在 Bash 4.3.11 测试:
POSIX vs Bash 扩展:
[ 是 POSIX
[[ is 一个 Bash 扩展
常规命令 vs 魔法
[ 只是一个有着奇怪名字的常规命令。
] is just an argument of [ that prevents further arguments from being used.只是一个 [ 的避免更多的参数被使用的参数。
Ubuntu 16.04 actually has an executable for it at /usr/bin/[ provided by coreutils, but the bash built-in version takes precedence.
Nothing is altered in the way that Bash parses the command.
In particular, < is redirection, && and || concatenate multiple commands, ( ) generates subshells unless escaped by \, and word expansion happens as usual.
[[ X ]] is a single construct that makes X be parsed magically. <, &&, || and () are treated specially, and word splitting rules are different.
There are also further differences like = and =~.
In Bashese: [ is a built-in command, and [[ is a keyword: https://askubuntu.com/questions/445749/whats-the-difference-between-shell-builtin-and-shell-keyword
<
[[ a < b ]]: lexicographical comparison
[ a < b ]: Same as above. required or else does redirection like for any other command. Bash extension.
I could not find a POSIX alternative to this, see: https://stackoverflow.com/questions/21294867/how-to-test-strings-for-less-than-or-equal
&& and ||
[[ a = a && b = b ]]: true, logical and
[ a = a && b = b ]: syntax error, && parsed as an AND command separator cmd1 && cmd2
[ a = a -a b = b ]: equivalent, but deprecated by POSIX
[ a = a ] && [ b = b ]: POSIX recommendation
(
[[ (a = a || a = b) && a = b ]]: false
[ ( a = a ) ]: syntax error, () is interpreted as a subshell
[ ( a = a -o a = b ) -a a = b ]: equivalent, but () is deprecated by POSIX
([ a = a ] || [ a = b ]) && [ a = b ] POSIX recommendation
word splitting
x='a b'; [[ $x = 'a b' ]]: true, quotes not needed
x='a b'; [ $x = 'a b' ]: syntax error, expands to [ a b = 'a b' ]
x='a b'; [ "$x" = 'a b' ]: equivalent
=
[[ ab = a? ]]: true, because it does pattern matching (* ? [ are magic). Does not glob expand to files in current directory.
[ ab = a? ]: a? glob expands. So may be true or false depending on the files in the current directory.
[ ab = a? ]: false, not glob expansion
= and == are the same in both [ and [[, but == is a Bash extension.
printf 'ab' | grep -Eq 'a.': POSIX ERE equivalent
[[ ab =~ 'ab?' ]]: false, loses magic with ''
[[ ab? =~ 'ab?' ]]: true
=~
[[ ab =~ ab? ]]: true, POSIX extended regular expression match, ? does not glob expand
[ a =~ a ]: syntax error
printf 'ab' | grep -Eq 'ab?': POSIX equivalent
Recommendation
I prefer to always use [].
There are POSIX equivalents for every [[ ]] construct I've seen.
If you use [[ ]] you:
lose portability
force the reader to learn the intricacies of another bash extension. [ is just a regular command with a weird name, no special semantics are involved.
没翻译完，待续