When I first started working in systems integration, I was primarily a PC support person. I spent a lot of time installing and supporting Windows applications in various PC LAN configurations, running various versions (and vendors) of TCP/IP transports. Since then, I have successfully ditched DOS and moved on. Now, after working on various versions of Unix for a few years, I porting some of our networking and data manipulation libraries to other platforms and environments, such as the AS/400 minicomputer and the Macintosh. This ongoing experience has given me a chance to appreciate just how powerful the tools we take for granted with Linux really are.
Searching for a word (or any other value) in a group of files is a very common task. Whether it's searching for a function in a group of source code modules, trying to find a parameter in a set of configuration files, or simply looking for a misplaced e-mail message, text searching and matching operations are common in all environments.
Unfortunately, this common task doesn't have an easy solution on all platforms. On most, the best solution available is to use the search function in an editor. But when it comes to Linux (and other Unix descendants), you have many solutions. One of them is grep.
grep is an acronym for “global regular expression print,” a reference to the command in the old ed line editor that prints all of the lines in a file containing a specified sequence of characters. grep does exactly that: it prints out lines in a file that contain a match for a regular expression. We'll gradually delve into what a regular expression is as we go on.
First, let's look at a quick example. We will search for a word in the Configure script provided with Linux for setting up the Linux kernel source, which is usually installed in the /usr/src/linux directory. Change to that directory and type (the $ character is the prompt, don't type it):
$ grep glob Configure
You should see:
# Disable filename globbing once and for all.
glob is in bold to illustrate what grep matched. grep does not actually print matches in bold.
grep looked for the sequence of characters glob and printed the line of the Configure file with that sequence. It did not look for the word glob. It looked for g followed by l followed by o followed by b. This points out one important aspect of regular expressions: they match sequences of characters, not words.
Before we dig any deeper into the specifics of pattern matching, let's look at grep's “user interface” with a few examples. Try the following two commands:
$ grep glob < Configure $ cat Configure | grep glob
both of these two commands should print
# Disable filename globbing once and for all.
which probably looks familiar.
In all of these commands, we have specified the regular expression as the first argument to grep. With the exception of any command line switches, grep always expects the regular expression as the first argument.
However, we presented grep with three different situations and received the same response. In the first exercise, we provided grep with the name of a file, and it opened that file and searched it. grep can also take a list of filenames to search.
In the other two exercises we illustrated a feature that grep shares with many other utilities. If no files are specified on the command line, grep reads standard input. To further illustrate standard input let's try one more example:
$ grep foo
When you run that, grep appears to “hang” waiting for something. It is. It's waiting for input. Type:
and press return. Nothing happens. Now type:
and press enter. This time, grep sees the string foo in foobar and echos the line foobar back at you, which is why foobar appears twice. Now type ctrl-d, the “end-of-file” character, to tell grep that it has reached the end of the file, whereupon it exits.
You just gave grep an input file that consisted of tttt, a newline character, foobar, a newline character, and the end-of-file character.
Piping input into grep from standard input also has another frequent use: filtering the output of other commands. Sometimes cutting out the unnecessary lines with grep is more convenient than reading output page by page with more or less:
$ ps ax | grep cron
efficiently gives you the process information for crond.
Practical Task Scheduling Deployment
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.
Join Linux Journal's Mike Diehl and Pat Cameron of Help Systems.
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- The Firebird Project's Firebird Relational Database
- Stunnel Security for Oracle
- My +1 Sword of Productivity
- Non-Linux FOSS: Caffeine!
- Managing Linux Using Puppet
- SUSE LLC's SUSE Manager
- Murat Yener and Onur Dundar's Expert Android Studio (Wrox)
- Doing for User Space What We Did for Kernel Space
- Google's SwiftShader Released
- SuperTuxKart 0.9.2 Released
With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide