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  • Note that in bash, it would return yes for " 123 " for instance. You wouldn't want to use that approach in ksh where it would not only work but also be a command injection vulnerability. Commented Jan 25, 2021 at 10:50
  • The entire matter of testing with "[[" vs "[", and the subtle differences therein which can often be lost to memory after reading up on the matter after only a few months, is precisely demonstrated in this answer. Therefore, I consider any type of pattern that is not reliable in any type of shell to be a "bad". As a result, I tend to make all code within a script use only one or the other type of test (i.e. if using bash, every test should be "[[" and would be considered "bad" if ever using "[" anywhere. Likewise for bourne shell, you can only use "[" and this type of test would be OK there) Commented Aug 10, 2023 at 20:16