How do I tell if a program is printing to stderr or stdout in the terminal?

Question

NOTE: a newline appears but od -c doesn't display it.

My music player's status command (quodlibet --status | od -c) has three states, and I'm trying to write a conditional based on that output. The outputs from that command are like this (may differ depending on the configuration).

1. I think this is printing to stderr because od -c doesn't display properly, even though it does for the other commands.

   not-running
   0000000
   

2. paused

   0000000   p   a   u   s   e   d       P   a   n   e   d   B   r   o   w
   0000020   s   e   r       1   .   0   0   0       s   h   u   f   f   l
   0000040   e       o   n       0   .   2   2   8  \n
   0000053
   

3. playing

   0000000   p   l   a   y   i   n   g       P   a   n   e   d   B   r   o
   0000020   w   s   e   r       1   .   0   0   0       s   h   u   f   f
   0000040   l   e       o   n       0   .   2   3   2  \n
   0000054
   

My goal is to remove everything except "not-running", "paused", or "playing" and use that in a conditional, like this:

#!/bin/bash

status=$(quodlibet --status | awk '{split($0,m," "); printf "%s",m[1]}' | tr -d '\000\007\010\n')
if [ "$status" = "playing" ]; then
  quodlibet --print-playing '<artist>: <title>' | cut -c1-45
else
  echo -n "$status"
fi

In the case where the player isn't running though this always prints a newline in the terminal after "not-running." Even if I do something like this

echo -n "$(quodlibet --print-playing '<artist>: <title>' | cut -c1-45)"

If I pipe that output to od -c, I get the same

not-running
0000000

which isn't right. The newline doesn't show up, but its their in the terminal.

Questions

• What's causing this?
• Is the command printing to standard error when the program isn't running?
• How can I determine this?

Answer 1

Couple of ways to approach this.

1. merge streams

   You could by pass determining the difference all together and simply merge STDERR and STDOUT.

   Example

   quodlibet --status 2>&1 | ...
   

2. use grep

   You could chop the output down by using the -o & -E switches to grep.

   Example

   $ echo "...blah not-running blah..." | grep -Eo "not-running|paused|playing"
   not-running
   
   $ echo "...blah paused blah..." | grep -Eo "not-running|paused|playing"
   paused
   
   $ echo "...blah playing blah..." | grep -Eo "not-running|paused|playing"
   playing
   

   This will cut everything out except for the strings that match the regex argument to grep.

3. determine the stream's type

   You can use the -t switch to determine the type of the file descriptor stream.

   excerpt from Bash man page

   -t fd True if file descriptor fd is open and refers to a terminal.

   where fd is one of:

   0: stdin
   1: stdout
   2: stderr

   Example

   This detects if the output is coming from STDOUT.

   $ if [ -t 1 ]; then echo "from STDOUT"; fi
   from STDOUT
   

   Returns "from STDOUT" since the output is coming through while:

   $ (if [ -t 1 ]; then echo "from STDOUT"; fi) | cat
   

   Returns nothing, since the output is being directed to cat.