最近甲同事权利代码审查该[[ ]]构建体是优选的超过[ ]中样构建体
if [ "`id -nu`" = "$someuser" ] ; then
echo "I love you madly, $someuser"
fi他无法提供理由。有一个吗？
[[具有较少的惊喜，并且通常更安全使用。但是它不是可移植的 - POSIX 没有指定它的功能，只有一些 shell 支持它（除了 bash，我听说 ksh 也支持它）。例如，您可以
[[ -e $b ]]测试文件是否存在。但是，使用[ ，必须引用$b ，因为它会拆分参数并扩展诸如"a*" （其中[[实际上是接受它的地方）。这还与怎么办[可外接程序，并接受它的参数通常只是像其他程序（虽然它也可以是一个内置的，但后来仍然没有这种特殊处理）。
[[还具有其他一些不错的功能，例如与=~匹配的正则表达式以及类似 C 的语言中已知的运算符。这是一个很好的页面： test， [和[[什么区别？和重击测试
Behavior differences
Some differences on Bash 4.3.11:
POSIX vs Bash extension:
[ is POSIX[[ is a Bash extension inspired from Korn shellregular command vs magic
[ is just a regular command with a weird name.
] is just the last argument of [.
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.expr x"$x" \< x"$y" > /dev/null or [ "$(expr x"$x" \< x"$y")" = 1 ]: POSIX equivalents, see: How to test strings for lexicographic less than or equal in Bash?&& and ||
[[ a = a && b = b ]]: true, logical and[ a = a && b = b ]: syntax error, && parsed as an AND command separator cmd1 && cmd2[ a = a ] && [ b = b ]: POSIX reliable equivalent[ a = a -a b = b ]: almost equivalent, but deprecated by POSIX because it is insane and fails for some values of a or b like ! or ( which would be interpreted as logical operations(
[[ (a = a || a = b) && a = b ]]: false. Without ( ), would be true because [[ && ]] has greater precedence than [[ || ]][ ( a = a ) ]: syntax error, () is interpreted as a subshell[ \( a = a -o a = b \) -a a = b ]: equivalent, but (), -a, and -o are deprecated by POSIX. Without \( \) would be true because -a has greater precedence than -o{ [ a = a ] || [ a = b ]; } && [ a = b ] non-deprecated POSIX equivalent. In this particular case however, we could have written just: [ a = a ] || [ a = b ] && [ a = b ] because the || and && shell operators have equal precedence unlike [[ || ]] and [[ && ]] and -o, -a and [word splitting and filename generation upon expansions (split+glob)
x='a b'; [[ $x = 'a b' ]]: true, quotes not neededx='a b'; [ $x = 'a b' ]: syntax error, expands to [ a b = 'a b' ]x='*'; [ $x = 'a b' ]: syntax error if there's more than one file in the current directory.x='a b'; [ "$x" = 'a b' ]: POSIX 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.case ab in (a?) echo match; esac: POSIX equivalent[[ ab =~ 'ab?' ]]: false, loses magic with '' in Bash 3.2 and above and provided compatibility to bash 3.1 is not enabled (like with BASH_COMPAT=3.1)[[ ab? =~ 'ab?' ]]: true=~
[[ ab =~ ab? ]]: true, POSIX extended regular expression match, ? does not glob expand[ a =~ a ]: syntax error. No bash equivalent.printf 'ab\n' | grep -Eq 'ab?': POSIX equivalent (single line data only)awk 'BEGIN{exit !(ARGV[1] ~ ARGV[2])}' ab 'ab?': POSIX equivalent.Recommendation: always use []
There are POSIX equivalents for every [[ ]] construct I've seen.
如果使用[[ ]] ，则：
[只是一个具有奇怪名称的常规命令，不涉及任何特殊的语义。感谢StéphaneChazelas 所做的重要更正和补充。
[[ ]]具有更多功能 - 建议您阅读Advanced Bash 脚本指南，以获取更多信息，尤其是第 7 章中的扩展测试命令部分。测试。
顺便说一句，作为指南， [[ ]]是在 ksh88（1988 年版本的 Korn shell）中引入的。