argument precedence, output redirection
Hi all, I'm not sure this is a bug or please let me know the concept: What is the difference between: ls -R /etc/ 2>&1 1>/dev/null and ls -R /etc/ 1>/dev/null 2>&1 the second one redirect everything to /dev/null but the first one, still prints errors (run as a non root user would unveil the problem) it the order of arguments important? If yes, what is the idea/concept behind this behavior? Best Regards smime.p7s Description: S/MIME cryptographic signature
Re: argument precedence, output redirection
On Fri, Dec 03, 2010 at 06:16:06PM +0330, Payam Poursaied wrote: > What is the difference between: > ls -R /etc/ 2>&1 1>/dev/null > and > ls -R /etc/ 1>/dev/null 2>&1 http://mywiki.wooledge.org/BashFAQ/055 In a sentence, redirections are done in order, from left to right.
Re: argument precedence, output redirection
On 12/03/2010 07:46 AM, Payam Poursaied wrote: > > Hi all, > I'm not sure this is a bug or please let me know the concept: > What is the difference between: > ls -R /etc/ 2>&1 1>/dev/null > and > ls -R /etc/ 1>/dev/null 2>&1 POSIX requires that redirections are evaluated from left to right. The first line duplicates fd 2 from 1 (that is, stderr is now shared with stdout), then changes fd 1 onto /dev/null (so you've silenced stdout, and errors from ls will show up on your stderr). The second line changes fd 1 onto /dev/null, then duplicates fd 2 from 1 (that is, stderr is now shared with /dev/null, and you've silenced all output to either stream). > the second one redirect everything to /dev/null but the first one, still > prints errors (run as a non root user would unveil the problem) > it the order of arguments important? If yes, what is the idea/concept behind > this behavior? Yes the order is important, and the idea behind the behavior is that left-to-right evaluation order can be easily documented and relied on. -- Eric Blake ebl...@redhat.com+1-801-349-2682 Libvirt virtualization library http://libvirt.org signature.asc Description: OpenPGP digital signature
Re: argument precedence, output redirection
> I'm not sure this is a bug or please let me know the concept: > What is the difference between: > ls -R /etc/ 2>&1 1>/dev/null > and > ls -R /etc/ 1>/dev/null 2>&1 > > the second one redirect everything to /dev/null but the first one, still > prints errors (run as a non root user would unveil the problem) > it the order of arguments important? If yes, what is the idea/concept behind > this behavior? Redirections are processed left-to-right (or, if you prefer, beginning to end). The manual page has this to say: Note that the order of redirections is significant. For example, the command ls > dirlist 2>&1 directs both standard output and standard error to the file dirlist, while the command ls 2>&1 > dirlist directs only the standard output to file dirlist, because the standard error was duplicated as standard output before the standard output was redirected to dirlist. -- ``The lyf so short, the craft so long to lerne.'' - Chaucer ``Ars longa, vita brevis'' - Hippocrates Chet Ramey, ITS, CWRUc...@case.eduhttp://cnswww.cns.cwru.edu/~chet/
Re: argument precedence, output redirection
Le 03/12/2010 14:46, Payam Poursaied a écrit : > > Hi all, > I'm not sure this is a bug or please let me know the concept: > What is the difference between: > ls -R /etc/ 2>&1 1>/dev/null > and > ls -R /etc/ 1>/dev/null 2>&1 > > the second one redirect everything to /dev/null but the first one, still > prints errors (run as a non root user would unveil the problem) > it the order of arguments important? If yes, what is the idea/concept behind > this behavior? I was confused by this for a long time and kept reading again and again the correct but *too long* answers others have posted, until I started using this *short* reading trick: 2>&11>/dev/null output2 := output1; output1 := NULL When you apply this from left to right it should be quite obvious why output2 is not NULL. And now you can finally understand things like this: exec 3>&1 4>&2# save exec 1>/dev/null 2>&1 # discard everything ... exec 1>&3 2>&4# restore
argument precedence, output redirection
Hi all, I'm not sure this is a bug or please let me know the concept: What is the difference between: ls -R /etc/ 2>&1 1>/dev/null and ls -R /etc/ 1>/dev/null 2>&1 the second one redirect everything to /dev/null but the first one, still prints errors (run as a non root user would unveil the problem) it the order of arguments important? If yes, what is the idea/concept behind this behavior? Best Regards smime.p7s Description: S/MIME cryptographic signature
