Category Archives: How to

Autostart services in Linux

chkconfig is a simple command-line tool that helps a Linux administrator configure,  maintain,  autostart and manage the configuration of the symlinks located in /etc/rc[0-6].d path.

First of all let me show you which are the most used services in a Linux distributuion.

You can find out what services can be started in your server by typing:

chkconfig --list

The output should look something like this:

~ # chkconfig --list
acpid           0:off   1:off   2:on    3:on    4:on    5:on    6:off
cgconfig        0:off   1:off   2:off   3:off   4:off   5:off   6:off
cgred           0:off   1:off   2:off   3:off   4:off   5:off   6:off
cmdavd          0:off   1:off   2:off   3:off   4:off   5:off   6:off
cmdmgd          0:off   1:off   2:off   3:off   4:off   5:off   6:off
crond           0:off   1:off   2:on    3:on    4:on    5:on    6:off
htcacheclean    0:off   1:off   2:off   3:off   4:off   5:off   6:off
httpd           0:off   1:off   2:on    3:on    4:on    5:on    6:off
ip6tables       0:off   1:off   2:on    3:on    4:on    5:on    6:off
iptables        0:off   1:off   2:on    3:on    4:on    5:on    6:off
mailman         0:off   1:off   2:on    3:on    4:on    5:on    6:off
mdmonitor       0:off   1:off   2:on    3:on    4:on    5:on    6:off
mysqld          0:off   1:off   2:on    3:on    4:on    5:on    6:off
named           0:off   1:off   2:on    3:on    4:on    5:on    6:off
netconsole      0:off   1:off   2:off   3:off   4:off   5:off   6:off
netfs           0:off   1:off   2:off   3:on    4:on    5:on    6:off
network         0:off   1:off   2:on    3:on    4:on    5:on    6:off
ntpd            0:off   1:off   2:off   3:off   4:off   5:off   6:off
ntpdate         0:off   1:off   2:off   3:off   4:off   5:off   6:off
portreserve     0:off   1:off   2:on    3:on    4:on    5:on    6:off
psa             0:off   1:off   2:on    3:on    4:on    5:on    6:off
qmail           0:off   1:off   2:on    3:on    4:on    5:on    6:off
rdisc           0:off   1:off   2:off   3:off   4:off   5:off   6:off
restorecond     0:off   1:off   2:off   3:off   4:off   5:off   6:off
rsyslog         0:off   1:off   2:on    3:on    4:on    5:on    6:off
saslauthd       0:off   1:off   2:off   3:off   4:off   5:off   6:off
spamassassin    0:off   1:off   2:on    3:on    4:on    5:on    6:off
squid           0:off   1:off   2:off   3:off   4:off   5:off   6:off
sshd            0:off   1:off   2:on    3:on    4:on    5:on    6:off
xinetd          0:off   1:off   2:on    3:on    4:on    5:on    6:off

xinetd based services:
        chargen-dgram:  off
        chargen-stream: off
        daytime-dgram:  off
        daytime-stream: off
        discard-dgram:  off
        discard-stream: off
        echo-dgram:     off
        echo-stream:    off
        ftp_psa:        on
        poppassd_psa:   on
        rsync:          off
        smtp_psa:       on
        smtps_psa:      on
        submission_psa: on
        tcpmux-server:  off
        time-dgram:     off
        time-stream:    off

The left column contains the name of the process, to the right you have 7 columns, each one represents a Linux runlevel. Usually you will use runlevels 3,4,5. Runlevel 0 and 6 are related to shutdown (0) and reboot (6), so you should never use these.

The “on” and “off” reffer to the fact that a specific service will autostart (on) or will not autostart (off) after a reboot of the server.

The manual of chkconfig can be accesed via the command:

~ # chkconfig --help
chkconfig version 1.3.49.3 - Copyright (C) 1997-2000 Red Hat, Inc.
This may be freely redistributed under the terms of the GNU Public License.usage:   chkconfig [--list] [--type ] [name]
         chkconfig --add 
         chkconfig --del 
         chkconfig --override 
         chkconfig [--level ] [--type ]  <on|off|reset|resetpriorities>
root@nyxware~ #

To autostart a service after each reboot you would use a command like this:

chkconfig --level 345 httpd on

or just

chkconfig httpd on

To stop a process from starting after each reboot enter the command:

chkconfig httpd off

or stop it from running at a specific runlevel:

# chkconfig --level 3 httpd off
root@nyxware#

Use grep to see the status of a specific service:

~ # chkconfig --list | grep ssh
sshd            0:off   1:off   2:on    3:on    4:on    5:on    6:off
root@nyxware
~ #

  * replace httpd with your desired service name.

Posted in BASH, How to. Tagged with , , , .

Show only specific columns in Linux log files

To troubleshoot specific application errors, a sysadmin needs to check his logs for warning, errors or other useful information.
However, most server logs are quite unreadable or contain too much information. This kind of situation is unacceptable and you need to extract only specific columns from that log.
Let’s say you are checking the Apache error log and you want to extract only a specific column. I’m trying to search the log for the word error, the display the IP’s that are getting that error and count how many times the error occurs per IP.
Here’s how you can do it.

First you need to find out where the Apache log file is. In my case it is /etc/httpd/logs/error_log (Redhat, CenOS). If you use Debian or Ubuntu the location will be /var/log/apache2/error.log.

Now that you know the log location, put the path in the command below:

cat /etc/httpd/logs/error_log | grep error | cut -d" " -f8 | uniq -c | sort

You should see something like this:

[root@nyx ~]# cat /etc/httpd/logs/error_log | grep error | cut -d" " -f8 | uniq -c | sort
      1 109.166.141.29]
      1 111.162.148.116]
      1 118.113.227.137]
      1 166.78.10.25]
      1 187.33.2.88]
      1 207.46.13.77]
      1 66.249.78.221]
      1 66.249.78.3]
      6 61.19.246.190]
      6 95.111.68.120]

So, the command syntax is pretty simple: I concatenate the log file, match the “error” word, then print the 8th column, remove duplicates, count the occurrence per IP and sort the list. Voila!

Now you know which IPs are receiving errors when they are visiting your site. You might want to do some checks and see what the problem is. Maybe those IPs are just running bots that are searching for vulnerabilities and it might be a good idea to block them in your firewall.

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How to delete files in Linux

During everyday usage of the Linux operating system, you will encounter situations when you will need to delete one or more files from the file system.

The easiest way to delete a file from BASH is to issue the rm command.

In most Linux distributions the rm command is an alias for “rm -i”, so when you enter rm in the shell, the alias points to rm -i (the “i” stands for interactive).

[root@nyx backup]# rm wordpress.sql
rm: remove regular file `wordpress.sql'? y
[root@nyx backup]#

Delete the file without a confirmation:

[root@nyx backup]# rm -f wordpress.sqlwordpress.sql
[root@nyx backup]#

Delete all the files in the folder called “backup”.  -r stand for recursively, -f  is forcing the delete command without a confirmation. Be careful when you use “-r -f” !

[root@nyx backup]# rm -r -f ./backup/
[root@nyx backup]#

rm

 

 

 

Find files older than x days and delete them:

find /home/backup/* -mtime +30 -exec rm {} \;  
## find files older than 30 days from the /home/backup directory and delete them
Posted in BASH, How to. Tagged with , , , , .