Linux Bash：单引号/双引号中的变量扩展
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Bash: Variable Expansion in Single Quote / Double Quote
提问by Susan Mayer
I want to add a variable ${date}in the following bash script:
我想${date}在以下 bash 脚本中添加一个变量：
ffmpeg -i in.flv -vf drawtext="fontfile=Sans.ttf:text='Today is ${date}':fontsize=6" out.flv
Obviously, ${date}won't expand in single quote, please also note that there is a double quote beyond the single quote, which makes it even more complicated.
显然，${date}不会在单引号中展开，还请注意单引号之外还有一个双引号，这使得它更加复杂。
Thanks a lot. I am on CentOS 6.
非常感谢。我在 CentOS 6 上。
采纳答案by Matteo
${date}is expanded because it is between double quotes (the single quotes inside the double quotes are just characters)
${date}展开是因为它在双引号之间（双引号内的单引号只是字符）
Test it with:
测试它：
$ export date=SOMEVALUE
$ echo ffmpeg -i in.flv -vf drawtext="fontfile=/usr/share/fonts/dejavu/DejaVuLGCSans.ttf:text='Today is ${date}':fontsize=6" out.flv
ffmpeg -i in.flv -vf drawtext=fontfile=/usr/share/fonts/dejavu/DejaVuLGCSans.ttf:text='Today is SOMEVALUE':fontsize=6 out.flv
回答by fge
Your ${date}WILL expand correctly. As you said yourself, you surround the whole string with double quotes, and bash willexpand variables into double quotes.
你的${date}遗嘱正确展开。正如你自己所说，你用双引号将整个字符串括起来，bash会将变量扩展为双引号。
The fact that there are inner single quotes does not matter at all:
有内部单引号的事实根本无关紧要：
fg@erwin ~ $ ritchie="Goodbye world"
fg@erwin ~ $ echo "When Dennis passed away, he said '$ritchie'"
When Dennis passed away, he said 'Goodbye world'
