How to make bash script output compatible with both terminal and file?

Question

In bash, if I want to produce periodical status messages, I can replace the previous one by emitting a hotkey combination, e.g. as seen here, here or here. When a user executes the script directly, this is nice, because it keeps them informed without cluttering their screen.

However, if they want to use the same script in an automated context, the output will (must) be redirected into a file:

./test.sh >log.txt

At this point, none of \r, \b, \033[0K work.

How do I make this work for both modes?

(E.g. it would be straightforward enough to omit the intermediate messages, iff I could detect whether the output stream is a file.)

• If possible, I would like to avoid introducing an argument to the script. The real-world version already comes with several.
• If possible, I would like to avoid handling the output streams inside the script itself, e.g. as per this question, so it does not express surprising behaviour when embedded into other scripts.

Sample code test.sh:

#!/bin/bash

PREVIOUS_JOB_STATUS=""
SECONDS=0          # auto-incremented by Bash; divide by INTERVAL_SECONDS to get number of retries
INTERVAL_SECONDS=3 # interval between polls to the server
TIMEOUT_SECONDS=10 # how long until we give up, because the server is clearly busy (or dead)
while true; do
  RETRY_SECONDS=$SECONDS
  if [ $RETRY_SECONDS -lt $INTERVAL_SECONDS ]; then # in the real world, here would be      the polling operation
    JOB_STATUS='queued'
  else
    JOB_STATUS='processing'
  fi
  RESULT=1                                          # in the real world, here would be ? of the polling operation
  if [ $RESULT -eq 0 ]; then                            # success
    exit $RESULT
  elif [ $RETRY_SECONDS -gt $TIMEOUT_SECONDS ]; then    # failure (or at least surrender)
    echo "Timeout exceeded waiting for job to finish. Current Job Status is '$JOB_STATUS'. Current result code is '$RESULT'."
    exit $RESULT
  elif [ "$PREVIOUS_JOB_STATUS" = "$JOB_STATUS" ]; then # no change yet,      replace last status message and try again
    echo -en '\033[1A\r\033[K' # move one line up                                : \033[1A ,
                               # move to pos1                                    : \r      ,
                               # delete everything between cursor and end-of-line: \033[K  ,
                               # omit line break                                 : -n
  else                                                  # job status changed, write   new  status message and try again
    PREVIOUS_JOB_STATUS=$JOB_STATUS
  fi
  SLEEP_MESSAGE='Waiting for job to finish. Waited '`printf "%02g" $SECONDS`" s. Current job status is '$JOB_STATUS'. Current result code is '$RESULT'."
  echo $SLEEP_MESSAGE
  sleep $INTERVAL_SECONDS
done

Final output of ./test.sh:

Waiting for job to finish. Waited 00 s. Current job status is 'queued'. Current result code is '1'.
Waiting for job to finish. Waited 09 s. Current job status is 'processing'. Current result code is '1'.
Timeout exceeded waiting for job to finish. Current Job Status is 'processing'. Current result code is '1'.

Final output of ./test.sh >log.txt 2>&1:

Waiting for job to finish. Waited 00 s. Current job status is 'queued'. Current result code is '1'.
Waiting for job to finish. Waited 03 s. Current job status is 'processing'. Current result code is '1'.
^[[1A^M^[[KWaiting for job to finish. Waited 06 s. Current job status is 'processing'. Current result code is '1'.
^[[1A^M^[[KWaiting for job to finish. Waited 09 s. Current job status is 'processing'. Current result code is '1'.
Timeout exceeded waiting for job to finish. Current Job Status is 'processing'. Current result code is '1'.

Desired final output of ./test.sh >log.txt 2>&1: equal to final output of ./test.sh.

Answer 1

Use test -t 1 or test -t 2 to test if (respectively) stdout or stderr is associated with a terminal.

-t file_descriptor
True if file descriptor number file_descriptor is open and is associated with a terminal. False if file_descriptor is not a valid file descriptor number, or if file descriptor number file_descriptor is not open, or if it is open but is not associated with a terminal.

^(source)

Proof of concept:

sh -c '
test -t 1 && echo foo 
echo bar
'

Run the above code as-is and you will see foo and bar. Redirect the output away from the terminal (e.g. to a regular file, or pipe to cat) and foo will not be printed.

To avoid testing each time when you need to print an intermediate message, use this trick:

if test -t 1; then exec 5>&1; else exec 5>/dev/null; fi

Now print every intermediate message to the file descriptor 5 (e.g. echo baz >&5). It will go to stdout if it's a terminal, to /dev/null otherwise.