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issue
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[root@localhost .ssh]# cat 1 >> authorized_keys
-bash: authorized_keys: Permission denied



******************************************************************
solution :
     chattr -i filename
******************************************************************
[root@localhost .ssh]# lsattr authorized_keys
----i----------- authorized_keys

[root@localhost .ssh]# chattr -i authorized_keys
---------------- authorized_keys
....edit file....

[root@localhost .ssh]# chattr +i authorized_keys
----i----------- authorized_keys