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 turns non-printable characters into a printable form. 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 non-printable 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 aor an executable program from /bin.) A directory usually has some long lines, so it's a good idea to pipe cat's output through :
Each entry in a V7-type directory is 16 bytes long (that's also 16 characters, in thesystem). 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 (that's byte 16 in decimal, the way most of us count). And so on. Enough about od for now, though. We'll come back in a minute. Time to dissect the cat -v output:
You've probably seen sequences like
Those are control characters.
(Find them in the cat -v output, please.)
Another character like this is
^@, the character NUL (ASCII 0).
There are a lot of NULs in the directory; more about that below.
A DEL character (ASCII 177 octal) is shown as
cat -v has its own symbol for characters outside the ASCII range
with their high bits set,
also called metacharacters.
cat -v prints those as
M- followed by another character.
There are two of them in the cat -v output:
To get a metacharacter, you add 200 octal.
Let's look at
The octal value of the letter
a is 141.
When cat -v prints
M-a, it means the character you get
by adding 141+200, or 341 octal.
You can decode the character cat prints as
M-^? in the same way.
^? stands for the DEL character, which is octal 177.
Add 200+177 to get 377 octal.
If a character isn't
it's a regular printable character.
The entries in the directory (
hist) are all made of regular ASCII
If you're wondering where the entries
are in the ls listing, the answer is: they aren't.
Those entries have been deleted from the directory.
two NUL (ASCII 0, or
^@) bytes in front of the name when
a file has been deleted.
cat has two options, -t and -e, for displaying white space in a line. The -v option doesn't convert TAB and trailing space characters to a visible form without those options. See article 25.6.
Next, time for od -c; it's easier to explain than cat -v:
od -c shows some characters starting with a backslash (
It uses the standard UNIX and C abbreviations for
where it can.
\n stands for a newline character,
for a tab, etc.
There's a newline at the start of the
comp entry - see it in the
od -c output?
That explains why the cat -v output was broken onto a new line
at that place: cat -v doesn't translate newlines when it finds them.
\0 is a NUL character (ASCII 0).
It's used to pad the ends of entries in V7 directories when a name isn't the
full 14 characters long.
od -c shows the octal value of other characters as three digits.
For instance, the
007 means "the character 7 octal."
cat -v shows this as
Metacharacters, the ones with octal values 200 and above, are shown as
something by cat -v.
In od -c, you'll see their octal values - like
Each directory entry on a
UNIX Version 7 filesystem
starts with a two-byte "pointer" to its location in the
disk's inode table.
When you type a filename, UNIX uses this pointer to find the actual file information
on the disk.
The entry for this directory (named
Its parent (named
..) is at
And comp's entry is
Find those in the cat -v output, if you want - and compare the two
Like cat -v, regular printable characters are shown as is by od -c.