This is a reply that simply discribes redirecting output
All unix programs have three IO streams by default
Code:
0 - input, formally standard input. it is referred to as SDTIN
1 - output, formally standard output. it is referred to as SDTOUT
2 - error, formally standard error. it is referred to as SDTERR
by default all input comes from the keyboard and all output (STDOUT and STRERR) goes to the screen.
Example:
Quote:
cat /tmp/file
if the file exists, the cat command will sent the contents of the file to standard out, the screen.
if the the file does not exist, cat will send an error message to standard error, still the screen
Often is is useful to redirect an output stream into a file. the output redirection operator is >.
1> redirects SDTOUT and 2> redirects STDERR
STDOUT example
Quote:
cat /tmp/file 1> newfile
if the file exists, the cat command will sent the contents of the file to standard out, newfile.
if the the file does not exist, cat will send an error message to standard error, still the screen since we did not redirect STDERR with 2>
STDERR example
Quote:
cat /tmp/file 2> newfile
if the file exists, the cat command will sent the contents of the file to standard out, the screen.
if the the file does not exist, cat will send an error message to standard error, newfile
STDERR and STDOUT example
Quote:
cat /tmp/file 1> newfile 2> errorfile
if the file exists, the cat command will sent the contents of the file to standard out, newfile .
if the the file does not exist, cat will send an error message to standard error, errorfile
STDERR and SDTOUT example (special case, both sent to same file)
In this example, we are redirecting both standard out and erro to the same file.
Read the cat command like this: send the contents of /tmp/file to STDOUT (which is newfile) and send STDERR to the same place that STDOUT is going
Quote:
cat /tmp/file 1> newfile 2>&1
if the file exists, the cat command will sent the contents of the file to standard out, newfile .
if the the file does not exist, cat will send an error message to standard error, which is the same as STDOUT, newfile.
CAREFUL!!! left to right order matters!!!
the following command line is NOT like the previous:
Quote:
cat /tmp/file 2>&1 1> newfile
read this command left to right....
while running the cat command send STDERR to where STDOUT is going (STDOUT is STILL WRITING TO THE SCREEN!!!!!) and send the contents of the file to STDOUT, which is finally send to newfile.
basically, at the time STDERR was sent to STDOUT, SDTOUT was still the screen, only later is STDOUT send to a file. STDERR is not effected by the late redirection of STDOUT
One more more note. inplace of 1> you can simply use >. > without a number is implicitly STDOUT.
STDIN is a lot simpler. many programs can take file input for STDIN reather then from keyboard. also may programs can take STDIN inplace of command line parms.
back to cat.
everyone knows that this writes the output of a file to STDOUT
Code:
cat filename
the exact same thing can be done as
Code:
cat < filename
or it use cat to copy a file
Code:
cat < originalfile > copyOfFile
Hope this helps someone...ask questions, I can help if you need.
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That's a nice little write up. You should put that up on the wiki (UnderstandingRedirection maybe?) and link to it from the LinuxTips page.