命令替换不适用于makefile中的echo [英] command substitution doesn't work with echo in makefile

问题描述

这是一个简单的命令，可以在我的bash shell中正常工作:

Here's a simple command that works fine in my bash shell:

echo "Created at: $(date)" >> README.md

它将Created at: Wed Jan 24 10:04:48 STD 2018附加到README.md.

It appends Created at: Wed Jan 24 10:04:48 STD 2018 to README.md.

但是，ii我在我的makefile文件中包含了相同的命令，其行为是不同的.

However, ii I include the same command in my makefile, the behavior is different.

makefile:

README.md:
    echo "Created at: $(date)" >> README.md

运行make README.md会将命令替换视为空字符串，如下所示:

Running make README.md will treat the command substitution as an empty string like this:

echo "Created at: " >> README.md

在README.md后面附加的是Created at:.

What's appended to README.md is Created at:.

如何获取命令替换以在makefile中正确显示回声?

How do I get command substitution to output properly with echo in a makefile?

推荐答案

如果您希望Make调用的shell接收以下内容:

If you want the shell that Make invokes to receive the following:

echo "Created at: $(date)" >> README.md

然后，您需要在规则内使用另一个$转义$:

Then, you need to escape the $ with another $ inside the rule:

README.md:
    echo "Created at: $$(date)" >> README.md

否则，将扩展Make的变量date，这将成为echo作为参数的内容，因为makefile中的$(date)会扩展变量date.

Otherwise, the Make's variable date is expanded and that will be what echo gets as argument, since $(date) in a makefile expands the variable date.

请注意，如果Make变量date的定义如下，则它将按预期工作，而无需在规则中引用$:

Note that, if the Make's variable date is defined as below, it will however work as expected without quoting the $ in the rule:

date = $$(date)

README.md:
    echo "Created at: $(date)" >> README.md

原因是变量date(在规则的配方中使用)将由Make扩展为$(date)，并将被传递到外壳程序.

The reason is that the variable date (used in the rule's recipe) will be expanded by Make to $(date) and that will be passed to the shell.

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