Linux run script/service after few mins of reboot

You can use systemd timers to execute script a minute after boot.

First, create service file (/etc/systemd/system/myscript.service):



Then create timer (/etc/systemd/system/myscript.timer):

Description=Runs myscript every hour

# Time to wait after booting before activation

Now enable and run it:

# systemctl enable myscript.timer
# systemctl start myscript.timer

Why Nginx faster then apache?

The main difference is its architecture. How it was designed to handle HTTP request.

Apache creates processes and threads to handle additional connections.
We can configure the server to control the maximum number of allowable processes. But if this is creased outside of server capacity, too many processes exhaust memory and can cause the machine to swap memory to disk, severely degrading performance. Plus, when the limit of processes is reached, Apache refuses additional connections.

Nginx handles the HTTP request asynchroneously and its event based.
It also do not create new process per request instead it has no of process defined (.e.g 4 total nginx processes) during start time and it creates threads out of it. Each of these processes is single-threaded. Each worker can handle thousands of concurrent connections. It does this asynchronously with one thread, rather than using multi-threaded programming.

Difference between prefork and worker apache mpm module

MPM stands for Multi Processing Module which extends apache’s capability to implement hybrid multi processing multi threading in apache web server.

Default mpm can be checked with httpd -l or apachectl -l

The default MPM for Unix is the Prefork module.
The Worker MPM was introduced in Apache2.

Before I explain the difference between Prefork and worker its necessary to understand how it works. So let’s see how it works.

  • Prefork MPM

Working operation : – A single control process is responsible for launching child processes which listen for connections and serve them when they arrive. Apache always tries to maintain several spare or idle server processes, which stand ready to serve incoming requests. In this way, clients do not need to wait for a new child processes to be forked before their requests can be served.
We can adjust this spare process through the apche conf. For a normal server which is having 256 simultaneous connections can use the default prefork settings.

Perfork is the default module given by apache.

# StartServers: number of server processes to start
# MinSpareServers: minimum number of server processes which are kept spare
# MaxSpareServers: maximum number of server processes which are kept spare
# MaxClients: maximum number of server processes allowed to start
# MaxRequestsPerChild directive sets the limit on the number of requests that an individual child server process will handle. After MaxRequestsPerChild requests, the child process will die. If MaxRequestsPerChild is 0, then the process will never expire

As the name suggests this will pre fork necessary child process while starting apache. It is suitable for websites which avoids threading for compatibility for non-thread-safe libraries . It is also known as the best mpm for isolating each request.

  • Worker MPM


spec and RPMS

SPEC file

The “Recipe” for creating an RPM package is a spec file. Spec files end in the “.spec” suffix and contain the package name, version, RPM revision number, steps to build, install, and clean a package, and a changelog. Multiple packages can be built from a single RPM spec file, if desired. RPM packages are created from RPM spec files using the rpmbuild tool.

Spec files are usually distributed within SRPM files, which contain the spec file packaged along with the source code.


A typical RPM is pre-compiled software ready for direct installation. The corresponding source code can also be distributed. This is done in an SRPM, which also includes the “SPEC” file describing the software and how it is built. The SRPM also allows the user to compile, and perhaps modify, the code itself.

A software package may contain only scripts that are architecture-independent. In such a case only an SRPM may be available; this is still an installable RPM.



Rebuild the SRPM in One Step

The quickest way to rebuild the SRPM is to use the rpmbuild --rebuild ... command. This command will unpack the SRPM file into the specfile and the source files, and then it will build the RPM from the instructions on the specfile.

For example, to rebuild the /tmp/mypackage-1.0.0-1.src.rpm package, run this command:


[user@host ~]$ rpmbuild --rebuild /tmp/mypackage-1.0.0-1.src.rpm

If everything goes right, you will end up with a mypackage-1.0.0-1.i386.rpm file under the ~/rpmbuild/RPMS/i386 directory (if your architecture is i386, otherwise the name of the file and the directory will change accordingly).


For later Fedora RPMS, or any that produce an error like “error: unpacking of archive failed on file /builddir/build/SOURCES/mypackage-1.0.0.tar.gz;4dc983a7: cpio: MD5 sum mismatch”:


[user@host ~]$ rpm --nomd5 -i /tmp/mypackage-1.0.0-1.src.rpm

Once you unpacked the SRPM, you will notice that now there is a specfile (in this case, it will typically be called mypackage.spec) in the ~/rpmbuild/SPECS directory. That is the file you will use to build the RPM. To do that, use the following command:


[user@host ~]$ cd ~/rpmbuild/SPECS
[user@host SPECS]$ rpmbuild -ba mypackage.spec

The rpmbuild -ba command will run through all the steps of the RPM building process, and at the end it will create an RPM package file (which will be saved under~/rpmbuild/RPMS/i386, or the directory appropriate for your architecture), and also a new SRPM file (which will be saved under ~/rpmbuild/SRPMS).

The advantage of unpacking the SRPM first and then using rpmbuild -ba to rebuild it from the specfile is that you can modify the specfile (and maybe add some patches or even upgrade the source tarball) to suit your needs. This is a more complex situation than just rebuilding the SRPM, though, and if you are going down this route you should probably read more on the subject, as explained below, but the process goes like this:

  1. cd ~/rpmbuild/SPECS/
  2. rpmbuild -bp mypackage.spec
  3. cd ~/rpmbuild/BUILD/
  4. cp existing_directory existing_directory.orig
  5. cd existing_directory
  6. find the file you wish to change, modify it.
  7. cd ~/rpmbuild/BUILD/
  8. diff -Npru existing_directory.orig exiting_directory > name_of_your_patch_file.patch
  9. cp name_of_your_patch_file.patch ~/rpmbuild/SOURCES/
  10. cd ~/rpmbuild/SPECS/
  11. edit the mypackage.spec file to add the definition of name_of_your_patch_file.patch and the application of your_patch_file — please look in the file to see how that is done.
  12. rpmbuild -ba mypackage.spec

These are excellent resources to help you if you decide to customize a specfile to your needs:

Jboss interview questions

What’s the difference between a tar file and a war file?

–          TAR – derived from tape archive is a file format to archive bitstreams and name of a program to handle such files.  It is commonly used to collect many files into one large file for distributing or archiving while preserving the filesystem info.

–          WAR – web application archive is a jar file used to distribute a collection of JavaServer Pages, Java Servlets, Java classes, xml files, tag libraries, static web pages (html & others) and others resources that constitute a web application.


How do you look inside a jar file?

–          jar –tf <filename.jar>

–          What is a .war, .ear or a .jar file??

  • WAR – web application archive is a jar file used to distribute a collection of JavaServer Pages, Java Servlets, Java classes, xml files, tag libraries, static web pages (html & others) and others resources that constitute a web application.
  • JAR – Java Archive, is an archive file format typically used to aggregate many Java class files and associated metadata and resources into one file to distribute application software, libs on the Java platform.
  • EAR – Enterprise Archive is used to package one or more moduels into a single archive so the deployment of the various modules into app server happen simultaneiously and coherently.


What is jboss EAP?

–          JBoss Enterprise Application Platform is an open source implementation of the Java EE suite of services


How do you deploy an application to JBoss?

–          Choose the server profile to which you want to deploy the application

–          Copy your application (for example: .war or .ear or a .jar file) to JBOSS_HOME/server/<profilename>/deploy folder.

–          Start the server: (./

–          JBoss will deploy your application when it boots up.

–          That’s it!


How do you perform a hot deployment?

–          JBoss has a built-in hot deployer which can:

  • Detect new applications in the deploy folder and trigger an application deployment
  • Detect an application which was removed from the deploy folder and trigger an application undeployment
  • Detect that the deployment descriptor of an application (for example, the web.xml of .war or application.xml of .ear) has changed and trigger an application redeployment


How do you enable/disable a hot deployment?

–          To enable, put file hdscanner-jboss-beans.xml in  JBOSS_HOME/server/<profilename>/deploy

–          To disable hot deployment, remove the hdscanner-jboss-beans. xml from the deploy folder or rename it to hdscanner-jbossbeans. xml.bak (.bak files are ignored).


How can you configure the time for the hot deployer?

–          “scanPeriod” in hdscanner-jboss-beans.xml


How do you setup JBoss cluster??

–          Getting started with JBoss clustering is very simple. If two JBoss server instances using the all configuration are started on the same network, those servers will detect each other and automatically form a cluster.

–          Two application servers running on the same network detect each other auto and form cluster.


What do you check if you are having problems starting up JBoss?

–          First thing to check is JAVA_HOME=????, then logs

Delete mails from exchange server

First you will need to install fecthmail.
You need to create one hidden file with email user’s details with “.fetchmailrc” name.

poll protocol IMAP:
user YOUR_USER_ID_HERE with password YOUR_PASSWORD_HERE some text

Then to fetch the mail you will have to fire this in order to flush the mail from your exchange server.

#/usr/local/bin/fetchmail -a -K -v -F –limitflush –limit 5

Linux High IO load.. what to check for trouble shooting?

When you look at the CPU activity of your computer, one of the parameters is the iowait. This value shows how much time your CPU wastes while it is waiting for I/O operations for complete. These include disk read/write operations, network, IPC, etc. Is this behavior a problem and, if so, what causes it and how to fix it? One one of the popular Unix-related forums one “genius” wrote:

The iowait “problem” is funny. It’s like when people complain that Linux is “using all my memory”. Yeah, no shit. You should be upset if you are copying files and your computer is /not/ in 100% iowait.

In reality, 100% iowait indicates that there is a problem and in most cases – a big problem that may even lead to data loss. Essentially, there is a bottleneck somewhere in the system. Maybe one of your disks is getting ready to die; or, perhaps, the NIC firmware is having problems with the latest kernel upgrade you installed. The troubleshooting process starts with the potentially more serious possibility: bad disk.

Take a quick look at /etc/messages, /etc/dmesg, /etc/boot.log and any other system log files. You are looking for disk I/O errors, failed read/write operations, bad sectors – anything that indicates a hardware problem with a disk. If you don’t find anything, look for IRQ and disk controller errors. Also look for memory errors and kernel panics. The three most likely culprits of high iowait are: bad disk, faulty memory and network problems.

If you still see nothing relevant, it is time to test your system. If possible, kick all the users off the box, shut down Web server, database and any other user application. Log in via command line and stop XDM.

Open three shell windows: run “top” in one, “iostat -x 1? in the other and “find /etc -type f -print” in the third. Make sure you can see all three windows at the same time. This is a simple test that should generate some I/O activity on the system disk. Repeat this process for other disks. If you see iowait hovering near 100%, chance are you have a problem but we don’t know what it is yet. However, now we do know that network is probably not the cause.

deathstar:/ # iostat -x 1
Linux 2.6.5-7.201-default (deathstar) 12/20/08

avg-cpu: %user %nice %sys %iowait %idle
2.83 0.42 1.45 9.11 86.20

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util
hda 40.63 66.34 27.45 6.04 936.50 581.23 468.25 290.61 45.32 2.42 72.16 2.22 7.42
hdc 0.01 0.00 0.01 0.00 0.03 0.00 0.02 0.00 4.02 0.00 1.17 1.17 0.00
sda 0.09 2.32 4.15 1.33 71.56 29.23 35.78 14.62 18.37 0.65 118.49 6.39 3.51
sdb 3.47 0.00 1.90 0.00 15.32 0.01 7.66 0.01 8.08 0.74 391.31 5.68 1.08
fd0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.00 0.00 45.00 45.00 0.00

deathstar:/ # top
top – 21:28:28 up 1:22, 2 users, load average: 0.09, 0.14, 0.16
Tasks: 77 total, 1 running, 76 sleeping, 0 stopped, 0 zombie
Cpu(s): 2.8% us, 1.3% sy, 0.4% ni, 86.2% id, 9.1% wa, 0.1% hi, 0.0% si
Mem: 508644k total, 503612k used, 5032k free, 34052k buffers
Swap: 1020088k total, 458980k used, 561108k free, 16012k cached

1 root 16 0 640 56 28 S 0.0 0.0 0:05.14 init
2 root 34 19 0 0 0 S 0.0 0.0 0:00.00 ksoftirqd/0
3 root 5 -10 0 0 0 S 0.0 0.0 0:00.09 events/0
4 root 5 -10 0 0 0 S 0.0 0.0 0:00.00 khelper

Next step, lets stress out your CPU but not the disks. The command below will try to create an endless zip file in /dev/null. This generates no disk activity, but loads the CPU. Continue running “top” and “iostat -x 1? in the other two windows.

cat /dev/zero | bzip2 -c > /dev/null

If you see high CPU load but low iowait, we can eliminate CPU issues, IRQ conflicts, and faulty memory. Just to be on the safe side, let’s test memory anyway:

deathstar:/ # free
total used free shared buffers cached
Mem: 508644 503504 5140 0 37036 48968
-/+ buffers/cache: 417500 91144
Swap: 1020088 516196 503892

This server has 508644Kb of RAM. Use the corresponding value for the following test:

deathstar:/ # dd if=/dev/hda2 bs=508644 of=/backups/memtest count=1050
1050+0 records in
1050+0 records out

deathstar:/ # md5sum /backups/memtest ; md5sum /backups/memtest ; md5sum /backups/memtest
04762ff36b2231aac75754ab9c1a564a /backups/memtest
04762ff36b2231aac75754ab9c1a564a /backups/memtest
04762ff36b2231aac75754ab9c1a564a /backups/memtest

The three MD5 values above should be identical. If they are not – your system has a faulty RAM chip.

When you have eliminated hardware problems as possible causes of high iowait, the next step is to review firmware and drivers. You are particularly interested in disk controller firmware: unstable performance and no error messages are the signs of a firmware problem. Try really hard to remember if you made any system changes recently, especially something that required a reboot – like kernel upgrade, for example. If this is the case, roll back the upgrade or search for upgrade firmware. You should grab a copy of Sysinfo (free 30-day trial) to help you identify makes and models of your disks, controllers, etc.

While your disks and controllers may be tip-top, your may have a problem with a filesystem. Even if you see high iowait when accessing any filesystem, you should still check out the partition where /var is mounted and swap – if there is a problem, it will manifest itself regardless of what your system is doing. But here you will run into a little problem: fsck will not scan a mounted partition and you cannot unmount /var. Let’s say these are your partitions:

deathstar:/ # more /etc/fstab
/dev/hda2 / reiserfs acl,user_xattr 1 1
/dev/hda1 swap swap pri=42 0 0

You need to fsck /dev/hda2 because this is where your /var is mounted. Download KNOPPIX or Ubuntu LiveCD, boot from CD (without installing) and “fsck /dev/hda2? from there. If everything looks clean, shut down your system, take the CD out and boot normally. The next step is to check out swap. If you just run fsck on the swap partition, it will fail:

deathstar:/ # fsck /dev/hda1
fsck 1.34 (25-Jul-2003)
fsck: fsck.swap: not found
fsck: Error 2 while executing fsck.swap for /dev/hda1

You need to disable swap on /dev/hda1 before you can scan it. Before you can do this, you need to add another swap area: you cannot run without any swap space. So, to add swap on the fly, create a swap file (1Gb in this example):

deathstar:/ # dd if=/dev/zero of=/swapfile bs=1024 count=1048576
1048576+0 records in
1048576+0 records out

deathstar:/ # chmod 600 /swapfile

deathstar:/ # ls -lash /swapfile
1.1G -rw——- 1 root root 1.0G Dec 20 22:48 /swapfile

Now you can set up and activate the new swap file:

deathstar:/ # mkswap /swapfile
Setting up swapspace version 1, size = 1073737 kB
deathstar:/ # free
total used free shared buffers cached
Mem: 508644 500996 7648 0 38912 147332
-/+ buffers/cache: 314752 193892
Swap: 1020088 521784 498304
deathstar:/ # swapon /swapfile
deathstar:/ # free
total used free shared buffers cached
Mem: 508644 502232 6412 0 39400 147392
-/+ buffers/cache: 315440 193204
Swap: 2068656 521784 1546872

Now we need to deactivate the original swap partition. This operation may take a couple minutes to complete:

deathstar:/ # swapoff /dev/hda1
deathstar:/ # free
total used free shared buffers cached
Mem: 508644 501624 7020 0 31712 10416
-/+ buffers/cache: 459496 49148
Swap: 1048568 167032 881536

The next step is to create a standard filesystem on the old swap partition, so that fsck has something to scan:

deathstar:/ # mke2fs -c /dev/hda1
mke2fs 1.34 (25-Jul-2003)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
127744 inodes, 255024 blocks
12751 blocks (5.00%) reserved for the super user
First data block=0
8 block groups
32768 blocks per group, 32768 fragments per group
15968 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376
Checking for bad blocks (read-only test): done
Writing inode tables: done
Writing superblocks and filesystem accounting information: done

The previous operation already ran fsck and so, if you see no errors, you can now re-activate your original swap space and remove the temporary swap you created:

deathstar:/ # mkswap /dev/hda1
Setting up swapspace version 1, size = 1044574 kB
deathstar:/ # swapon /dev/hda1
deathstar:/ # swapoff /swapfile
deathstar:/ # rm /swapfile
deathstar:/ # free
total used free shared buffers cached
Mem: 508644 503172 5472 0 33668 9256
-/+ buffers/cache: 460248 48396
Swap: 1020088 156300 863788

Anothe command commonly used for analyzing system bottlenecks is vmstat. The following example runs vmstat five times at 2-second intervals:

deathstar:~ # vmstat -S M 2 5
procs ———–memory———- —swap– —–io—- –system– —-cpu—-
r b swpd free buff cache si so bi bo in cs us sy id wa
0 0 15 174 70 58 0 0 189 50 5 6 1 3 94 1
0 0 15 174 70 58 0 0 0 0 1005 35 4 0 96 0
0 1 15 174 70 58 0 0 0 258 1515 45 0 6 88 7
0 0 15 173 71 58 0 0 0 194 1083 24 0 1 83 16
0 0 15 173 71 58 0 0 0 0 1003 19 0 0 100 0

Explanation of vmstat columns:

(a) procs is the process-related fields are:

* r: The number of processes waiting for run time.
* b: The number of processes in uninterruptible sleep.

(b) memory is the memory-related fields are:

* swpd: the amount of virtual memory used.
* free: the amount of idle memory.
* buff: the amount of memory used as buffers.
* cache: the amount of memory used as cache.

(c) swap is swap-related fields are:

* si: Amount of memory swapped in from disk (/s).
* so: Amount of memory swapped to disk (/s).

(d) io is the I/O-related fields are:

* bi: Blocks received from a block device (blocks/s).
* bo: Blocks sent to a block device (blocks/s).

(e) system is the system-related fields are:

* in: The number of interrupts per second, including the clock.
* cs: The number of context switches per second.

(f) cpu is the CPU-related fields are:

These are percentages of total CPU time.

* us: Time spent running non-kernel code. (user time, including nice time)
* sy: Time spent running kernel code. (system time)
* id: Time spent idle. Prior to Linux 2.5.41, this includes IO-wait time.
* wa: Time spent waiting for IO. Prior to Linux 2.5.41, shown as zero.

If you failed to identify the cause of the iowait problem, you should consider the possibility that there is no problem: perhaps your system is handling extra load and running short on resources. Take a look at the running processes and see what’s eating up memory. Perhaps you upgraded an application and now it is using more RAM, which leads to high swapping, which leads to high disk activity, which leads to high iowait.

The solutions are simple:

1. Install more RAM
2. Move swap to another disk or – even better – move it to another disk on a separate controller.
3. Move user applications to another disk/controller and specify default log locations outside of the system disk.

– Jayesh

../../../libraries/libldap/error.c:273: ldap_parse_result: Assertion `r != ((void *)0)’ failed

If you are getting error as mentioned below while doing some operation your linux server bash shell.
../../../libraries/libldap/error.c:273: ldap_parse_result: Assertion `r != ((void *)0)’ failed

Then its due to nss-ldap software running on your server. One of the reason I found and fixed with was nscd service was down on my server restarting it fixed the issue.

Error I saw in logs were..


Oct 28 03:01:27 HOSTNAME nscd: nss_ldap: reconnected to LDAP server ldap:// after 1 attempt
Nov 10 02:49:58 HOSTNAME nscd: nss_ldap: reconnecting to LDAP server (sleeping 4 seconds)…
Nov 10 02:50:14 HOSTNAME nscd: nss_ldap: reconnected to LDAP server ldap:// after 2 attempts
Jan 18 07:45:09 HOSTNAME kernel: nscd[5114]: segfault at 00002b1c735dee78 rip 00002b1b6d4fe885 rsp 000000004185c6d0 error 4

Fix :
[root@HOSTNAME webdocs]# /etc/init.d/nscd status
nscd dead but subsys locked
You have new mail in /var/spool/mail/root
[root@HOSTNAME webdocs]# /etc/init.d/nscd restart
Stopping nscd: [FAILED]
Starting nscd: [ OK ]
[root@HOSTNAME webdocs]# /etc/init.d/nscd status
nscd (pid 30292) is running…
You have new mail in /var/spool/mail/root
[root@HOSTNAME webdocs]#

strings: ‘/lib/’: No such file centos

If you are getting above error while installing siteminder agent then its due to glibc not installed on centos as its installed with “minimal install” option..

# ./nete-wa-6qmr5-cr035-rhas30-x86-64.bin -i console
Preparing to install…
Extracting the JRE from the installer archive…
Unpacking the JRE…
Extracting the installation resources from the installer archive…
Configuring the installer for this system’s environment…
strings: ‘/lib/’: No such file

Launching installer…

./nete-wa-6qmr5-cr035-rhas30-x86-64.bin: /tmp/install.dir.18984/Linux/resource/jre/bin/java: /lib/ bad ELF interpreter: No such file or directory
./nete-wa-6qmr5-cr035-rhas30-x86-64.bin: line 2479: /tmp/install.dir.18984/Linux/resource/jre/bin/java: Success

Fix: install yum and then “yum install glibc”

– Cheers