What command do you run to check file system consistency under UNIX or Linux?

Q. What command I need to run to check file system consistency under UNIX or Linux like operating systems?

A. You need to run fsck [file system consistency check] command to check file system consistency and repair a Linux / UNIX file system. Microsoft Windows/DOS equivalent programs are scandisk and chkdsk.

Normally, the fsck program will try to run filesystems on different physical disk drives in parallel to reduce total amount time to check all of the filesystems. Also note that fsck is run automatically at boot time when the system detects that a file system is in an inconsistent state, indicating a non-graceful shutdown, such as a crash or power loss.

You must unmout file system before running fsck command. fsck should always be run in a single user mode.

How do I use fsck program?

Following is general sequence for running fsck:
Goto single user mode:
# init 1
Unmount file system (for example /dev/sdb1 under Linux or under FreeBSD use /dev/ad0s1f etc) :
# umount /dev/sdb1
Now run fsck command:
# fsck /dev/sdb1
Now mount filesystem
# mount -a
Go backto multiuser mode 2/3:
# init 2