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Viewing the Content of Text Files
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When administering your server, you will find that you often need to modify configuration files, which take the form of ASCII text files. Therefore, it s very important to be able to browse the content of these files. You have several ways of doing this: cat: Displays the contents of a file tac: Does the same as cat, but displays the contents in an inverse order tail: Shows just the last lines of a text file head: Displays the first lines of a file less: Opens an advanced file viewer more: Like less, but not as advanced
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First is the cat command. This command just dumps the contents of a file on the screen (see Listing 2-4). This can be useful, but, if the contents of the file does not fit on the screen, you ll see some text scrolling by and, when it stops, you ll only see the last lines of the file displayed on the screen. As an alternative to cat, you can use tac as well. Not only is its name opposite to cat, its result is too. This command will dump the contents of a file to the screen, but with the last line first and the first line last. Listing 2-4. The cat Command Is Used to Display the Contents of a Text File. root@RNA:/boot# cat /etc/hosts 127.0.0.1 localhost 127.0.1.1 RNA.lan RNA # The following lines are desirable for IPv6 capable hosts ::1 ip6-localhost ip6-loopback fe00::0 ip6-localnet ff00::0 ip6-mcastprefix ff02::1 ip6-allnodes ff02::2 ip6-allrouters ff02::3 ip6-allhosts Another very useful command is tail. If no options are used, this command will show the last ten lines of a text file. The command can also be modified to show any number of lines on the bottom of a file; for example, tail -2 /etc/passwd will display the last two lines of the configuration file in which user names are stored. Also very useful for monitoring what happens on your system is the option to keep tail open on a given log file. For example, if you use tail -f /var/log/messages, the most generic log file on your system is opened, and, when a new line is written to the bottom of that file, you will see it immediately. The opposite of tail is the head command, which displays the top lines of a text file. The last two files used to view the contents of text files are less and more. The most important thing you need to remember about them is that you can do more with less. Contrary to common sense, the less command is actually the improved version of more. Both commands will open your configuration file in a viewer as you can see in Listing 2-5. In this viewer you can browse down in the file by using the Page Down key or the spacebar. Only less offers the option to browse up as well. Also, both commands have a search facility. If the less utility is open and displays the content of your file, use /sometext from within the less viewer to locate sometext in the file. To quit both utilities, use the q command. Listing 2-5. The less Command Can Be Used As a Viewer to View File Contents. 127.0.0.1 127.0.1.1 localhost RNA.lan RNA
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# The following lines are desirable for IPv6 capable hosts ::1 ip6-localhost ip6-loopback fe00::0 ip6-localnet ff00::0 ip6-mcastprefix ff02::1 ip6-allnodes
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ff02::2 ip6-allrouters ff02::3 ip6-allhosts /etc/hosts (END)
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Finding Files That Contain Specific Text
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As a Linux administrator, you ll sometimes need to search for a specific file by some word or phrase within the file. Because most configuration files created on your server are ASCII text files (in the /etc directory or one of its subdirectories), it is rather easy to search for text within them using the grep utility, which is one of the Linux s most useful utilities. Let s start with a rather basic example in which you want to get a list of all files that contain the text Linda in /etc. You can just use grep linda /etc/*, and, if you want to make sure that you can search files that are readable for root only, use sudo grep linda /etc/*. Notice that the grep command is case sensitive. If you want it to be case insensitive, you should include the -i option: grep -i linda /etc/*. This command will produce a list of file names, followed by the line in which the text you were looking for is shown; see Listing 2-6. Listing 2-6. The grep Utility Is Useful for Searching Files That Contain a Certain Word or Phrase. sander@RNA:~$ sudo grep linda /etc/* /etc/group:linda:x:1001: /etc/gshadow:linda:!:: /etc/passwd:linda:x:1001:1001::/home/linda:/bin/sh /etc/shadow:linda:!:13671:0:99999:7::: If the output gets a little longer, you may find it confusing to see the lines that contain the text you were looking for as well. If that s the case, use the -l (list) option. You ll see only file names when using this option. Another disadvantage is that grep does not search subdirectories by default, but you can tell it to do so by using the -r option. If, however, you want fast results, be careful with the -r option because searching an entire directory tree structure for the occurrence of some word in a file is very labor intensive.
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