$()允许嵌套。

out=$(echo today is $(date))

我觉得倒勾是不允许的。

反引号('…')是只有最古老的不兼容POSIX的boure -shell才需要的遗留语法，而$(…)是POSIX，出于以下几个原因更受欢迎:

Backslashes (\) inside backticks are handled in a non-obvious manner: $ echo "`echo \\a`" "$(echo \\a)" a \a $ echo "`echo \\\\a`" "$(echo \\\\a)" \a \\a # Note that this is true for *single quotes* too! $ foo=`echo '\\'`; bar=$(echo '\\'); echo "foo is $foo, bar is $bar" foo is \, bar is \\ Nested quoting inside $() is far more convenient: echo "x is $(sed ... <<<"$y")" instead of: echo "x is `sed ... <<<\"$y\"`" or writing something like: IPs_inna_string=`awk "/\`cat /etc/myname\`/"'{print $1}' /etc/hosts` because $() uses an entirely new context for quoting which is not portable as Bourne and Korn shells would require these backslashes, while Bash and dash don't. Syntax for nesting command substitutions is easier: x=$(grep "$(dirname "$path")" file) than: x=`grep "\`dirname \"$path\"\`" file` because $() enforces an entirely new context for quoting, so each command substitution is protected and can be treated on its own without special concern over quoting and escaping. When using backticks, it gets uglier and uglier after two and above levels. Few more examples: echo `echo `ls`` # INCORRECT echo `echo \`ls\`` # CORRECT echo $(echo $(ls)) # CORRECT It solves a problem of inconsistent behavior when using backquotes: echo '\$x' outputs \$x echo `echo '\$x'` outputs $x echo $(echo '\$x') outputs \$x Backticks syntax has historical restrictions on the contents of the embedded command and cannot handle some valid scripts that include backquotes, while the newer $() form can process any kind of valid embedded script. For example, these otherwise valid embedded scripts do not work in the left column, but do work on the rightIEEE: echo ` echo $( cat <<\eof cat <<\eof a here-doc with ` a here-doc with ) eof eof ` ) echo ` echo $( echo abc # a comment with ` echo abc # a comment with ) ` ) echo ` echo $( echo '`' echo ')' ` )

Therefore the syntax for $-prefixed command substitution should be the preferred method, because it is visually clear with clean syntax (improves human and machine readability), it is nestable and intuitive, its inner parsing is separate, and it is also more consistent (with all other expansions that are parsed from within double-quotes) where backticks are the only exception and ` character is easily camouflaged when adjacent to " making it even more difficult to read, especially with small or unusual fonts.

来源:为什么$(…)比'…”(引号)?在BashFAQ

参见:

POSIX标准节“2.6.3命令替换” 包含$()语法的POSIX原理 命令替换 Bash-hackers:命令替换

除了其他答案，

$(...)

在视觉上更突出

`...`

反撇号看起来太像撇号了;这取决于你使用的字体。

(而且，正如我刚刚注意到的，在内联代码示例中输入反撇号要困难得多。)

最主要的一个是嵌套它们的能力，命令中的命令，而不会失去理智，试图弄清楚某种形式的转义是否对反节拍有效。

一个例子，虽然有点做作:

deps=$(find /dir -name $(ls -1tr 201112[0-9][0-9]*.txt | tail -1l) -print)

它会给你一个/dir目录树中所有与2011年12月以来最早的文本文件同名的文件的列表(a)。

另一个例子是获取父目录的名称(而不是完整路径):

pax> cd /home/pax/xyzzy/plugh
pax> parent=$(basename $(dirname $PWD))
pax> echo $parent
xyzzy

(a)既然那个特定的命令可能实际上不起作用，我还没有测试它的功能。所以，如果你投我的反对票，你已经失去了意图:-)这只是一个说明你如何嵌套，而不是作为一个没有bug的生产准备片段。

来自man bash:

       $(command)
or
       `command`

Bash performs the expansion by executing command and replacing the com-
mand  substitution  with  the  standard output of the command, with any
trailing newlines deleted.  Embedded newlines are not deleted, but they
may  be  removed during word splitting.  The command substitution $(cat
file) can be replaced by the equivalent but faster $(< file).

When the old-style backquote form of substitution  is  used,  backslash
retains  its  literal  meaning except when followed by $, `, or \.  The
first backquote not preceded by a backslash terminates the command sub-
stitution.   When using the $(command) form, all characters between the
parentheses make up the command; none are treated specially.

我想出了一个完全有效的$(…)除以'…'的例子。

我正在使用一个运行Cygwin的Windows远程桌面，并希望迭代一个命令的结果。遗憾的是，由于远程桌面或Cygwin本身的原因，反勾字符无法输入。

合理的假设是，在这种奇怪的设置中，美元符号和圆括号会更容易输入。