评估字符串中的变量 [英] Evaluating variables in a string

问题描述

此脚本中的最后一行将无法正常运行:

The last line in this script won't work as I expect:

myfile="afile.txt"
mycmd='cat $myfile'
eval $mycmd
echo eval $mycmd

此处echo显示'eval cat $ myfile'.如何打印"eval cat afile.txt"?

Here echo prints 'eval cat $myfile'. How can I print 'eval cat afile.txt'?

推荐答案

让我们一步一步来做吧:

Let's take things step by step:

执行此操作时:

mycmd='cat $myfile'

可以防止外壳插值$myfile.因此:

You prevent the shell from interpolating $myfile. Thus:

$ echo $mycmd
cat $myfile

如果要允许插值，可以使用双引号:

If you want to allow the interpolation, you can use double quotes:

$ mycmd="echo $myfile"  #Double quotes!
$ echo "$mycmd"
cat afile.txt

当然，当您执行eval时，这会冻结解释$mycmd.

This, of course, freezes the interpretation of $mycmd when you do an eval.

$ myfile="afile.txt"
$ mycmd="echo $myfile"
$ echo $mycmd
cat afile.txt
$ eval $mycmd   #Prints out afile.txt
$ myfile=bfile.txt
$ eval $mycmd   #Still prints out afile.txt and not bfile.txt

将此与以下内容进行比较:

Compare this to:

$ myfile="afile.txt"
$ mycmd='cat $myfile'   #Single quotes hide $myfile from the shell
echo $mycmd
cat $myfile             #Shell didn't change "$myfile", so it prints as a literal
$ eval $mycmd           #Prints out afile.txt
$ myfile=bfile.txt
$ eval $mycmd           #Now prints out bfile.txt

您可能想要做的是在回显语句时在回显语句中评估$mycmd:

What you probably want to do is to evaluate the $mycmd in an echo statement when you echo it:

$ echo $(eval "echo $mycmd")
$ cat afile.txt
$ myfile=bfile.txt
$ echo $(eval "echo $mycmd")
cat bfile.txt

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