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 ?
Asked
Active
Viewed 83 times
0
terdon
- 242,166
-
Are you asking if two opposite values are the same? O.o – muru Jun 16 '19 at 17:42
-
@muru No I mean are results true ? – Batuhan Bakar Jun 16 '19 at 17:43
-
But shouldn't the answer be 1 if the result is true? – Batuhan Bakar Jun 16 '19 at 17:45
-
1https://stackoverflow.com/q/2933843/2072269 – muru Jun 16 '19 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' Jun 16 '19 at 21:20
-
If any of the answers solves your issue, please consider "accepting" the answer. This is the best way to show gratitude on this site. Accepting an answer not only marks the question as resolved, but also signals to future readers that the accepted answer actually solved the issue. More information about this is available here: https://unix.stackexchange.com/help/someone-answers – Kusalananda Jun 16 '19 at 22:45
3 Answers
4
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
Kusalananda
- 333,661
3
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)
terdon
- 242,166
-
-
With the caveat, of course, that in
(()), booleans are C-style (0 is falsey) – D. Ben Knoble Jun 17 '19 at 01:05
0
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.
ctrl-alt-delor
- 27,993