shell里输出结果的重定向(redirect)
2016-02-11 03:06阅读：
标准输入/输出的重定向:
Standard Input and Output Redirection The shell and many UNIX
commands take their input from
standard input
(
stdin), write output to
standard output
(
stdout), and write error output to
standard
error (
stderr). By default, standard input is
connected to the terminal keyboard and standard output and error to
the terminal screen.
The way of indicating an end-of-file on the default standard input,
a terminal, is usually .
Redirection of I/O, for example to a file, is accomplished by
specifying the destination on the command line using a
redirection metacharacter followed by the desired
destination.
C Shell Family
Some of the forms of redirection for the C shell family are:
Character
|
Action
|
>
|
Redirect standard
output
|
>&
|
Redirect standard output and standard error
|
<</code>
|
Redirect standard input
|
>!
|
Redirect standard output; overwrite file if it
exists
|
>&!
|
Redirect standard output and standard error; overwrite file if
it exists
|
|
|
Redirect standard output to another command (pipe)
|
>>
|
Append standard output
|
>>&
|
Append standard output and standard error
|
The form of a command with standard input and output redirection
is:
%
command -[
options] [
arguments]
<
input file >
output file
If you are using
csh and do not have the
noclobber
variable set, using
> and
>& to
redirect output will overwrite any existing file of that name.
Setting
noclobber prevents this. Using
>!
and
>&! always forces the file to be
overwritten. Use
>> and
>>& to append output to existing
files.
Redirection may fail under some circumstances: 1) if you have the
variable
noclobber set and you attempt to redirect output to
an existing file without forcing an overwrite, 2) if you redirect
output to a file you don't have write access to, and 3) if you
redirect output to a directory.
Examples:
% who > names- Redirect standard output to a file named
names
% (pwd; ls -l) > out- Redirect output of both commands to a file named
out
% pwd; ls -l > out- Redirect output of
ls command only to a file named
out
Input redirection can be useful, for example, if you have written a
FORTRAN program which expects input from the terminal but you want
it to read from a file. In the following example,
myprog, which was written to read standard input and
write standard output, is redirected to read
myin and
write
myout:
% myprog < myin > myout
You can suppress redirected output and/or errors by sending it to
the
null device,
/dev/null. The example shows
redirection of both output and errors:
% who >& /dev/null
To redirect standard error and output to different files, you can
use grouping:
% (cat myfile > myout) >& myerror
Bourne Shell Family
The Bourne shell uses a different format for redirection which
includes numbers. The numbers refer to the file descriptor numbers
(0 standard input, 1 standard output, 2 standard error). For
example,
2> redirects file descriptor 2, or
standard error.
&n is the syntax for
redirecting to a specific open file. For example
2>&1 redirects 2 (standard error) to 1
(standard output); if 1 has been redirected to a file, 2 goes there
too. Other file descriptor numbers are assigned sequentially to
other open files, or can be explicitly referenced in the shell
scripts. Some of the forms of redirection for the Bourne shell
family are:
Character
|
Action
|
>
|
Redirect standard
output
|
2>
|
Redirect standard error
|
2>&1
|
Redirect standard error to standard output
|
<</code>
|
Redirect standard input
|
|
|
Pipe standard output to another command
|
>>
|
Append to standard output
|
2>&1|
|
Pipe standard output and standard error to another
command
|
Note that
< and
> assume standard
input and output, respectively, as the default, so the numbers 0
and 1 can be left off. The form of a command with standard input
and output redirection is:
$
command -[
options] [
arguments]
<
input file >
output file
Redirection may fail under some circumstances: 1) if you have the
variable
noclobber set and you attempt to redirect output to
an existing file without forcing an overwrite, 2) if you redirect
output to a file you don't have write access to, and 3) if you
redirect output to a directory.
Examples:
$ who > names- Direct standard output to a file named
names
$ (pwd; ls -l) > out- Direct output of both commands to a file named
out
$ pwd; ls -l > out- Direct output of
ls command only to a file named
out
Input redirection can be useful if you have written a program which
expects input from the terminal and you want to provide it from a
file. In the following example,
myprog, which was
written to read standard input and write standard output, is
redirected to read
myin and write
myout.
$ myprog < myin > myout
You can suppress redirected output and/or error by sending it to
the
null device,
/dev/null. The example shows
redirection of standard error only:
$ who 2> /dev/null
To redirect standard error and output to different files (note that
grouping is not necessary in Bourne shell):
$ cat myfile > myout 2> myerror
原文（http://sc.tamu.edu/help/general/unix/redirection.html）
