The operators (in order of increasing precedence) are:
Parentheses are used for grouping in the usual manner. The keywords cannot be used as strings.
To add 1 to the shell variable a :
The following may be used to print the non-directory part of the file name stored in variable a (the value in a need not contain `/'):
Note the quoted shell metacharacters.
expr returns the following exit status:
0 if the expression is neither null nor 0,
1 if the expression is null or 0,
2 for invalid expressions.