How to Create a swap file in Linux partition

Linux divides its physical RAM (random access memory) into chucks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.

Swapping is necessary for two important reasons. First, when the system requires more memory than is physically available, the kernel swaps out less used pages and gives memory to the current application (process) that needs the memory immediately. Second, a significant number of the pages used by an application during its startup phase may only be used for initialization and then never used again. The system can swap out those pages and free the memory for other applications or even for the disk cache.

Steps to create swap file

1. Use the "dd" command to create a file.

dd if=/dev/zero of=/aSwapFile bs=1024 count=65536

where dd: - is used to copy a specified number of bytes from an Input file (if) to an Output file (of).
Here the dd command copy null characters from the special file "/dev/zero" and copy it to the output file "aSwapFile" in the "/" directory. The "bs" specifies that the characters are read as BYTES. The "count" specifies the size of the bytes block that is to be created in the output file.

2. Use the "mkswap" command to set up the Linux swap area on the file created.

mkswap /aSwapFile

3. Use the "swapon" command to activate the swap file created.

swapon /aSwapFile

4. Edit the fstab "/etc/fstab" to enable the swap after a reboot.

vi /etc/fstab
(Add the following line to the fstab file.)
/aSwapFile swap swap defaults 0 0

Batch file renaming

Batch file renaming can be pretty handy as it saves loads of time when you have to rename thousands of files. There are many simple ways to do it though.

If you have files with same ending pattern it can be done as:

e.g. if you want all .htm to be renamed to .html

rename .htm .html *.htm

and On other Unixes, we'd have to do something like this to batch rename files:

for i in *.html
do
j=`echo $i | sed 's/.htm$/.html/'`
# or, in this simple case even just: j=$"i"l
mv $i $j
done

Though if we had bash or ksh, we could make that a little less cumbersome:

for i in *.htm
do
j=${i%.htm}.html
# or: j=${i}l
mv $i $j
done

The Linux "rename" isn't going to handle to more complex cases though. For example, I had to transfer mail files from one system to another recently. On the old system, each message would be named something like "1124993500.7359464636.e-smith". On the new system, they'd be "00000001.eml", with hexadecimal numbering going on up, so you'd get "00000009.eml" and the next message would be "0000000a.eml", and so on. There's no way for "rename" to do that, but a loop can:

for i in *
do
j=`printf "%0.9x.eml\n" $x`
mv $i $j
x=$((x+1))
done

And if you dont want to follow any pattern and simply add new extension to all the files in a directory:

for i in $(ls -lArt /path/to/directory |awk '{print $9}');do mv $i $i.txt; done

adding to it if files are placed recursively in directories:

for i in $(find /path/to/root -type f -print);do mv $i $i.txt; done

Using Foremost

Foremost Syntax

foremost [-h][-V][-d][-vqwQT][-b][-o

] [-t][-s][-i]

Available Options

-h Show a help screen and exit.
-V Show copyright information and exit.
-d Turn on indirect block detection, this works well for Unix file systems.
-T Time stamp the output directory so you don’t have to delete the output dir when running multiple times.
-v Enables verbose mode. This causes more information regarding the current state of the program to be dis-played on the screen, and is highly recommended.
-q Enables quick mode. In quick mode, only the start of each sector is searched for matching headers. That is,the header is searched only up to the length of the longest header. The rest of the sector, usually about 500 bytes, is ignored. This mode makes foremost run con- siderably faster, but it may cause you to miss files that are embedded in other files. For example, using quick mode you will not be able to find JPEG images embedded in Microsoft Word documents.

Quick mode should not be used when examining NTFS file systems. Because NTFS will store small files inside the Master File Table, these files will be missed during quick mode.

-Q Enables Quiet mode. Most error messages will be sup-pressed.
-w Enables write audit only mode. No files will be extracted.
-a Enables write all headers, perform no error detection in terms of corrupted files.
-b number Allows you to specify the block size used in foremost. This is relevant for file naming and quick searches. The default is 512. ie. foremost -b 1024 image.dd

-k number Allows you to specify the chunk size used in foremost.This can improve speed if you have enough RAM to fit the image in. It reduces the checking that occurs between chunks of the buffer. For example if you had > 500MB of RAM. ie. foremost -k 500 image.dd

-i file The file is used as the input file. If no input file is specified or the input file cannot be read then stdin is used.

-o directory Recovered files are written to the directory directory.

-c file Sets the configuration file to use. If none is speci-fied, the file “foremost.conf” from the current direc-tory is used, if that doesn’t exist then “/etc/fore-most.conf” is used. The format for the configuration file is described in the default configuration file included with this program. See the CONFIGURATION FILE section below for more information.

-s number Skips number blocks in the input file before beginning the search for headers. ie. foremost -s 512 -t jpeg -i /dev/hda1

Foremost examples

Search for jpeg format skipping the first 100 blocks

sudo foremost -s 100 -t jpg -i image.dd

Only generate an audit file, and print to the screen (verbose mode)

sudo foremost -av image.dd

Search all defined types

sudo foremost -t all -i image.dd

Search for gif and pdf

sudo foremost -t gif,pdf -i image.dd

Search for office documents and jpeg files in a Unix file sys-tem in verbose mode.

sudo foremost -v -t ole,jpeg -i image.dd

Run the default case

sudo foremost image.dd

image.dd means you need to enter your hardisk mount point i.e /dev/sda1 or /dev/sda2

Recover deleted files in Linux

Although there's no common "undelete" command for a Linux EXT3 file system, you can recover many types of accidentally erased files, including documents, graphics, and system files using the "Foremost" console application.

When you delete a file, the data is not really overwritten. The pointer in the filesystem to the file is simply removed so the disk area can be overwritten when necessary. The more the disk is written to after the file is deleted, the larger the chance it will be overwritten and become unrecoverable.

Foremost is a command line utility for finding and recovering deleted files based on their type. It was origionally developed for the US Air Force Office of Special Investigations. It can recover files from a number of filesystems, including fat, ext3 and NTFS. It can be installed and run from the live cd.

Foremost can recover files with the following extensions:
jpg, gif, png, bmp, avi ,exe, mpg, wav, riff, wmv, mov, pdf, ole, Excel, Access, doc, zip, XML, SXW, SXC, SXI, SX, rar, htm, cpp

For other file extensions we may need to edit /etc/foremost.conf which can be found in man page of Foremost (man foremost)

How to Install:

Enable the universe repository and install foremost:

sudo apt-get install foremost

Assuming the lost files are on a USB drive (sda), you need to create a writeable directory on another drive where you can put the recovered files

sudo mount /dev/sdb1 /recovery
sudo mkdir /recovery/foremost

And then run foremost:

sudo foremost -i /dev/sda -o /recovery/foremost

or for specific file format e.g. video (avi):

sudo foremost -t avi -i /dev/sda -o /recovery/foremost

The recovered files will then be owned by root. Change their ownership so that you can use them:

sudo chown -R youruser:yourgroup /recovery/foremost



Please note that there's no guarantee that foremost will succeed in recovering your files, but at least there's a chance.