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13.11. More Commands for Reading Text Files

You have already been introduced to several basic shell prompt commands for reading files in text editors. Here are a few more.

13.11.1. The head Command

You can use the head command to look at the beginning of a file. The command is:

head <filename>

head can be a useful command, but because it is limited to the first several lines, you will not see how long the file actually is. By default, you can only read the first ten lines of a file. You can change the number of lines displayed by specifying a number option as shown in the following command:

head -20 <filename>

13.11.2. The tail Command

The reverse of head is tail. Using tail, you can view the last ten lines of a file. This can be useful for viewing the last 10 lines of a log file for important system messages. You can also use tail to watch log files as they are updated. Using the -f option, tail automatically print new messages from an open file to the screen in real-time. For example, to actively watch /var/log/messages, type the following at a shell prompt as the root user:

tail -f /var/log/messages

13.11.3. The grep Command

The grep command is useful for finding specific character strings in a file. For example, if you want to find every reference made to "coffee" in the file sneakers.txt, you would type:

grep coffee sneakers.txt

You would see every line in that file where the word "coffee" is found.

TipTip
 

Unless otherwise specified, grep searches are case sensitive. That means that searching for Coffee is different than searching for coffee. Among grep's options is -i, which allows for a case-insensitive search through a file. Read the grep man page for more about this command.

13.11.4. I/O Redirection and Pipes

You can use pipes and output redirection when you want to store and/or print information to read at a later time.

You can, for example, use grep to search for particular contents of a file, then have those results either saved as a file or sent to a printer.

To print the information about references to "coffee" in sneakers.txt, for example, just type:

grep coffee sneakers.txt | lpr

13.11.5. Wildcards and Regular Expressions

What if you forget the name of the file you are looking for? Using wildcards or regular expressions, you can perform actions on a file or files without knowing the complete filename. Just fill out what you know, then substitute the remainder with a wildcard. Wildcards are special symbols that you can substitute for letters, numbers, and symbols that make finding particular directories and files easier than examining long directory listings to find what you are searching for.

TipTip
 

To read more about wildcards and regular expressions, take a look at the bash man page (man bash). Remember that you can save the file to a text file by typing man bash | col -b > bash.txt. Then, you can open and read the file with less or vi (vi bash.txt). If you want to print the file, be aware that it is quite long.

We know the file is called "sneak____.txt," so type:

ls sneak*.txt

and there is the name of the file:

sneakers.txt

You will probably use the asterisk (*) most frequently when you are searching. The asterisk will search out everything that matches the pattern you are looking for. So even by typing:

ls *.txt

or:

ls sn*

You would find sneakers.txt and any other files whose name ends with .txt or begin with sn. It helps to narrow your search as much as possible.

One way to narrow a search is to use the question mark symbol (?). Like the asterisk, using ? can help locate a file matching a search pattern.

In this case, though, ? is useful for matching a single character, so if you were searching for sneaker?.txt, you would get sneakers.txt as a result, and/or sneakerz.txt, if there were such a filename.

Regular expressions are more complex than the straightforward asterisk or question mark.

When an asterisk, for example, just happens to be part of a filename, as might be the case if the file sneakers.txt was called sneak*.txt, that is when regular expressions can be useful.

Using the backslash (\), you can specify that you do not want to search out everything by using the asterisk, but you are instead looking for a file with an asterisk in the name.

If the file is called sneak*.txt, type:

sneak\*.txt

Here is a brief list of wildcards and regular expressions:

  • * — Matches all characters

  • ? — Matches one character in a string

  • \* — Matches the * character

  • \? — Matches the ? character

  • \) — Matches the ) character

 
 
  Published under the terms of the GNU General Public License Design by Interspire