I am running this on Linux:
$ number=4 $ test $number -eq 4 $ echo $? 0 $ test $number -lt 2 $ echo $? 1 Are the results (0 and 1) true ?
- Are you asking if two opposite values are the same? O.omuru– muru2019-06-16 17:42:48 +00:00Commented Jun 16, 2019 at 17:42
- @muru No I mean are results true ?Batuhan Bakar– Batuhan Bakar2019-06-16 17:43:32 +00:00Commented Jun 16, 2019 at 17:43
- But shouldn't the answer be 1 if the result is true?Batuhan Bakar– Batuhan Bakar2019-06-16 17:45:44 +00:00Commented Jun 16, 2019 at 17:45
- 1stackoverflow.com/q/2933843/2072269muru– muru2019-06-16 17:54:01 +00:00Commented Jun 16, 2019 at 17:54
- See also What are the shell’s control and redirection operators? (specifically terdon’s answer, section A.2) and Using “reserved” codes for exit status of shell scripts.G-Man Says 'Reinstate Monica'– G-Man Says 'Reinstate Monica'2019-06-16 21:20:22 +00:00Commented Jun 16, 2019 at 21:20
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3 Answers
On Unix systems, utilities return a zero exit-status if what they did went well, if it succeeded without errors etc. If a utility fails, it returns a non-zero exit status.
This way, it's possible to tell a user with the exit-status what when wrong (see e.g. the "EXIT VALUES" or similar section of some manuals, for example the manual of rsync). In short, if the operation succeeded, the exit-status is zero and if it's not zero, it failed and the exit-status may say why it failed.
The test utility, which you use, follow this pattern in that if a comparison succeeded (it's "true"), it returns zero. If the comparison (or whatever operation it carries out) fails (it's "false"), it returns a non-zero exit-status.
The if keyword takes a utility and acts on its exit-status. If the utility returns a zero exit-status, it takes the default branch, otherwise it takes the else branch:
if test 4 -eq 3; then echo 4 is 3 else echo 4 is not 3 fi if can take any utility:
if grep -q 'secret' file.txt; then echo file.txt contains secret else echo file.txt does not contain secret fi if mkdir mydir; then echo created directory mydir else echo failed to create directory mydir fi 
- Thank you for your interest.Batuhan Bakar– Batuhan Bakar2019-06-16 21:48:56 +00:00Commented Jun 16, 2019 at 21:48
By convention, 0 means the operation was a success and anything other than 0 is an error. So, if $? is 0, then the test was successful and if it is not 0, for example if it is 1, the test failed.
You can see this a bit more clearly perhaps if you run this:
$ number=4 $ if test $number -eq 4; then echo "YES ($?)"; else echo "NO ($?)"; fi YES (0) $ if test $number -lt 2; then echo "YES ($?)"; else echo "NO ($?)"; fi NO (1) - Thank you @terdon you explained clearly.Batuhan Bakar– Batuhan Bakar2019-06-16 21:48:39 +00:00Commented Jun 16, 2019 at 21:48
- With the caveat, of course, that in
(()), booleans are C-style (0 is falsey)D. Ben Knoble– D. Ben Knoble2019-06-17 01:05:35 +00:00Commented Jun 17, 2019 at 1:05
The shell is not C. In C 0 is false, everything else is true. In the shell 0 is true/success, everything else is false. This is because there are many ways to fail, but one way to succeed.
Specifically from the info page, of test.
Exit status: 0 if the expression is true, 1 if the expression is false, 2 if an error occurred. 
- Thank you so much you explained well.Batuhan Bakar– Batuhan Bakar2019-06-16 21:47:51 +00:00Commented Jun 16, 2019 at 21:47