Bash括号使用

How to use double or single brackets, parentheses, curly braces in Bash

1. A single bracket ([) usually actually calls a program named [; man test or man [ for more info. Example:

   $ VARIABLE=abcdef
   $ <span class="hljs-keyword">if</span> [ <span class="hljs-variable">$VARIABLE</span> == abcdef ] ; <span class="hljs-keyword">then</span> <span class="hljs-built_in">echo</span> <span class="hljs-built_in">yes</span> ; <span class="hljs-keyword">else</span> <span class="hljs-built_in">echo</span> no ; <span class="hljs-keyword">fi</span>
   <span class="hljs-built_in">yes</span>
   

2. The double bracket ([[) does the same thing (basically) as a single bracket, but is a bash builtin.

   $ VARIABLE=abcdef
   $ <span class="hljs-keyword">if</span> [[ <span class="hljs-variable">$VARIABLE</span> == 123456 ]] ; <span class="hljs-keyword">then</span> <span class="hljs-built_in">echo</span> <span class="hljs-built_in">yes</span> ; <span class="hljs-keyword">else</span> <span class="hljs-built_in">echo</span> no ; <span class="hljs-keyword">fi</span>
   no
   

3. Parentheses (()) are used to create a subshell. For example:

   $ <span class="hljs-built_in">pwd</span>
   /home/user 
   $ (<span class="hljs-built_in">cd</span> /tmp; <span class="hljs-built_in">pwd</span>)
   /tmp
   $ <span class="hljs-built_in">pwd</span>
   /home/user
   

   As you can see, the subshell allowed you to perform operations without affecting the environment of the current shell.

4. (a) Braces ({}) are used to unambiguously identify variables. Example:

   $ VARIABLE=abcdef
   $ <span class="hljs-built_in">echo</span> Variable: <span class="hljs-variable">$VARIABLE</span>
   Variable: abcdef
   $ <span class="hljs-built_in">echo</span> Variable: <span class="hljs-variable">$VARIABLE123456</span>
   Variable:
   $ <span class="hljs-built_in">echo</span> Variable: <span class="hljs-variable">${VARIABLE}</span>123456
   Variable: abcdef123456
   

   (b) Braces are also used to execute a sequence of commands in the current shell context, e.g.

   $ { <span class="hljs-built_in">date</span>; top -b -n1 | <span class="hljs-built_in">head</span> ; } &gt;logfile 
   <span class="hljs-comment"># 'date' and 'top' output are concatenated, </span>
   <span class="hljs-comment"># could be useful sometimes to hunt for a top loader )</span>
       
   $ { <span class="hljs-built_in">date</span>; make 2&gt;&amp;1; <span class="hljs-built_in">date</span>; } | <span class="hljs-built_in">tee</span> logfile
   <span class="hljs-comment"># now we can calculate the duration of a build from the logfile</span>
   

There is a subtle syntactic difference with ( ), though (see bash reference) ; essentially, a semicolon ; after the last command within braces is a must, and the braces {, } must be surrounded by spaces.

Source: StackOverflow