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June
9, 2003 -
The
Standard UNIX File Descriptors - Standard
Input (stdin), Standard Output (stdout),
and Standard Error (stderr)
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If
you are familiar with UNIX I/O
redirection, syntax similar to the
following should not be new to you:
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command >
file 2>&1
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Briefly, when
command runs it sends "normal"
output to file, and any error messages
generated by command are also written to
file. "2>&1"
handles the latter.
Have you ever wondered where the numbers 2
and 1 come from? This may be common
knowledge to our more experienced readers,
buy may need some explaining to those who
are relatively new to the UNIX operating
system environment. The 2 and 1 are
file descriptors. Okay, so what's a
file descriptor?
When a UNIX program wants to use a file,
it must first open that file. When
it does so, UNIX will associate a number
with the file. This number, which is
used by the program when reading from and
writing to the file, is the file
descriptor.
A typical UNIX program will open three
files when it starts. These files
are:
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- standard
input (also known as stdin)
- standard output (also known as stdout)
- standard error (also known as stderr)
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Standard input
has a file descriptor of 0, standard output
uses 1, and the number 2 is used by standard
error. Are you starting to see where
this is headed?
Looking at our command again,
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command >
file 2>&1
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you should now
recognize that 2>&1 instructs
the shell to send messages headed to stderr
(2) to the same place messages to stdout (1)
are sent. In our example, that place
is file.
If you are wondering, > is equivalent to
1>, and < is short for <0.
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more...
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Tips...
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