Yet another way to turn on line-buffering output mode for the long_running_command is to use the script command that runs your long_running_command in a pseudo terminal (pty).

script -q /dev/null long_running_command | print_progress      # FreeBSD, Mac OS X
script -c "long_running_command" /dev/null | print_progress    # Linux

Yet another way to turn on line-buffering output mode for the long_running_command is to use the script command that runs your long_running_command in a pseudo terminal (pty).

script -q /dev/null long_running_command | print_progress      # (FreeBSD, Mac OS X)
script -q -c "long_running_command" /dev/null | print_progress # (Linux)