The command patch -R -p0 -s -f --dry-run < abc.patch fails with exit code 1 and prints a error message from patch command but when used with negation in a if condition it passes (i.e prints Pass), below is the code
if ! patch -R -p0 -s -f --dry-run < abc.patch; then
echo "Pass"
fi
What I tried ? I redirected string dev/null no change in result
Please help me understand what's going wrong in here ?
Since you negate the exit status of patch for the if statement, the if statement could be read as "if patch fails, output the string Pass".
The if statement tests whether the given command succeeds (true) or fails (false). You can remember this and forget about the rest in this answer if you wish.
When the command fails, its exit status is non-zero, possibly allowing for a closer inspection of the reason for failure. Provided that the utility assigns any meaning to the actual value returned (as e.g. curl and rsync do, see the very end of their manuals), a script may choose to handle an error appropriately.
If the command's exit status is zero, it signals the shell that the utility finished successfully.
The exit status is more of "an error code" rather than "a boolean value" or "true or false". The true/false boolean test is performed internally by the if statement testing the exit status to see whether the utility had a failure condition. If not (exit status is zero), the test is true, i.e., the utility did not fail.
You obviously wanted to say something like "unless X is wrong, do Y", came to a negated "if" to express "unless", and in the end negated effectively the "wrong", so your "if" succeeds exactly if (and only if) X is wrong (see @they's answer). This has nothing to do with exit codes, just with complexities of negation.
trueandfalsebuiltins to understand and compare the outcomes ofif true ; then echo A ; fi,if ! true ; then echo A ; fi,if false ; then echo A ; fi, andif ! false ; then echo A ; fi, respectively.