if flags in UNIX shell scripting

/bin/[ is another name for /bin/test. It evaluates its arguments as a boolean expression, and exits with an exit code of 0 if it is true, or 1 if it is false.
If test is invoked as [, then it requires a closing bracket ] as its last argument. Otherwise, there must be no closing bracket.
test understands the following expressions, among others:

-e filename

True if filename exists.

-d filename

True if filename exists and is a directory.

-f filename

True if filename exists and is a plain file.

-h filename

True if filename exists and is a symbolic link.

-r filename

True if filename exists and is readable.

-w filename

True if filename exists and is writable.

-n string

True if the length of string is non-zero.

-z string

True if the length of string is zero.

string

True if string is not the empty string.

s1 = s2

True if the strings s1 and s2 are identical.

s1 != s2

True if the strings s1 and s2 are not identical.

n1 -eq n2

True if the numbers n1 and n2 are equal.

n1 -ne n2

True if the numbers n1 and n2 are not equal.

n1 -gt n2

True if the number n1 is greater than n2.

n1 -ge n2

True if the number n1 is greater than or equal to n2.

n1 -lt n2

True if the number n1 is less than n2.

n1 -le n2

True if the number n1 is less than or equal to n2.

! expression

Negates expression, that is, returns true iff expression is false.

expr1 -a expr2

True if both expressions, expr1 and expr2 are true.

expr1 -o expr2

True if either expression, expr1 or expr2 is true.

( expression )

True if expression is true. This allows one to nest expressions.