http://linoxide.com/linux-command/password-expire-chage-command/

I had a problem after a reboot of an AWS EC2 instance, because the server changed the hostname after reboot. Because we send millions of e-mails each hour, a lot of e-mail with sender like ‘person@ip-10-20-30-40.localdomain’ was “bounced”.

The first thing I had to do was change the hostname to correct the problem cause (solution found here). I’m using Red Hat 7 on AWS EC2 (RHEL7):

1. Edit the file /etc/cloud/cloud.cfg.d/defaults.cfg deleting/commenting the above lines:
   hostname: myhostname
   fqdn: myhostname.mydomainname
2. Then edit the /etc/hostname adding your hostname
3. Run the command hostname <hostname>
   EX: hostname myhost.domain.com
4. Restart postfix

Then I need to correct the hostname of bounced messages. Unfourtunally, change the messadges using SED doesn’t work. If you try to do this, the messasges will be moved to the “corrupt” folder. So, the sollution is do a “remap” of all senders (solution found here):

1. Create “stmp_generic_maps parameter”:
   1. Add or uncomment the following line on /etc/postfix/main.cf:
      smtp_generic_maps = hash:/etc/postfix/generic
   2. Edit the file /etc/postfix/generic adding the “mappers”, one each line (from –> to):
      person@ip-10-20-30-40.localdomain        person@hostname myhost.domain.com
   3. Create the hash file of the mapping:
      postmap /etc/postfix/generic
   4. Reload Postfix:
      service postfix reload
2. Wait the message “requeue” or force the message flushing (sse above).

Some instructions to deal with “bounced” messages on Postfix (found here and here):

• Dump entire mail queue:
  mailq
• Try to deliver a single message:
  postqueue -i E7B5114648B
• Read a message from mail queue:
  postcat -qv BC6798D96E2
• Flush the mail queue (try to deliver all queued messages):
  postqueue -f
• Delete a message from queue:
  postsuper -d BC6798D96E2

Remember to read the log, on /var/log/maillog. So, a great tip is “tail -f /var/log/maillog” when you need to understand the errors and try the actions above.

Follow the Link: http://www.tecmint.com/things-to-do-after-installing-fedora-21-workstation/

Nice tool to customize Grub2 setting, such as sequence, add or remove options, etc: Grub Customizer.

Grub Customizer

Use the first command to test and the second to delete:

sudo find /tmp/ -type f -name 'ftpfile_*' -mtime +2 -exec ls -ltr {} \;

sudo find /tmp/ -type f -name 'ftpfile*' -mtime +2 -exec rm -rf {} \;

To configure keyboard

As superuser, edit file /etc/X11/xorg.conf.d/00-keyboard.conf, and change the item XkbModel to thinkpad60:

Option “XkbModel” “thinkpad60”

To configure Wireless

First install RPMFUSION free and nonfree.

Second: yum install kmod-ndiswrapper kmod-wl

Third: reboot

Useful tool to build regular expressions: http://gskinner.com/RegExr/

Just use the split command:

split --bytes=1G large_file_name short_dir/short_file_prefix_

This will create a lot of files with 1 G in short_dir. The files will named short_file_prefix_aa, short_file_prefix_ab, short_file_prefix_ac …

Found in Techie Corner.

Find files and save a text file named “script.sh”:

find . | xargs grep -l "text string here" | awk '{print "rm "$1}' > script.sh

Then chmod +x the file and run. Solution found in Stack Overflow.

Just use the smart_host:

1. Edit your  /etc/mail/sendmail.mc

2. Add a line like this:

define(`SMART_HOST', `smtp.your.provider')dnl

3. Apply the changes:

m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

Then restart the service.