Adam’s Tech Blog

With cheaper storage you might find yourself literally submersed in storage devices. It is possible to have your block devices change (from sdc to sde) when using multiple controllers of the same model (I’ve experienced this with 3Ware controllers). So how do you know where to mount your devices after a reboot? Use e2label to label your disks (much like NTFS labels in Windows).

 

Labeling Your Device

To label your device follow these steps:

# e2label /dev/sdb1 awesomedisk

 

Modifying Your fstab

Now you can modify your fstab to reflect your label changes. Instead of using the path to the block device (/dev/sdb1) you can use this syntax: (LABEL=awesomedisk)

Example:

LABEL=awesomedisk                 /mnt/awesomedisk                       ext3    defaults        1 1

 

Check Current Label

To check the label currently set for a block device use this command:
# tune2fs -l /dev/sdb1 | grep "Filesystem volume name:"

Working with Linux over the years has really exposed me to many useful and unique tools. I’ll show you a few of the tools that help me get through my day a bit quicker and more efficiently. 

Moving/Grabbing FIles

Getting files to your Linux system can be easily achieved using the tools below.

wget

The wget command allows you to retrieve files from an FTP/HTTP or HTTPs source. Typically use this command to get new packages or new tarballs to your server.

Usage:
# wget <source>
# wget http://us2.php.net/get/php-5.2.6.tar.gz/from/this/mirror

scp

SCP allows you to transfer files over SSH (TCP Port 22). You can even transfer files between two servers over SSH.

Usage:

# scp <source> user@remotehost:/path/on/remote/host

Looking for FIles

Lose a file? Need to list all files modified after a certain date? Want to remove all files with "foobar" in the name? Check out the commands below.

find

The find command allows you to search for a specific file by name, size or modified time. The criteria mentioned before is just part of what you can do with find. To see full functionality type "man find" to view the manual page.

# find / -name foo.atxt (this command finds all files named "foo.txt" starting in the root directory)

Another usage of find which is quite handy is the -exec flag.

# find / -name foo.txt -exec ls -lth {} \;

This command will list all files named foo.txt and run the command "ls -lth" on each file. This can be useful to apply a command to all files that match a certain string.

locate

Use the locate command to find a specific file quickly. This is more primitive than the before mentioned find command. It relies on a database of files which it performs its lookups. To update this database run a command, appropriately named, "updatedb".

Usage:
# locate foo.txt

Sorting Results

Since a Linux Administrator typically needs to sort through massive log files, show certain fields and manipulate output; an admin needs to know about the tools below.

sort

The sort command allows you to sort the results of your output. To sort the results by the first regular character (not a symbol). Here is an example of how to use sort with the locate command:

# locate bin | sort

Piping the output of locate to sort will show all results in order.

uniq

Uniq is short for, you guessed it, unique. Uniq accepts input from STDIN (Standard Input) and shows only unique lines of text. This command basically strips duplicates.

# cat foo.txt | uniq

awk

Awk is a pattern scanning engine. The most common usage is to separate rows of text into chunks. The chunks, by default, are separated by spaces. For instance this sentence: "The quick brown fox" when piped to this command: "awk {’print $2′}" will print simply "quick".

# echo "The quick brown fox" | awk {’ print $3 ‘}
brown

This command can be very useful when combining it with long field-driven lists.

wc

wc (Word Count) will count the number of words, newlines and bytes in a file. Typically this command is used with the "-l" flag to count the lines if output. This command accepts input from STDIN (Standard Input).

# cat foo.txt | wc -l
300

This means that the file "foo.txt" contains 300 lines.

Running Commands (that may take a while)

Since the command line interface (CLI) usually offers no progress bar (except for certain apps like wget), you’ll sometimes need to run a program for a long time. Searching through millions of files? Removing a directory containing large quantities of data/files? What if you get disconnected while running one of these commands in the foreground? Use one of the solutions below to run your commands.

Starting Commands in the Background

It is generally useful to start a command, which takes a long time, in the background. Think of these programs as "service" programs which run without a terminal (ssh session, or physical terminal) attached. To start a program in the background simply add a space and the ampersand (&) after the command.

# sleep 1000 &

The above command will be executed in the background. The command waits 1000 seconds to end. Check the progress if your command by issuing "ps aux | grep sleep" to view the PID and status of your command.

To force a command currently running in the foreground to run in the background press "CTRL + Z". You will then be presented the PID. Disregard the "Stopped" message. This indicates the foreground process has stopped and is now running in the background.

screen

Using the program called "screen" allows you to run multiple virtual terminals behind the scenes. This can be useful for programs (and commands) that can’t be ran in the background. Instead of running multiple SSH sessions you can simply type "screen" to initiate a new screen. Press "CTRL + AD" to detach the current screen. Type "screen -r" to reattach to the screen(s) currently active. To install screen on CentOS/RHEL issue the following command:

# yum -y install screen

This sometimes can be an issue. I want PHP5 but I only have CentOS 4.6. What can I do? It seems like the CentOS repository doesn’t have PHP5, only PHP4. Enter “–-enablerepo centosplus”. Just add “—enablerepo centosplus” after yum and you’ll be able to install PHP 5, Apache 2.0 and MySQL 5 in no time.

# yum –enablerepo=centosplus –y install php httpd mysql php-mysql php-gd

The above also works for upgrades by substituting “install” for “upgrade”.

Sometimes it’s necessary to quickly determine the difference between two files. The command “diff” can be particularly useful if you find yourself in this situation.

The most simple usage of diff is “diff <oldfile> <newfile>”.

The output looks like this:

The output of diff is really how to alter the original file to match the new file. “1a2” means to add a new line to the old file after line 1. This line should be “bar” as signified by the “>” mark.

Often patches are distributed using the diff file. PHPBB usually offers this as a way to patch their open source code. In short, using diff can save you time and offer a way to compare to configuration files to see their differences.

Generally speaking, many search engines like Google, MSN, and Yahoo like to see your URLs be as pretty as possible. Using applications like PHP will often produce URLs that look like this: http://somesite.com/index.php?section=section1&data=test. These URLs are typically frowned upon by the search engines and can produce mixed results when attempting to achieve the  highest search engine ranking.

There are two methods we can use to make our URL’s a bit more friendly. Many applications today already provide pre-packaged .htaccess files. It’s important for me to explain that there are two methods for setting up mod_rewrite rules. The first (and most common) method is to use a file called “.htaccess” placed in the root directory of your web site. This file is read in to the Apache configuration in real time (The AllowOverride option in your Apache configuration controls this). We’ll cover the .htaccess method here.

The .htaccess Method

This is the most common way to implement mod_rewrite rules. First, open a text file with your favorite editor (if you do not have shell access simply use notepad or vi on your local machine). Remember that a dot (.) before the file name designates this file as hidden in Linux.

For our example we’re going to redirect http://somesite.com/index.php?section=section1&data=test to look like this: http://somesite.com/section1/test.

1. Edit the .htaccess file you wish to
   # vi /var/www/html/.htaccess
2. Place the following in the new .htaccess file:
   RewriteEngine On
   RewriteRule ([^/]+)/([^/]+)?$ index\.php?section=$1&data=$2 [L]
3. Save the file and exit your editor.
   (* Note: If you upload this file with an FTP/SCP program your file will not appear as it is hidden, if you need to delete the file due to a misconfiguration, issue “DELE .htaccess” in the directory you uploaded the file.)

The [L] directive tells mod_rewrite to redirect the request with a 301 redirect.

Troubleshooting

1. I receive a 500 server error after uploading.
   Check to make sure no typo’s were made. Check the apache error_log for any clues.
2. The redirect does not take place but the site still shows.
   This is usually because your host does not support mod_rewrite or is not allowing .htaccess files. To check if .htaccess is being parsed open a new .htaccess and put the following inside:
   Redirect /test “http://www.google.com”
   If this works you can safely assume that .htaccess files are being parsed.