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File-Viewing Commands Actions for 'File-Viewing Commands'
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Three file-viewing commands (head, tail, and less) are used to display a portion of a file. These are especially useful when you have a very large text file you want to view and you want to narrow you display of it.
Provide me one or more command syntax examples using each of the three file-viewing commands. You must provide me using at least one command option (ex. with head command you can specify the number of lines to display using "-n num" option).
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Explanation / Answer
Commands/File Viewing:
cat:
cat displays the contents of a file to screen. It can also display multiple files concatenated together, and using the shell, its standard output can be redirected from the screen to the file.
Some useful options are:
-n line numbering
-s squeeze sequences of blank lines into one
-v show non-printing characters
Examples
$ cat example.txt
The contents of the file
example.txt are now
displayed.
$ cat -n example.txt
1 The contents of the file
2 example.txt are now
3 displayed.
Concatenating multiple files (in this example, the same file twice):
$ cat example.txt example.txt
The contents of the file
example.txt are now
displayed.
The contents of the file
example.txt are now
displayed.
Concatenate the files, but use the shell to redirect the result to a new file.
$ cat example.txt example.txt > double.txt
We can concatenate binary files, too. Some programs limit downloads to 2 GB; this is bad for larger files like some DVD images. Suppose a web site helpfully divides the file "sodalinux.dvd" into three parts, 2 GB or less, for downloading and later concatentation. We combine the files, and use the > shell redirection output to put the DVD image in a file:
$ cat sodalinux.dvd1 sodalinux.dvd2 sodalinux.dvd3 > sodalinux.dvd
If we want to type less, then most shells also allow this:
$ cat sodalinux.dvd{1,2,3} > sodalinux.dvd
The -v option is useful for viewing control characters embedded mostly in text. In this example, the file "/usr/share/man/cat1/pax.0" is mostly text but contains control characters which the pagers "less" and "more" use to make text bold. Using cat -v we can see the control characters. Here are the first four lines:
$ cat -v /usr/share/man/cat1/pax.0 | head -4
PAX(1) OpenBSD Reference Manual PAX(1)
N^HNA^HAM^HME^HE
p^Hpa^Hax^Hx - read and write file archives and copy directory hierarchies
Using "cat" with no arguments makes it copy standard input to standard output. Combined with shell redirection, this makes it easy to write a very short text file. All one needs to know is to press Control-D (^D) to indicate end of input, finish the file, and return to the shell. Here is how to write "example.txt":
$ cat > example.txt
The contents of the file
example.txt are now
displayed.
^D $
If you put "cat" with no arguments in a pipe, it only copies standard input to standard output. This might seem useless. For example, the following two pipes have the same function:
$ dmesg | less
$ dmesg | cat | less
However, "cat" can be used as insulation to make programs think that they are not running on terminals. In the next example, GNU bc does not print its copyright message on startup. We enter one calculation ("3 + 9") and then quit (^D):
$ bc | cat
3 + 9
12
^D
tac:
tac (cat spelled backwards) works like cat, but reverse the order of the lines (last line is written out first). When multiple files are given, they are printed out in the order they appear.
$ tac foo
Third Line
Second Line
First Line
more:
more paginates output. The problem with "cat" is that if a file is too long, then it falls beyond the top of the screen. The job of "more" is to stop and wait when it fills the screen. Most users find it easier to use "less", but on some systems "more" has all of the features of "less".
Keys:
· return read next line
· space bar read next screen
· q quit
Examples: The pager will act like "cat" if the file is short enough.
$ more hello.txt
Hello World
less:
less paginates output. The program is called "less" because of the joke that "less is more", "less" actually has several features which "more" lacks.
Keys:
· h read help. You might forget the other commands, but remember this one!
· j go down one line. The down-arrow key might also work.
· k go up one line. The up-arrow key might also work.
· d go down one-half screen.
· u go up one-half screen.
· f go forward one screen.
· b go back one screen.
· p return to the top of the file.
· q quit the pager.
Number arguments:
· 0 through 9: type a number. The number will be used as the argument N to the next command.
· j go down N lines.
· k go up N lines.
· p jump to the N% position, where 0% is the first line and 100% the last line of the file.
Examples:
Read some file:
$ less example.txt
Pipe "dmesg" into "less" so that the dmesg does not scroll off the screen:
$ dmesg | less
od:
od is a utility that lets you view binary files.
Examples:
View a file in octal format:
$ od wordlist.dat
0000000 064506 071562 006564 051412 061545 067157 006544 040412
0000020 070160 062554 005015 072522 061155 062554 005015 061501
View a file in hex format:
$ od -x wordlist.dat
0000000 6946 7372 0d74 530a 6365 6e6f 0d64 410a
0000020 7070 656c 0a0d 7552 626d 656c 0a0d 6341
View a file in character format:
$ od -c wordlist.dat
0000000 F i r s t S e c o n d A
0000020 p p l e R u m b l e
head:
head displays 10 lines from the head (top) of a given file
Examples:
$ head wordlist.dat
First
Second
Third
Fourth
Fifth
Sixth
Seventh
Eighth
Ninth
Tenth
Display the top two lines:
$ head -2 wordlist.dat
First
Second
tail:
tail displays last 10 lines of the file
Examples:
$ tail wordlist.dat
Ninety-First
Ninety-Second
Ninety-Third
Ninety-Fourth
Ninety-Fifth
Ninety-Sixth
Ninety-Seventh
Ninety-Eighth
Ninety-Ninth
One Hundredth
Display the bottom two lines:
$ tail -2 wordlist.dat
Ninety-Ninth
One Hundredth
Tips: The -f option displays the tail, then waits for and displays any new options to the file. This is normally used to watch log files. (The next example has only three lines from tail, but the 80-column terminal was too narrow, so the lines were broken into five lines.)
$ tail -f /var/log/messages
Apr 14 00:05:33 redserver sshd[1575]: Accepted password for rumbear from 24.52.1
45.23 port 33372 ssh2
Apr 14 00:05:34 redserver sshd[1594]: subsystem request for sftp
Apr 14 00:06:35 redserver sshd[1594]: Received disconnect from 242.122.35.47: 11
: Disconnect requested by Windows SSH Client.
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