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Note that the test command has a bad design which is fixed by the ksh/bash alternative.

rdebath
2017-06-25 14:12:21 +01:00
parent d736a1ce11
commit f97d5375ec
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SC2166.md

@@ -14,12 +14,14 @@ And likewise, prefer `[ p ] || [ q ]` over `[ p -o q ]`.
[ "$1" = "test" ] && [ -z "$2" ]
```
### Rationale:
`-a` and `-o` to mean AND and OR in a `[ .. ]` test expression is not well defined. They are obsolescent extensions in [POSIX](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html) and their behavior is almost always undefined.
`-a` and `-o` to mean AND and OR in a `[ .. ]` test expression is not well defined. They are obsolescent extensions in [POSIX](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html) and the lack of additional quoting levels means that expressions involving these operators are often ambiguous. Values that begin with hyphens and the `!` string are the usual issues.
Using multiple `[ .. ]` expressions with shell AND/OR operators `&&` and `||` is well defined and therefore preferred (but note that they have equal precedence, while `-a`/`-o` is unspecified but often implemented as `-a` having higher precedence).
Using multiple `[ .. ]` expressions with shell AND/OR operators `&&` and `||` is well defined and therefore preferred (but note that they have equal precedence, while `-a`/`-o` is unspecified but usually implemented as `-a` having higher precedence).
### Exceptions:
None.
If the shell variant being used is ksh derived (such as the bash shell) it will have the shell builtin command `[[ ... ]]`. This has the operators `&&`, `||`, `(`, `)`, `!` which safely avoid the ambiguity by noting which arguments were quoted and requiring the operators to be unquoted (except by the `[[ ... ]]` construct itself).