单词分裂的不良影响

$ a='I am a string with spaces'
$ [ $a = $a ] || echo "didn't match"
bash: [: too many arguments
didn't match

[ $a = $a ] 被解释为 [ I am a string with spaces = I am a string with spaces ]。[ 是 test 命令，I am a string with spaces 不是单个参数，而是 6 个参数！

$ [ $a = something ] || echo "didn't match"
bash: [: too many arguments
didn't match

[ $a = something ] 被解释为 [ I am a string with spaces = something ]

$ [ $(grep . file) = 'something' ]
bash: [: too many arguments

grep 命令返回带有空格的多行字符串，因此你可以想象有多少个参数……：D

了解基础知识的内容，时间和原因 。