Redirect stdin, stdout, stderr of a daemon to /dev/pts/#

fdhijack.sh

#!/bin/sh
# 
# Redirect stdin, stdout, stderr of a daemon to /dev/pts/# 
#
#################################################################################################################
# Ref: stdio buffering: http://www.pixelbeat.org/programming/stdio_buffering/
# Default Buffering modes:
#    stdin  -> is always buffered
#    stderr -> is always unbuffered
#    if stdout is a terminal then buffering is automatically set to line buffered, else it is set to buffered
#################################################################################################################
# Therefore if we set stdout, stderr unbuffered by setvbuf(), we don't need flush (-f) feature.
# However buffered mode stdio is more efficient.
#################################################################################################################

if [ "$2" = "" ]; then
    echo "Usage: $0 PID /path/to/pts"
    echo "Example: $0 \`pidof Daemon\` /dev/pts/1"
    echo "         $0 \`pidof Daemon\` \`tty\`"
    exit 0
fi

SLEEP_DURATION=5

GDB=${GDB:-gdb}

if ${GDB} --version > /dev/null 2>&1; then
    true
else
    echo "Unable to find gdb."
fi

pid=$1
dst=$2

# -f : flushes all open output streams to avoid buffered IO
if [ "${2}" = "-f" ]; then
    (
        echo "attach $pid" 
        echo 'call fflush(0)'
        echo 'detach' 
        echo 'quit' 
        sleep 5
    ) | ${GDB} -q -x -
else
    (
        echo "attach $pid" 
        echo 'call open("'$dst'", 66, 0666)' 
        #echo 'call dup2($1,0)' # stdin
        echo 'call dup2($1,1)'  # stdout, $1 derived from previous open() by gdb
        echo 'call dup2($1,2)'  # stderr
        # NOTE: setvbuf cause SIGSEGV in some situation, 
        #       uncomment them if you needed.
        #echo 'p setvbuf(stdout, 0, 2, 0)' #set _IONBF unbuffered
        #echo 'p setvbuf(stderr, 0, 2, 0)' #set _IONBF unbuffered
        echo 'call close($1)'
        echo 'detach'
        echo 'quit'
        sleep $SLEEP_DURATION
    ) | ${GDB} -q -x -
fi