给定一个命令，其输出正由 grep 处理，我还希望将实际输出包含在日志文件中以进行调试。
例如，如果我有以下内容
useful=$(curl -s http://example.com/some/data | grep 'useful line')
echo "useful=$useful"
我想看看
This page has a number of lines in it, but'
useful lines are the only ones I care about
except when something goes wrong and then
I'd really like to see what the heck was in
the output that grep consumed.
useful=useful lines are the only ones I care about
这可以像这样用 T 恤完成
useful=$(curl -s http://example.com/some/data | tee /proc/$$/fd/1 | grep 'useful line')
echo …