Which statement correctly distinguishes bash's `[[ ... ]]` from `[ ... ]`?
LPIC-1 Exam 102-500, objective 105. Shells and shell scripting hard
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The options
Not correct `[[ ... ]]` is the POSIX form and works in any /bin/sh, while `[ ... ]` is a bash extension.
Wrong, and reversed. `[` is the POSIX test command and exists as a builtin in every POSIX shell and as /usr/bin/[ on disk. `[[ ]]` is a bash and ksh extension that plain /bin/sh (dash) does not accept.
Correct `[[ ... ]]` is shell syntax, so word splitting and filename expansion are not applied to unquoted variables inside it, whereas `[` is an ordinary command whose arguments are expanded normally.
Correct. Because `[[` is parsed as a compound command, `[[ $file = *.txt ]]` works even when $file is empty or contains spaces, while `[ $file = x ]` can break into the wrong number of arguments.
Not correct `[ ... ]` needs no closing bracket; the trailing ] is optional.
Wrong. `[` is a command whose final argument must be `]`. Omitting it is a syntax error from the test builtin itself.
Not correct `[[ ... ]]` compares only integers, so strings must always be tested with `[`.
Wrong. `[[ ]]` handles strings well, including pattern matching with = and regex matching with =~. Numeric comparison inside it still uses -eq, -lt and friends, exactly as in `[`.
Why
`[` is literally a command: `[ -f /etc/passwd ]` is the test builtin with `]` as its last argument, which is why the spaces around the brackets are mandatory. `[[ ]]` is a bash keyword, so it can also offer && and || inside the test and unquoted pattern matching. For portable /bin/sh scripts use `[`; for bash-only scripts `[[ ]]` is safer against unquoted variables.
Where this comes from
- Cited
- LPI exam objective 105.2
- What it says
- Use test and conditional expressions in shell scripts.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 105 and space the ones you get wrong.