何时在Bash if语句中需要方括号？ [英] When are square brackets required in a Bash if statement?

问题描述

通常，我在if语句中使用方括号：

Usually, I use square brackets in the if statement:

if [ "$name" = 'Bob' ]; then ...

但是，当我检查 grep 成功我不使用方括号：

But, when I check if grep succeeded I don't use the square brackets:

if grep -q "$text" $file ; then ...

中何时需要方括号如果声明？

推荐答案

方括号是测试的同义词命令。 if 语句检查命令的退出状态以决定采用哪个分支。 grep -q$ text是一个命令，但$ name='Bob'不是 - 这只是一个表达。 test 是一个命令，它接受一个表达式并对其进行评估：

The square brackets are a synonym for the test command. An if statement checks the exit status of a command in order to decide which branch to take. grep -q "$text" is a command, but "$name" = 'Bob' is not--it's just an expression. test is a command, which takes an expression and evaluates it:

if test "$name" = 'Bob'; then ...

因为方括号是测试的同义词命令，然后您可以将其重写为原始语句：

Since square brackets are a synonym for the test command, you can then rewrite it as your original statement:

if [ "$name" = 'Bob' ]; then ...

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