Shell Scripting Learning Notes-string test expressions

1.1 String test operator

The string test operator has the function of comparing whether two strings are the same, whether the length of a string is zero, whether the string is null (NOTE: Bash distinguishes between 0-length strings and empty strings, etc.)
The following table is a common string operator

You can also view the string test operator with the man test

String test operator Hints:

(1)-N compares whether the string length is nonzero, or true if nonzero: [-n ' $myvar]

(2)-Z compares whether string length equals zero, if equal to zero is true, such as: [-Z "$myvar"]
Special Note: Test the operation symbol for the string in the table above. For example: [-N ' $myvar "] to enclose the string in" "double quotation marks.

Precautions:

(1) string or string variable comparisons are followed by double quotation marks before being compared.

(2) string or string variable comparison, the comparison symbol at both ends of the best have spaces, learning shell Multi-reference system script.

"=" compares two strings in the same way, equivalent to "= =", such as ["$a" = "$b"] where a variable of $ A is best used "", because if there are spaces in the middle, the sign may go wrong, but the better way is ["${a}" = "${b}"].

Example Demo:

Error case with no spaces at both ends of the equals sign:

System Script Case:

Shell Scripting Learning Notes-string test expressions