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Command line quick tips: Reading files different ways
Fedora is delightful to use as a graphical operating system. You can point and click your way through just about any task easily. But you’ve probably seen there is a powerful command line under the hood. To try it out in a shell, just open the Terminal application in your Fedora system. This article is one in a series that will show you some common command line utilities.
In this installment you’ll learn how to read files in different ways. If you open a Terminal to do some work on your system, chances are good that you’ll need to read a file or two.
The whole enchilada
The cat command is well known to terminal users. When you cat a file, you’re simply displaying the whole file to the screen. Really what’s happening under the hood is the file is read one line at a time, then each line is written to the screen.
Imagine you have a file with one word per line, called myfile. To make this clear, the file will contain the word equivalent for a number on each line, like this:
one
two
three
four
five
So if you cat that file, you’ll see this output:
$ cat myfile
one
two
three
four
five
Nothing too surprising there, right? But here’s an interesting twist. You can also cat that file backward. For this, use the tac command. (Note that Fedora takes no blame for this debatable humor!)
$ tac myfile
five
four
three
two
one
The cat file also lets you ornament the file in different ways, in case that’s helpful. For instance, you can number lines:
$ cat -n myfile
1 one
2 two
3 three
4 four
5 five
There are additional options that will show special characters and other features. To learn more, run the command man cat , and when done just hit q to exit back to the shell.
Picking over your food
Often a file is too long to fit on a screen, and you may want to be able to go through it like a document. In that case, try the less command:
$ less myfile
You can use your arrow keys as well as PgUp/PgDn to move around the file. Again, you can use the q key to quit back to the shell.
There’s actually a more command too, based on an older UNIX command. If it’s important to you to still see the file when you’re done, you might want to use it. The less command brings you back to the shell the way you left it, and clears the display of any sign of the file you looked at.
Just the appetizer (or dessert)
Sometimes the output you want is just the beginning of a file. For instance, the file might be so long that when you cat the whole thing, the first few lines scroll past before you can see them. The head command will help you grab just those lines:
$ head -n 2 myfile
one
two
In the same way, you can use tail to just grab the end of a file:
$ tail -n 3 myfile
three
four
five
Of course these are only a few simple commands in this area. But they’ll get you started when it comes to reading files.
via: https://fedoramagazine.org/commandline-quick-tips-reading-files-different-ways/
作者：Paul W. Frields 选题：lujun9972 译者：译者ID 校对：校对者ID