为什么“ sh -c”返回与实际命令不同的结果？

Question

I'm trying to get the number of pages in a PDF file through the command line.

pdfinfo "/tmp/temp.pdf" | grep Pages: | awk '{print $2}'

produces

3

In Node.js, I need to use 'sh' because of the piping.

But

sh -c "pdfinfo '/tmp/temp.pdf' | grep Pages: | awk '{print $2}'"

produces

Pages:          3

Why am I getting different output?

Answer 1

The sh -c command is double quoted, which will expand the $2 which is likely empty, so the awk command becomes just print which prints the whole line. You could escape the $ to prevent it from being expanded:

sh -c "pdfinfo '/tmp/temp.pdf' | grep Pages: | awk '{print \$2}'"

Incidentally, awk can do pattern matching, so no need for both grep and awk :

pdfinfo "/tmp/temp.pdf" | awk '/Pages:/ {print $2}'