Copy Files To Dev Null Productions
You misread the advice, the idea is not to copy the large file to /dev/null, which wouldn't affect it in any way, outside putting it in cache if space is available. Cp bigfile /dev/null # useless The advice is not to remove the file then copy /dev/null to it, as it would keep the original inode unchanged and won't free any disk space as long as processes have that file open. The advice is to replace the file content with /dev/null one, which, given the fact /dev/null size is zero by design, is effectively truncating the file to zero bytes: cp /dev/null bigfile # works cat /dev/null bigfile # works It might be noted that if you use a shell to run these commands, there is no need to use /dev/null, a simple empty redirection will have the same effect and would be more efficient, cat /dev/null being a no-op anyway.: bigfile # better bigfile # even better if the shell used supports it. Short answer is no, it doesn't work. There are many flavour of unix abroad, may be for some this might work, can you provide a link? Now, you can test it:.
open two terminal in your system. copy to non root user a large file (like /var/log/messages ). vi it ( vi messages ) in first terminal. cp messages /dev/null on other.
quit vi on first. Messages should still be there.
(vi is not a good candidate, if you remove file, ( rm messages) file will be removed (at least on ubuntu 16.04), you can rewrite it from vi (using w but file will be read from.messages.swp on current dir).
The dd command stands for “ data duplicator” and used for copying and converting data. It is very powerful low level utility of Linux which can do much more like;.
Backup and restore the entire hard disk or partition. It can copy and convert magnetic tape format, convert between ASCII and EBCDIC formats, bytes and can also convert lower case to upper case. It can also be used by Linux kernel make files to make boot images. Only superuser can run this command because you can face a big data loss due to it’s improper usage, so you should be very careful while working with this utility. At that moment data loss can convert the dd utility as a “ data destroyer” for you.
That’s why it is recommended that beginners should not use this command on a production machine until they get familiarity on this. You must make sure that target location must have sufficient space while running this command. Syntax of dd command Before we start with some practical work we need to talk about it’s syntax. Dd if= of= Options We normally do not explain about syntax but this command syntax require some explanation. The syntax is totally different when compared to many Linux commands we know.
In this syntax dd is followed by two things if= –This is a source from where you want to copy data and ‘if’ stands for input-file. Of= –This is a source from where you want to write/paste data and ‘of’ stands for output-file. options –These options include, how fast data should be written, what format etc. Input(source file name) and Output(target file name) in syntax are disks, partitions, files and devices to which you want to write and read data from. There are many options which we will discuss in examples.
Learn Linux dd command with examples Example 1: Clone one hard disk to another hard disk. This is useful when we are building many machines with same configuration. We no need to install OS on all the machines. Just and required software on machine then clone with below example. Dd if=/dev/sda of=/dev/sdb Example 2: We can take backup of a partition/complete HDD for future restoration. Example 7: We can create virtual file system with dd command which can be used as swap.
To know more about you should know on. Dd if=/dev/zero of=/swapfile bs=1024 count=200000 where bs stands for block size and count is nothing but number of such blocks used to crate this swap file. Make sure you use in multiples of 1024 bytes which is equal to 1KB. Ff you do not specify block size, dd use a default block size of 512 bytes. Below conventions will work for block sizes. N and BYTES may be followed by the following multiplicative suffixes: c =1, w =2, b =512, kB =1000, K =1024, Mb =1000.1000, MB=1024.1024, Gb =1000.1000.1000, GB =1024.1024.1024, and so on for T, P, E, Z, Y. Example 8: We can even create ISO files from a CD-ROM or DVD-ROM using dd command.
Dd if=/dev/dvd of=/opt/mylinuximage.iso or with more dd if=/dev/sr0 of=/home/$user/mycdimage.iso bs=2048 conv=sync Some other examples: dd if=/dev/sda1 of=/dev/sdb1 bs=4096 conv=noerror,sync This will make clone of one partition sda1 to other sdb1 partition, also used sync option to synchronize the partition dd if=/dev/sdx of=/dev/sdy bs=64k conv=noerror,sync This will clone the entire drive, including MBR, all partitions and data where noerrr instructs dd to ignore all read errors while continuing operations. The snyc data offsets stay in sync And bs=sets block size which is set to 64k. Example 9: We can even check disk quota using dd command by creating huge files which eats up HDD in no time. Dd if=/dev/zero of=/usr/disk-img/disk-quota.ext3 count=40960 This will create 20MB file (disk image) at said path. Example 10: We can even create bootable USB’s using dd command. Dd if=/home/$user/bootimage.img of=/dev/sdc This will create boot-able USB drive where /dev/sdc is an USB drive.
Example 11: Data recovery using ddrescue command. Ddrescue tool is used for cloning and recovering data. This is not installed by default in many Linux machines. Below are the commands to install for respective distributions.
Installing ddrescue in Debian based machines apt-get install gddrescue Installing ddrescue in Redhat based machines yum install gddrescue It can copy data from one file/block device(hard disk or CD-ROM) to another while trying to rescue the data i.e read error to maximizing the recovered data. To recover data ddrescue command needs to run in two steps followed by running fsck command. Step 1: It copies each and every block without reading errors and logging these errors in log file. Step 2: It will copy only bad block and try to read 3 times to source before it gives up. Than you can run command to check file system for corruption.
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Step 3: Running fsck command. Ddrescue –f –n /dev/sdX /dev/sdY rescue.log ddrescue –d –f –r3 /dev/sdX /dev/sdY rescue.log fsck –f /dev/sdY Example 12: Computer forensics using dcfldd is the enhanced version of dd having useful features used for computer forensics and security as well. It takes dd’s parameters. You can use dcfldd command when you need to know that a copy and subsequent copies are identical to the original. Dcfldd has some additional features like;.
Hashing on-the-fly. Status output. Image/wipe verify.
Split output. hashing Again We have to install this command as well, if that is not present. Installing dcfldd in Debian based machines apt-get install dcfldd Installing dcfldd in Redhat based machines yum install dcfldd Example dcfldd if=/dev/source hash=md5,sha512 hashwindow=1G md5log=md5.txt sha512log=sha512.txt hashconv=after bs=512 conv=noerror,sync split=1G splitformat=aa of=image.dd The above command will read one Gb from the source drive and write to a file called image.dd.aa. It will also calculate the MD5 hash and the sha512 has of each Gigbyte read.