eval "tail | grep" | cat

01-output

$ ~/foo.sh 2>/dev/null
I want this:
eval "tail -n100 -f /tmp/tmp.85E0vH3e5d | grep 'GET./foo'" | cat
(real use case is running `tail -f | grep` in this: https://github.com/marek-saji/task-runner.sh/blob/b876596dab86c30c57ee341ca4511e079a2ae4ff/src/task-runner.sh#L89-L91)
... but that doesn't work

Removing `| cat` makes it work:
eval "tail -n100 -f /tmp/tmp.85E0vH3e5d" | grep 'GET./foo'
GET /foo

so does dropping `-f` from `tail`:
eval "tail -n100 /tmp/tmp.85E0vH3e5d | grep 'GET./foo'" | grep 'GET./foo'
GET /foo

or `grep`:
eval "tail -n100 -f /tmp/tmp.85E0vH3e5d" | cat
GET /foo
GET /bar

or changing that grep to cat
eval "tail -n100 -f /tmp/tmp.85E0vH3e5d | cat" | cat
GET /foo
GET /bar

02-test.sh

#!/bin/bash

LOG=$( mktemp )
echo 'GET /foo' >> ${LOG}
echo 'GET /bar' >> ${LOG}

echo 'I want this:'
echo "eval \"tail -n100 -f ${LOG} | grep 'GET./foo'\" | cat"
echo '(real use case is running `tail -f | grep` in this: https://github.com/marek-saji/task-runner.sh/blob/b876596dab86c30c57ee341ca4511e079a2ae4ff/src/task-runner.sh#L89-L91)'
( eval "tail -n100 -f ${LOG} | grep 'GET./foo'" | cat ; ) &
PID=$!
sleep 1
kill ${PID} 2>&1 > /dev/null
echo "... but that doesn't work"

echo

echo 'Removing `| cat` makes it work:'
echo "eval \"tail -n100 -f ${LOG}\" | grep 'GET./foo'"
( eval "tail -n100 -f ${LOG}" | grep 'GET./foo' ; ) &
PID=$!
sleep 1
kill ${PID} 2>&1 > /dev/null

echo

echo 'so does dropping `-f` from `tail`:'
echo "eval \"tail -n100 ${LOG} | grep 'GET./foo'\" | grep 'GET./foo'"
( eval "tail -n100 ${LOG} | grep 'GET./foo'" | grep 'GET./foo' ; )

echo

echo 'or `grep`:'
echo "eval \"tail -n100 -f ${LOG}\" | cat"
( eval "tail -n100 -f ${LOG}" | cat ; ) &
PID=$!
sleep 1
kill ${PID} 2>&1 > /dev/null

echo

echo 'or changing that grep to cat'
echo "eval \"tail -n100 -f ${LOG} | cat\" | cat"
( eval "tail -n100 -f ${LOG} | cat" | cat ; ) &
PID=$!
sleep 1
kill ${PID} 2>&1 > /dev/null

rm -rf ${LOG_DIR}