These days I am reading the book 'Advanced Bash-Scripting Guide' and I got confused about '&&' and '-a'.
The related part was here:
[ 1 -eq 1 ] && [ -n "`echo true 1>&2`" ] # true [ 1 -eq 2 ] && [ -n "`echo true 1>&2`" ] # (no output) # ^^^^^^^ False condition. So far, everything as expected. # However ... [ 1 -eq 2 -a -n "`echo true 1>&2`" ] # true # ^^^^^^^ False condition. So, why "true" output? # Is it because both condition clauses within brackets evaluate? [[ 1 -eq 2 && -n "`echo true 1>&2`" ]] # (no output) # No, that's not it. # Apparently && and || "short-circuit" while -a and -o do not.
The last line explained the reason, but I wanted to find more evidences from the BASH manual and I got it.
For && and ||:
Expressions may be combined using the following operators, listed in decreasing order of precedence:
( expression )
Returns the value of expression. This may be used to override the normal precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire condi-
tional expression.
For -a and -o:
Expressions may be combined using the following operators, listed in decreasing order of precedence. The evaluation depends on the number of
arguments; see below. Operator precedence is used when there are five or more arguments.
! expr True if expr is false.
( expr )
Returns the value of expr. This may be used to override the normal precedence of operators.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.
test and [ evaluate conditional expressions using a set of rules based on the number of arguments.
0 arguments
The expression is false.
1 argument
The expression is true if and only if the argument is not null.
2 arguments
If the first argument is !, the expression is true if and only if the second argument is null. If the first argument is one of the unary
conditional operators listed above under CONDITIONAL EXPRESSIONS, the expression is true if the unary test is true. If the first argument
is not a valid unary conditional operator, the expression is false.
3 arguments
The following conditions are applied in the order listed. If the second argument is one of the binary conditional operators listed above
under CONDITIONAL EXPRESSIONS, the result of the expression is the result of the binary test using the first and third arguments as oper‐
ands. The -a and -o operators are considered binary operators when there are three arguments. If the first argument is !, the value is
the negation of the two-argument test using the second and third arguments. If the first argument is exactly ( and the third argument is
exactly ), the result is the one-argument test of the second argument. Otherwise, the expression is false.
4 arguments
If the first argument is !, the result is the negation of the three-argument expression composed of the remaining arguments. Otherwise,
the expression is parsed and evaluated according to precedence using the rules listed above.
5 or more arguments
The expression is parsed and evaluated according to precedence using the rules listed above.So for '-a' and '-o', both sides will ALWAYS be executed.
