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Addition and subtraction arithmetics with Linux date command
This article features some simple examples on how to add or subtract a time from a current date. At first we use a native date sytax to do this trick and later we will see how this can be done manually by converting date to an epoch time. Let's start with a simple backup script based on the date Linux command:
#!/bin/bash tar cjf linuxconfig_$(date +%H%M-%d%m%Y).tar.bz2 ~/public_html
Every time the script is executed it will create a file with a current date included in the file name. The backup problem is solved. However, we do not usually want to keep all backup files indefinitely or until they consume all available free space. This is where the subtraction arithmetics with Linux date command comes handy. Let's see couple examples how to subtract time from a current date using date string: Subtract 10 years from a current date:
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Resetting Vodafone USB mobile broadband device with usb_modeswitch
First time I used my vodafone USB mobile broadband device on my Fedora Linux system it worked perfectly . However, after couple minutes I got disconnected I was not able to connect again even when the blue light on the Vodafone USB mobile broadband device was making a clear blue blinking signal that network was found and all I need to do is to just connect. Eventually after many attempts to make an Internet connection I had reseted it with a usb_modeswitch command to make it working again. It is not guaranteed solution but its worth to try.
If you are in the same situation you may also try to reset your Vodafone USB mobile broadband device. To do that we first need vendor and product ID in a hexadecimal form. To get this information execute:
$ lsusb
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The difference between parameter and variable in bash
Question:
Hi, in bash context what is the difference between parameter and variable?
Answer:
In a bash context a parameter is an entity that stores values. Furthermore, a parameter can be a name ( variable ), number ( positional parameter ) or special character ( special parameter ). Therefore, a variable is a parameter denoted by a name. ( see: man bash ). Let's have a closer look on all three bash parameters:
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Avoiding tedious remote login ssh syntax
Although you have exchanged public keys with your remote server to avoid that boring bit of entering your super long secure user password everytime you are about to login, you still need to face ssh syntax in order get someting done. Yes, it is just single line command with few words but typing that line 30 times a day can take you at least 10 minutes of problem solving time, bore you and more iportatbly it may even give you a headache. This article outlines two alternative options on how to make ssh login procedure easier and thus keeping you sane longer.
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Fedora - Missing /etc/syslog.conf a syslog configuration file
Question:
Hi, the book I'm reading makes a reference to /etc/syslog.conf as a syslog configuration file. I'm trying to locate it on my Fedora 14 Linux system and it appears that I do not have one. What is the location I need to look in to find this conf file.
Answer:
Fedora Linux does not include syslog with a default installation. You are more likely to have rsyslog installed on your system with /etc/rsyslog.conf as a configuration file. To confirm this run a rpm command to list all installed packages and pipe output to grep to search for syslog:
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How do I display user ID associated with a process?
Question:
command which displays the user who invoked the command?
Answer:
ps command will print any user ID associated with any process on the system. To see all processes currently running on a Linux system a "ps" command can be used. The most common options used by ps command are "aux":
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ls command with a long listing format output
Question:
When we execute the  ls command, the result contains first column like -rw-rw-r-- or lrwxrwxrwx. What does this mean?
Answer:
The output mentioned in your question can be produced with a following command:
ls -l filename
-l option of a ls command will instruct ls to display output in a long listing format which means that instead of output containing only a name(s) of file or directory the ls command will produce additional information. Example:
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Match beginning and end of the filename using meta characters and regex
Question:
What is the name of the command which search for all the files starting with 'A' and end with 'K'?
Answer:
ls | grep ^A.*K$
Long answer:
Rather than searching for a single command we need a combination of commands to do this trick. Before we are able to do such a trick we need to get acquainted with couple bash features and terms:
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How do I locate all files installed from a RPM package?
Question:
Hi, recently I have installed couple packages into my Fedora Linux system. I'm just wondering how do I locate all files which had installed into my system.
Answer:
The easiest way to check all files installed on your system is to check a RPM package manifest which shows all files and location for any particular RPM package. Let's say that I downloaded a telnet-server-1.2-137.1.i586.rpm RPM package from some online source and I wish to see what this package contains and what files will I install into my system. The following command will reveal all that information:
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Burning CD's over the Internet with or without ISO image
How many GUI CD/DVD burning applications do you know? Now, how many of them you can name that can burn an ISO image directly from ftp server or burn your remote directory over ssh? If you want to use a GUI burning software in such manner you would first need to mount your remote ftp or ssh directory as a part of a local system. However, this is possible without any extra effort by use of command line interface.
This article will illustrate some command line tricks on how to work with ISO images and how burning a data from a command line can safe you time. It really highlights a power of Linux command line interface.
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Creating an Incremental backup with Linux and pax archive tool
pax is an archive utility somewhere between cpio and tar. This is just because is independent of the specific archive format, and supports a wide variety of different archive formats. It can perform simple tasks as creating a compressed archive of a selected directory or it can as much easily create a daily incremental backups. In this short article we will see how to protect our daily work by creating an incremental backup with pax.
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Automatically mount USB external drive with autofs
Question:
My Debian (up to date) system will automount USB pen drives on the fly, but not a 1 Tb external USB drive (Iomega), which will mount but it has to be attached before booting. How to I change the behaviour for the Iomega usb large drive so I can mount it after the system is up? Thanks.
Answer:
If your 1 Tb external USB drive mounts when it is attached before booting you may have a line in your /etc/fstab configuration file which mounts it during the boot time. If this is the case and you plug in your external USB drive after the boot, execute as a root user:
# mount -a
This will mount all devices which are not currently mounted, including your Iomega USB drive.
However, this may not be the most reliable solution since the base device file name for your drive can be different every time you plugin your USB disk. Since you have already mentioned that you are using other USB pen drives, your base device file name for your Iomega USB disk can be anything like: /dev/sdb1, /dev/sdd1 or /dev/sdXn .
I have exactly the same problem with my external WD USB drive as it mounts automatically in Fedora but not in Debian. Therefore,  I recommend use autofs. autofs is very simple a neat solution. It takes little bit of configuration but its worth it !
Make USB device base name permanent
To avoid any confusion whether base name for your USB block device is /dev/sdb1, /dev/sdd1 or /dev/sdXn we make it permanently /dev/Iomega anytime you plug it in. This can be done with help of udev device manager. You should have udev already installed on your system, otherwise install it with:
# apt-get install udev
Next, search for a current base name of your external USB disk using fdisk command:
# fdisk -l
This will return something like this:
OUTPUT:
Disk /dev/sdc: 2000.3 GB, 2000396746752 bytes 255 heads, 63 sectors/track, 243201 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Disk identifier: 0x001425a0 Device Boot Start End Blocks Id System /dev/sdc1 1 243201 1953512001 b W95 FAT32
Where the base name for your external USB device is /dev/sdc. Next, use udevinfo command with /dev/sdc as an argument to get model attribute:
$ udevinfo -a -p /sys/block/sdc/ | grep model
ATTRS{model}=="Ext HDD 1021 "
Now, that we have model attribute, we can add it to /etc/udev/rules.d/custom.rules by following line:
SUBSYSTEM=="scsi", ATTRS{model}=="Ext HDD 1021 ", SYMLINK+="Iomega%n"
At this point all we need to do is restart udev device manager:
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Using rsync over ssh as an ultimate backup tool
There are many backup tools around and many ways how to use them. For example, it is possible to use gzip and ftp to make a local copy of your web site. This approach have couple drawbacks such us: data are transferred over the internet unencrypted and we are most likely transferring data which we had copied over the day before.
To solve an unencrypted transfer problem we can instead of ftp use scp. However, this time the transfer time will be even longer as scp will create an extra overhead of creating encrypted tunnel for our backup internet connection. To stop transferring a duplicate data we can use rsync. If we combine rsync with ssh, compression, bash and cron we can end up with a ultimate backup tool.
Let's create simple but powerful backup solution using rsync, ssh, compression and cron scheduler:
Passwordless ssh
At this point we need to create passwordless ssh login. By doing this we can avoid the need of entering password when doing our backup. This way we can make the whole backup process completely automatic. Please follow this tutorial to make ssh login to your server without password.
rsync installation
If you have not done so yet install rsync tool as a root user:
For Ubuntu, Debian enter as:
# apt-get install rsync
And Fedora, RHEL and CentOS:
# yum install rsync
Making a database backup
In case that your website is using database such as mysql we first need to make a database backup. Therefore. our backup bash script starts with following lines:
#!/bin/bash # create database backup /usr/bin/ssh user@server.remote '( mysqldump --password='pass' \ mydatabase > ~/public_html/mywebsite/mydatabase.sql )'
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Limit user environment with ulimit Linux command
Whether it is user intention or just accident it may happen, that a single user can eat up all available system resources such as RAM memory or disk space. Depends on the nature of you Linux system you may want to limit your users to only what they might actually need.
Let's start with something like a fork bomb:
:(){ :|:& };:
The line above can almost instantly consume all resources since it creates recursive function all to it self as it forks unlimited children processes. One does not even need a root privileges to crash your Linux system. What about to limit user by a number of process he/she can spawn:
NOTE: All limits are applied to a current shell session only. To make a permanent change system wide use /etc/profile .
$ ulimit -u 10
$ :(){ :|:& };:
bash: fork: retry: Resource temporarily unavailable
This takes care of the fork bomb problem. But what about disk space? Linux command ulimit can limit users to create files bigger than a certain size:
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Change tty font size with Grub 2 boot console resolution
Question:
I'm using ubuntu 10.04. How can I increase the font size in the ttys ?
Answer:
Increasing / decreasing font size on tty consoles is linked to a console resolution. Therefore, to increase your font size you need to decrease a console screen resolution by changing grub2 default settings. Please note that your Ubuntu Linux is using grub2 so looking inside /boot/grub/menu.lst would not do any help. Instead we concentrate on /etc/default/grub file.
Before making any permanent changes to your Ubuntu Linux system we first try to change a console screen resolution directly from a grub boot line option right after reboot.
NOTE: There is no guarantee that any of the options below will work. This are just generic and no hardware specific solutions.
Option 1:
First choose a resolution number from the table below:
|
 |
640x480 |
800x600 |
1024x768 |
1152x864 |
1280x1024 |
1600x1200 |
|
8bit |
769 |
771 |
773 |
353 |
775 |
777 |
|
15bit |
784 |
787 |
790 |
 |
793 |
796 |
|
16bit |
785 |
788 |
791 |
355 |
794 |
797 |
|
24bit |
786 |
789 |
792 |
 |
795 |
798 |
Let's say that we first try 795. Now reboot your system and edit a grub boot line with 'e' key and add after ro option:
vga=795
Once you altered line with the above vga option press CTRL+x to boot.
Option 2:
You can also try specify nomodeset instead of vga parameter. Reboot and edit grub boot line with "e' and add nomodeset after ro option:
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How do I print all arguments submitted on a command line from a bash script?
Question:
How do I print all arguments submitted on a command line from a bash script?
Answer:
There are couple ways how to print bash arguments from a script. Try some scripts below to name just few.
In this first script example you just print all arguments:
#!/bin/bash echo $@
If you intend to do something with your arguments within a script you can try somethign simple as the following script:
#!/bin/bash for i; do
echo $i done
Next we have some script which are doing the same as the previous bash script but employ different approach:
#/bin/bash for i in $*; do echo $i done
Let's print all bash arguments using shift:
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