Today, on my Centos server I tried to add a user but got the following error:
-bash: /usr/sbin/useradd: Input/output error
How do I fix it?
“Input/output error” indicates something that shouldn't happen somewhere in the input/output stack. The intended meaning is a hardware failure: your hard disk is failing.
Often this error turns up in cases where the problem is a software bug or a misuse of a special-purpose filesystem. But here, you're reading from a system directory, so a hardware failure is the most likely explanation.
RAM also tends to fail, so when you see an inexplicable error, do test your RAM.