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We want to check the filesystem on the disks as /deb/sdc ... /dev/sdg on each Red Hat Linux machine.

The target is to find what are the disks that require e2fsck ( as e2fsck -y /dev/sdb etc.)

According to man page

-n
Open the filesystem read-only, and assume an answer of 'no' to all questions. Allows e2fsck to be used non-interactively. This option may not be specified at the same time as the -p or -y options.

When we run the command (example)

 e2fsck -n /dev/sdXX

we get

e2fsck 1.42.9 (28-Dec-2013)
Warning!  /dev/sdc is mounted.
Warning: skipping journal recovery because doing a read-only filesystem check.
/dev/sdc: clean, 94/1310720 files, 156685/5242880 blocks

So what do we need to capture from e2fsck -n output, that requires us to run e2fsck (without -n)?

e2fsck process

init 1
umount /dev/sdXX
e2fsck -y /dev/sdXX  # (or e2fsck -C /dev/sdXX for full details) 
init 3
0

You use e2fsck so I assume we're talking about ext? filesystem. The command

tune2fs -l /dev/sdXX

will tell you the apparent state of the filesystem (which can be mounted, it's safe). You will get (among other things) either

Filesystem state:   clean

or something else than clean. Because grep returns false if the match is not found, your basic try can be like this:

tune2fs -l /dev/sdXX | grep "^Filesystem state:[ ]*clean$" || { commands; to; fix; the; filesystem; }

The above will work only if the filesystem detected its unclean state beforehand. Sometimes you want to check for problems anyway (that's why the desired behavior is to fsck every Nth mount or when such-and-such number of days have passed). If I get you right you're trying to get to know if you should e2fsck -y /dev/sdXX, by analyzing the output of e2fsck -n /dev/sdXX.

I say don't analyze the output. Check the exit status. See man 8 e2fsck to learn:

The exit code returned by e2fsck is the sum of the following conditions:
0 - No errors
1 - File system errors corrected
2 - File system errors corrected, system should be rebooted
4 - File system errors left uncorrected
8 - Operational error
16 - Usage or syntax error
32 - e2fsck canceled by user request
128 - Shared library error

Note e2fsck -n /dev/sdXX will do nothing useful (and it will return "no errors") if the filesystem seems clear; so this is another way to detect the current apparent state, like we did with tune2fs. To check anyway you need -f option. Then you want to know if the exit status includes 4. In bash this can be done with:

e2fsck -nf /dev/sdXX  # this is safe even if the filesystem is mounted
status=$?
[ $(( $status & 4 )) -eq 4 ] && { commands; to; fix; the; filesystem; }

Quick explanation:

  • $? is the exit status of the last command (e2fsck in this case). I save it to a separate variable so I can do multiple tests with it. It's not necessary in this simple example where there is one test only, but a good practice in general. The underlying reason is: after these lines $status still contains exit status of e2fsck and can be reused, while (the new) $? has nothing to do with e2fsck.
  • $(( ... )) does shell arithmetic
  • where & is bitwise AND,
  • then [ ... -eq 4 ] is in fact the test command to check if the result is 4.
  • If the test succeeds then the { ... } block will be executed.

There may be errors not within the filesystem itself but on deeper level, on the device. I think this is out of scope of this question but just to point you in the right direction in case you need it, these are useful commands (notice sdX, not sdXX):

  • smartctl -t long /dev/sdX
  • badblocks -n -b 512 /dev/sdX

Read their man pages before you use them.

  • thank you so much for the long and excellent answer , just one note why not do the --> [[ $? -eq 4 ]] && e2fsck -y /dev/sdXX , its more simple – King David Nov 13 '17 at 9:31
  • second - is it safe to run the e2fsck -nf /dev/sdXX while device is mounted ? – King David Nov 13 '17 at 9:41
  • @KingDavid [ $? -eq 4 ] misses e.g. 12 which is 4+8. You probably won't get this value. It's up to you what logic you will use, whether you'll check for 128 or not, etc. If you want to match exactly 4 then [ $? -eq 4 ] should do fine. e2fsck -nf /dev/sdXX is safe because -n makes it read-only, therefore the command cannot break anything. A separate question is: can the mounted state alter the conclusion? This I don't know. I assume you need to probe health of a filesystem without disrupting normal operations. Obviously you should umount before e2fsck -y. – Kamil Maciorowski Nov 13 '17 at 9:54
  • @KingDavid I have deliberately spoiled my (test) filesystem with some dd commands and got exit code 12 from e2fsck -nf. I think you shouldn't check for sole 4; use my bitwise-aware code better. – Kamil Maciorowski Nov 13 '17 at 10:41
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First check if it is Journaling file system.

"A journaling file system is a file system that keeps track of changes not yet committed to the file system's main part by recording the intentions of such changes in a data structure known as a "journal", which is usually a circular log. In the event of a system crash or power failure, such file systems can be brought back online more quickly with a lower likelihood of becoming corrupted."

Then you can try Gparted to check for consistency and see what the tool can do for you.

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