参考：https://medium.com/analytics-vidhya/simple-tutorial-to-install-use-gnu-parallel-79251120d618
Download Latest version of GNU Parallel
$ wget http://ftp.gnu.org/gnu/parallel/parallel-latest.tar.bz2
Unzip it, it creates a new folder parallel-yyyymmdd
$ sudo tar xjf parallel-latest.tar.bz2
Change to new folder
$ cd parallel-yyyymmdd
Build the software in your machine
$ sudo ./configure && make
若无管理员权限，则指定prefix=自己的文件夹
Install it
$ sudo make install
Switch to home directory
$ cd
How to Test Parallel ?
$ parallel
使用：
ls [input dir] | parallel --jobs 32 --eta tesseract [input dir]/{} [output dir]{.} --oem 1 hocr (可选--tmpdir=[temp dir])