boolean expression by using commands with "if-then-else"

Question

We know that it is possible to do if command ; then something ; fi, where the status returned from command is interpreted as boolean (0==true). We also know that is ok doing if [[ $a > $b || $c == yes ]] ; then something; fi.

But I'm curious because we are not allowed to combine commands like Boolean expressions using "if", as in if cmd1 || cmd2; then ; something; fi. Or can we?

There is other ways of doing this without "if" or even with nested if. However, I'm interested on using if :-).

Appreciate any comment.

chepner · Accepted Answer · 2016-10-21 16:32:17Z

3

That is correct. The condition to an if statement is defined to be a list, which very roughly speaking is either a single pipeline, or two lists joined by && or ||. These are not Boolean operators, although there is a resemblance. a && b runs a, then runs b if a succeeds. a || b runs a, then runs b if a fails.

Compare

if [[ $a > $b || $c == yes ]]; then   # conditional operators
if [[ $a > $b ]] || [[ $c == yes ]]; then   # shell operators

Given that [[ ... ]] is a built-in command, there is very little difference between the two. However, the shell operators have equal precedence, while the conditional operator && has higher precedence than conditional ||. That is

if a || b && c; then # same as (a || b) && c
if [[ a || b && c ]]; then # same as a || (b && c)

answered Oct 21, 2016 at 16:32

chepner

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hidefromkgb · Accepted Answer · 2016-10-21 15:34:20Z

0

I`ll just leave this here.

check_and() {
    if grep -q A <<<"$1" && grep -q C <<<"$1"; then
        echo YES
    else
        echo NO
    fi
}

check_or() {
    if grep -q A <<<"$1" || grep -q C <<<"$1"; then
        echo YES
    else
        echo NO
    fi
}

check_and ABC # YES
check_and ABD # NO
check_and ZBC # NO
check_and ZBD # NO

check_or ABC # YES
check_or ABD # YES
check_or ZBC # YES
check_or ZBD # NO

answered Oct 21, 2016 at 15:34

hidefromkgb

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