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Writing to the stderr of a script

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Top Forums Shell Programming and Scripting Writing to the stderr of a script
# 1  
Old 02-28-2010
Writing to the stderr of a script
I have Mac OS X, and I wrote a bash shell script that does some encryption of files. When the user of the script does something wrong I want it to send an error message to the stderror of the script. Here's the code snippet I have:

Code:
printf "$1: File not found\nNote: This script will only work on files.  It will not take bare strings.\n" > $0 1>&2;

$0 is (according to my sources) supposed to be the name of the script, but for some reason, for me it's always "bash". I think this may be a Mac thing. When I ran the script and sent its stderr to a file, I found that the file was empty afterward. What's happening is pretty obvious: the script sends the standard output of printf to the standard error of the bash shell, rendering it practically inaccessible.

So my question is how can I access the name of the process for the script, seeing as $0 doesn't work?
Last edited by Ultrix; 02-28-2010 at 06:37 PM..
# 2  
Old 02-28-2010
output to stderr
>&2 printf

oops i was answering the thread title.
how are you calling the script?
# 3  
Old 02-28-2010
Quote:
Originally Posted by bigearsbilly
>&2 printf
That's not the problem. It's printing the error message just fine, but it's not redirecting the output to the stderr of the script, just to the stderr of bash.
# 4  
Old 02-28-2010
Ok, one thing after the other...

$0 is the name of the program running. What's put there depends on how you call it. But you can't use that name in the sense of "redirect something to that program".

Redirections are done from left to right. So your line
Code:
printf "$1: File not found\nNote: This script will only work on files.  It will not take bare strings.\n" > $0 1>&2;

actually means "take the output of printf, redirect it to the file named in $0, and if there's still something on stdout redirect it to stderr"

What you want is
Code:
printf "$1: File not found\nNote: This script will only work on files.  It will not take bare strings.\n" 1>&2

Which means "take printf's output, and redirect it to stderr"
# 5  
Old 02-28-2010
no you are doing it wrong ultrix,
first you do:
> $0 (print output to the script!)
then over riding with >&2

order is important.
the canonical way is

>&2 printf thing > out.log
# 6  
Old 02-28-2010
Quote:
Originally Posted by pludi
Ok, one thing after the other...

$0 is the name of the program running. What's put there depends on how you call it. But you can't use that name in the sense of "redirect something to that program".

Redirections are done from left to right. So your line
Code:
printf "$1: File not found\nNote: This script will only work on files.  It will not take bare strings.\n" > $0 1>&2;

actually means "take the output of printf, redirect it to the file named in $0, and if there's still something on stdout redirect it to stderr"

What you want is
Code:
printf "$1: File not found\nNote: This script will only work on files.  It will not take bare strings.\n" 1>&2

Which means "take printf's output, and redirect it to stderr"
Yes, that works. It redirects to stderr. Thanks for the help.
# 7  
Old 03-01-2010
Code:
cmd  >> somestdout.redirect.file.append  # stderr not

cmd 2>> somestderr.redirect.file.append  # stdout not

cmd 2> somefile # overwrite somefile, only stderr

cmd > some.stdout  2>some.stderr   
cmd >> some.stdout  2>>some.stderr  

cmd >&2           # output redirect to the stderr

cmd >&2 | cmd2   # stderr also to the pipe
cmd 2>>err  |  cmd2  # only stdout to the pipe

# stdout redirect to the file and stderr redirect to the stdout = somefile
cmd  > somefile  2>&1   

# maybe never used: stderr redirect stdout and after that stdout redirect to 
# the somefile - stderr is not redirected to the somefile
cmd 2>&1 > somefile 

# after this all stderr has redirected to the file err.log, 
# very nice ex. in cgi using.
exec 2>>err.log 

cmd >/dev/null 2>&1   # output is not interesting

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