问题
Here's a simple command that works fine in my bash shell:
echo "Created at: $(date)" >> README.md
It appends Created at: Wed Jan 24 10:04:48 STD 2018 to README.md.
However, ii I include the same command in my makefile, the behavior is different.
makefile:
README.md:
echo "Created at: $(date)" >> README.md
Running make README.md will treat the command substitution as an empty string like this:
echo "Created at: " >> README.md
What's appended to README.md is Created at:.
How do I get command substitution to output properly with echo in a makefile?
回答1:
If you want the shell that Make invokes to receive the following:
echo "Created at: $(date)" >> README.md
Then, you need to escape the $ with another $ inside the rule:
README.md:
echo "Created at: $$(date)" >> README.md
Otherwise, the Make's variable date is expanded and that will be what echo gets as argument, since $(date) in a makefile expands the variable date.
Out of intererst
Note that, if the Make's variable date is defined as below, it will however work as expected without quoting the $ in the rule:
date = $$(date)
README.md:
echo "Created at: $(date)" >> README.md
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.
来源：https://stackoverflow.com/questions/48429075/command-substitution-doesnt-work-with-echo-in-makefile