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12.4. Show Nonprinting Characters with cat -v or od -c

Especially if you use an ASCII-based terminal, files can have characters that your terminal can't display. Some characters will lock up your communications software or hardware, make your screen look strange, or cause other weird problems. So if you'd like to look at a file and you aren't sure what's in there, it's not a good idea to just cat the file!

Instead, try cat -v. It shows an ASCII ("printable") representation of unprintable and non-ASCII characters. In fact, although most manual pages don't explain how, you can read the output and see what's in the file. Another utility for displaying nonprintable files is od. I usually use its -c option when I need to look at a file character by character.

Let's look at a file that's almost guaranteed to be unprintable: a directory file. This example is on a standard V7 (Unix Version 7) filesystem. (Unfortunately, some Unix systems won't let you read a directory. If you want to follow along on one of those systems, try a compressed file (Section 15.6) or an executable program from /bin.) A directory usually has some long lines, so it's a good idea to pipe cat's output through fold:

% ls -fa
% cat -v . | fold -62
% od -c .
0000000 377 016   .  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
0000020   > 007   .   .  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
0000040 341  \n   c   o   m   p  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
0000060  \0  \0   M   a   s   s   A   v   e   F   o   o   d  \0  \0  \0
0000100  \0  \0   h   i   s   t  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0

Each entry in a V7-type directory is 16 bytes long (that's also 16 characters, in the ASCII system). The od -c command starts each line with the number of bytes, in octal, shown since the start of the file. The first line starts at byte 0. The second line starts at byte 20 octal (that's byte 16 in decimal, the way most people count). And so on. Enough about od for now, though. We'll come back to it in a minute. Time to dissect the cat -v output:

cat has two options, -t and -e, for displaying whitespace in a line. The -v option doesn't convert TAB and trailing-space characters to a visible form without those options. See Section 12.5.

Next, od -c. It's easier to explain than cat -v:

The strings (Section 13.15) program finds printable strings of characters (such as filenames) inside mostly nonprintable files (such as executable binaries).

-- JP

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