Redirect stderr/-out of running application to my own bash script

Question

I am running an application called "hd-idle". It is spinning down disks after a specific time of inactivity. The output looks like this:

user@linux:~$ sudo /usr/sbin/hd-idle -i 10800
symlinkPolicy=0, defaultIdle=10800, defaultCommand=scsi, defaultPowerCondition=0, debug=false, logFile=, devices=
sda spindown
sdd spindown
sde spindown
sda spinup
sdd spinup
sdd spindown
[...]

I want to save this output to a logfile (while the application in running), add timestamps and change sd[a-z] to corresponding model/serial of the hard drive.

I wrote a small bash script that does what I want:

user@linux:~$ cat hd_idle_logger.sh
#!/bin/bash

DATUM=$(date '+%Y-%m-%d %H:%M:%S')
INPUT=$(cat)

REGEX='(sd[a-z])\s(spin(down|up))'

[[ $INPUT =~ $REGEX ]]

if [ -n ${BASH_REMATCH[1]} ]
then
    MODEL=$(lsblk /dev/${BASH_REMATCH[1]} -n -o MODEL)
    SERIAL=$(lsblk /dev/${BASH_REMATCH[1]} -n -o SERIAL)
fi

echo -e "$DATUM\t${MODEL}_$SERIAL (${BASH_REMATCH[1]})\t${BASH_REMATCH[2]}" >> /home/linux/hd_idle_logger.log

I can verify that it works:

user@linux:~$ echo "sdd spindown" |& ./hd_idle_logger.sh

user@linux:~$ cat hd_idle_logger.log
2023-02-12 12:14:54     WDC_WD120EMAZ-10BLFA6_1PAEL2ES (sdd)    spindown

But running the application and passing the output to my script doesn't work, the logfile doesn't produce any content and I don't see the output on console anymore:

user@linux:~$ sudo /usr/sbin/hd-idle -i 10800 |& /home/user/hd_idle_logger.sh

So what I am doing wrong?

`INPUT=$(cat)`, the variable will only be assigned a value once the stdin file descriptor has been released reaching EOF. So it only captures the output of hd-idle once it finishes execution. — Léa Gris, Feb 12 '23 at 11:44
See: this answer to [Is there a Unix utility to prepend timestamps to stdin?](https://stackoverflow.com/a/9813614/7939871) — Léa Gris, Feb 12 '23 at 11:46

Socowi · Accepted Answer · 2023-02-13T00:58:00.033

As long as hd-idle is running, your script will be stuck at INPUT=$(cat). Because $(cat) has to capture ALL output, it can online terminate once hd-idle terminated.

You need a script/program that processes hd-idle's output on the fly; e.g. line by line, while hd-idle is still running. You could do this with a while read loop:

#! /bin/bash
regex='(sd[a-z])\s(spin(down|up))'
while IFS= read -r line; do
  [[ $line =~ $regex ]] || continue
  model=$(lsblk /dev/"${BASH_REMATCH[1]}" -n -o MODEL)
  serial=$(lsblk /dev/"${BASH_REMATCH[1]}" -n -o SERIAL)
  printf '%(%Y-%m-%d %H:%M:%S)T\t%s_%s (%s)\t%s\n' \
    "$model" "$serial" "${BASH_REMATCH[1]}" "${BASH_REMATCH[2]}"
done >> /home/linux/hd_idle_logger.log

However, it would be more efficient to switch to utils like sed or awk and pre-compute the list of serial numbers or look for the required information in the /sys/block file system, so that you don't have to execute lsblk for each line.

Thank you very much! Now everything works as expected. Just had to correct some small errors in your code (e.g. $s instead of %s). — merlin, Feb 12 '23 at 19:17
@merlin You're welcome. Thanks for the hint. You probably meant »*%s instead of $s*«, right? I corrected that part. — Socowi, Feb 13 '23 at 00:58

Redirect stderr/-out of running application to my own bash script

So what I am doing wrong?

1 Answers1