Sometimes data arrives from sources in unusual formats. For example, every time I read a tape made on an SGI machine, the bytes are swapped. The dd command takes this in stride, swapping the bytes as required. The ability to use dd in a pipe with rsh means that the tape device on any *nix system is accessible, given the proper rlogin setup.
Example 3: Byte Swapping with Remote Access of Magnet Tape:rsh sgi.with.tape dd bs=256b if=/dev/rmt0 conv=swab | tar xvf -
The dd runs on the SGI and swaps the bytes before writing to the tar command running on the local host.
Murphy's Law was postulated long before digital computers, but it seems it was specifically targeted for them. When you need to read a floppy or tape, it is the only copy in the universe and you have a deadline past due, that is when you will have a bad spot on the magnetic media, and your data will be unreadable. To the rescue comes dd, which can read all the good data around the bad spot and continue after the error is encountered. Sometimes this is all that is needed to recover the important data.
Example 4: Error Handlingi:dd bs=265b conv=noerror if=/dev/st0 of=/tmp/bad.tape.image
The Linux kernel Makefiles use dd to build the boot image. In the Alpha Makefile /usr/src/linux/arch/alpha/boot/Makefile, the srmboot target issues the command:
Example 5. Kernel Image Makefile:dd if=bootimage of=$(BOOTDEV) bs=512 seek=1 skip=1
This skips the first 512 bytes of the input bootimage file (skip=1) and writes starting at the second sector of the $(BOOTDEV) device (seek=1). A typical use of dd is to skip executable headers and begin writing in the middle of a device, skipping volume and partition data. As this can cause your disk to lose file system data, please test and use these applications with care.
The dd command has been around since the 1970s, ported to many systems, rewritten many times, and tested by time as a useful tool. The current Linux version is GNU dd GNU fileutils 3.12, written by Paul Rubin, David MacKenzie, and Stuart Kemp, Copyright © 1985, 1990, 1991 Free Software Foundation, Inc.
GNU dd is found in the fileutils collection, with the current version at the URL ftp://prep.ai.mit.edu/pub/gnu/fileutils-3.12.tar.gz or a mirror near you.
Other major versions include SYSV and BSD, with the BSD source version 5.16 4/28/93 derived from software contributed to Berkeley by Keith Muller of the University of California, San Diego and Lance Visser of Convex Computer Corporation, Copyright © 1991 The Regents of the University of California.
Sam Chessman (SSC3) (firstname.lastname@example.org)
Practical Task Scheduling Deployment
July 20, 2016 12:00 pm CDT
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
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