How the does keyword “if” test if a value is true of false?

Question

In bash script

if [ 1 ]

then

   echo "Yes"

else

   echo "No"

fi

Output:

Yes

It represents that '1' is treated as true value.

But in code:

word = Linux

letter = nuxi

if echo "$word" | grep -q "$letter"

then

    echo "Yes"

else

    echo "No"

fi

Output: No

But

echo "$word" | grep -q "$letter"

will return 1, why the result is No.

how the keyword

if

test the value returned by the command after

if

?

Answer 1

The return value of a command is checked. [ 1 ] has a return value of 0 (true). Any other return value (like 1) indicates an error.

You can display the return value of the last executed command using the $? variable:

true
echo $?
# returned 0
false
echo $?
# returned 1
echo $?
# returned 0 as the last executed command is 'echo', and not 'false'

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