or \) is returned, the string containing the \ (and \) will return, or the mismatch returns null

Match STRING REGEXP

Class String:regexp Same

substr STRING POSITION LENGTH

Returns the substring from position (starting at 1), long length, if position and length are 0, negative, or non-negative, then returns null

Index STRING CHARSET

CharSet can contain more than one character, but only returns the position of the first character in string, and returns 0 if the characters in CharSet are not in string.

Length STRING

Returns the length of a string

+ TOKEN

The token is interpreted as a string, even if token is a keyword, such as expr index index n, because the second index is the keyword of the expr syntax, the error is handled as follows: Expr index + index n

All of the above expressions need to be aware that there must be spaces on both sides of the operator.

The expr command in the shell