userdel: user is currently logged in

userdel: user `username` is currently logged in

If you are trying to remove / delete a user 

# userdel username

and it shows following warning

userdel: user `username` is currently logged in

Now you are not able to remove the user.

Solution :

Run following command.

# pgrep -u username

In place of word username, put your username which is giving you trouble.
Now kill the instances using

# killall -u username

Now try to delete your user but you are still getting same message, check again

# pgrep -u username

You can't run delete user command successfully until you get the instances of the same user using pgrep command.
The result of pgrep command should be blank, No result.

Linux : crontab permission denied

Linux : user can't create/update crontabs
Crontab permission denied for Linux user
Allow user to create crontabs on Linux

By default the permission of file /usr/bin/crontab is

 -rwxr-xr-x 1 root root 34784 Apr 11 10:58 /usr/bin/crontab

Only root can create/update/delete crons.

If you want to allow user to create crons on Linux system, follow this.

a) If you are using CentOS, Fedora or RHEL
The permissions of crontab file should be

 -rwsr-xr-x 1 root root 34784 Apr 11 10:58 /usr/bin/crontab 

Owner and Group both should be root for the file as well as SUID bit should be set.
The ‘s’ in user field in place of ‘x’ indicates that SUID bit is set. 
To set SUID, Run following command

# chmod 4755 /usr/bin/crontab

or

# chmod u+s /usr/bin/crontab

b) If you are using Debian or ubuntu
The permissions of crontab file should be 

 -rwxr-sr-x 1 root crontab 34784 Apr 11 10:58 /usr/bin/crontab 

Owner should be root but the Group should be crontab as well as SGID bit should be set.
The ‘s’ in group in place of ‘x’ indicates that SGID bit is set.  
To make Group 'crontab', run following command.

# chgrp crontab /usr/bin/crontab

To set SGID, Run following command

# chmod 2755 /usr/bin/crontab

or

# chmod g+s /usr/bin/crontab

Filezilla - Response:530 Sorry, the maximum number of clients (n) from your host are already connected

Filezilla - Response:530 Sorry, the maximum number of clients (n) from your host are already connected

Solution 1 - 
Open filezilla.
File -> site manager -> select your 'My Sites' entry -> transfer settings -> uncheck the check box "Limit number of simultaneous connections"

Try again.

Solution 2 -
Close all connections of same site. Do not open multiple filezilla instances for same site.

Restart filezilla.