Lesson 4 - Redirecting I/O
Required Reading
-
Learning UNIX: Chapter 5 (Redirecting I/O)
Additional Notes
A related concept to standard output is standard error. In
addition to displaying results of a command, the UNIX shell
sometimes prints error messages that result from a command.
Any time you get an error message printed to the terminal,
that is considered standard error, and not standard out. If
you only redirect standard output, you will still see error
messages on the screen.
As you learned in your text, you can redirect standard out using
the > operator. Standard error, on the other hand,
can be redirected using the operator 2>. If you want
to combine both standard out and standard error, you can generally
use the operator >&. However, there are some variations
depnding on which UNIX flavor and shell you are using. For more
variations review the section on I/O redirection (5.4.2) in
this
article.
The exercise below will help clarify these differences. This
exercise is should be submitted as part of project
3.
| Task |
Command |
Comments |
| list the contents of two directories (one is valid,
one is invalid) |
ls
. jke |
Notice that you get an error message on the screen as
well as the directory listing. |
| redirect standard out to a file |
ls
. jke > myoutput
cat
myoutput
|
The error message is still displayed on the screen;
it is not written to the file with the listing results. |
| redirect standard error to a file |
ls
. jke 2> myerrors
cat
myerrors
|
The error message is written to the file, while the
listing results are displayed on the screen. |
| redirect both standard error and standard out to different
files |
ls
. jke > myoutput 2> myerrors
cat
myerrors
cat
myoutput
|
The errors are written to the file myerrors while the
listing results are written to the file myoutput. |
| redirect both standard error and standard out to the
same file |
ls
. jke >& myoutput
cat
myoutput
|
The error message is written to the file myoutput, along
with the directory listing. |
One nice thing about output redirection is that commands
run a whole lot faster when they aren't printing results to
the screen.
An interesting filter is the wc (word count) command.
Here's the syntax (from man wc)
NAME wc - print the number of bytes, words, and lines in
files SYNOPSIS wc [-clw] [--bytes] [--chars] [--lines] [--words]
[--help] [--version] [file...]
DESCRIPTION wc counts the number of bytes, whitespace-separated
words, and newlines in each given file, or the standard
input if none are given or when a file named `-' is given.
It prints one line of counts for each file, and if the file
was given as an argument, it prints the filename following
the counts. If more than one filename is given, wc prints
a final line containing the cumulative counts, with thefilename
`total'. The counts are printed in the order: lines, words,
bytes. By default, wc prints all three counts. Options can
specify that only certain counts be printed. Options do
not undo others previously given, so wc --bytes --words
prints both the byte counts and the word counts.
OPTIONS
-c, --bytes, --chars
Print only the byte counts.
-w, --words
Print only the word counts.
-l, --lines
Print only the newline counts.
The text discussion of the grep command is not very clear
about the different uses of grep. They do not elaborate on
how grep works without the use of pipes.
grep "cat" file.txt
will search the contents of the file "file.txt"
for the word "cat". That is different than the use of grep
with a pipe. When you use grep without a pipe, it searches
file contents. If you pipe something to grep, it searches
the text that you are piping to standard input instead. Therefore,
if you do the ls command and pipe it to grep (as in
the book examples) you are searching the text that is returned
by the ls command.
So
grep "test" *
searches the contents of all files in the current directory
for the word "test". This would find a list of files in the
working directory that contain a the word "test"
within their text.
where
ls * | grep "test"
searches the filenames of all files in the current directory
for the word "test". This would find, for example, a file
named "testfile.txt" in the working directory.
It's a subtle difference, but very important!
Next Step
Take the self-quiz
Complete project 3
Continue to lesson 5
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