Home / ubuntu / Questions / 1239899
Accepted
ariaman5
Asked: 2020-05-16 05:18:11 +0800 CST

piping grep to another grep doesn't work as expected

  • 772

I'm using grep on Ubuntu Desktop 20.04.

I want to get player movements from gnuchess game by using grep each turn. The first command i used is this :

gnuchess -mq | grep -a "Black ([0-9][0-9]*) : [0-9][0-9]*. [a-z][0-9]" | grep "[a-z][0-9]"

for the first grep, if movements are 'e3' and 'e5', the output is :

Black (1) : 1. e5

I expect the second grep give me e5 but it doesn't. Output is blank.

Yet when i redirect gnuchess output to a file and use the same command it works and shows e5. Why?

Edit:

The problem was with output buffering. Added my answer below.

command-line grep

2 Answers

  1. Best Answer

    turning off output buffering did the work.

    gnuchess -mq | stdbuf -o0 grep -a "Black ([0-9][0-9]*) : [0-9][0-9]*. [a-z][0-9]" | grep -o "[a-z][0-9]"

    For more information: this

    • 2

  2. grep returns the whole line where it finds a match, so the behaviour you experience is the expected one.

    If you are sure that what you want is the fifth token of the string, you can use awk:

    gnuchess -mq | grep -a "Black ([0-9][0-9]*) : [0-9][0-9]*. [a-z][0-9]" | awk '{print $5}'

    For example: echo 'Black (1) : 1. e5' | grep -a "Black ([0-9][0-9]*) : [0-9][0-9]*. [a-z][0-9]" | awk '{print $5}' returns only e5.

    • -1