pipe and 'read' oddity
in [Shell]
|
From: Robert Latest on 15 Jan 2010 15:48 Hello people, the snippet from an interactive bash session below should illustrate my "problem". Stuff on the right-hand side are my comments for the respective line. I attempted assigning hours and minutes from date's output to a couple of variables using 'read': $ date '+%H +%M' | read h m ; echo $h $m | First try | ... prints empty line $ read h m ; echo $h $m | Huh? Can't 'read' read from stdin?? 10 20 | This was typed in by hand 10 20 | Sure enough: correct output $ cat | read h m ; echo $h $m | Let's pipe stdin through cat 11 22 | This was typed in 10 20 | h and m still have the old values $ eval `date '+h=%H; m=%M'` ; echo $h $m | This is what I ended up doing 21 38 | ... with the correct result In short: Why doesn't 'read' assign any values to variables h and m when stdin comes in from a pipe? Thanks, robert
From: Edgardo Portal on 15 Jan 2010 15:56 On 2010-01-15, Robert Latest <boblatest(a)yahoo.com> wrote: > Hello people, > > the snippet from an interactive bash session below should illustrate my > "problem". Stuff on the right-hand side are my comments for the > respective line. I attempted assigning hours and minutes from date's > output to a couple of variables using 'read': > > > $ date '+%H +%M' | read h m ; echo $h $m | First try > | ... prints empty line > $ read h m ; echo $h $m | Huh? Can't 'read' read from stdin?? > 10 20 | This was typed in by hand > 10 20 | Sure enough: correct output > $ cat | read h m ; echo $h $m | Let's pipe stdin through cat > 11 22 | This was typed in > 10 20 | h and m still have the old values > $ eval `date '+h=%H; m=%M'` ; echo $h $m | This is what I ended up doing > 21 38 | ... with the correct result > > In short: Why doesn't 'read' assign any values to variables h and m > when stdin comes in from a pipe? > > Thanks, > robert FWIW, I noticed the same thing recently when porting some ksh scripts from an old-ish Solaris to bash on Linux. Turned out that bash doesn't work the way ksh does in this respect: jim(a)katie:~$ echo $SHELL /bin/bash jim(a)katie:~$ date +'%H %M' | read h m; echo $h $m jim(a)katie:~$ ksh u@h:w$ date +'%H %M' | read h m; echo $h $m 15 53 Didn't bother to debug it, just installed ksh on Linux and moved on.
From: pk on 15 Jan 2010 16:18 Robert Latest wrote: > the snippet from an interactive bash session below should illustrate my > "problem". Stuff on the right-hand side are my comments for the > respective line. I attempted assigning hours and minutes from date's > output to a couple of variables using 'read': > > > $ date '+%H +%M' | read h m ; echo $h $m | First try > | ... prints empty line > $ read h m ; echo $h $m | Huh? Can't 'read' read from > stdin?? > 10 20 | This was typed in by hand > 10 20 | Sure enough: correct output > $ cat | read h m ; echo $h $m | Let's pipe stdin through cat > 11 22 | This was typed in > 10 20 | h and m still have the old > values > $ eval `date '+h=%H; m=%M'` ; echo $h $m | This is what I ended up doing > 21 38 | ... with the correct result > > In short: Why doesn't 'read' assign any values to variables h and m > when stdin comes in from a pipe? See this page: http://mywiki.wooledge.org/BashFAQ/024
From: Jerry Peters on 15 Jan 2010 16:32 Robert Latest <boblatest(a)yahoo.com> wrote: > Hello people, > > the snippet from an interactive bash session below should illustrate my > "problem". Stuff on the right-hand side are my comments for the > respective line. I attempted assigning hours and minutes from date's > output to a couple of variables using 'read': > > > $ date '+%H +%M' | read h m ; echo $h $m | First try > | ... prints empty line > $ read h m ; echo $h $m | Huh? Can't 'read' read from stdin?? > 10 20 | This was typed in by hand > 10 20 | Sure enough: correct output > $ cat | read h m ; echo $h $m | Let's pipe stdin through cat > 11 22 | This was typed in > 10 20 | h and m still have the old values > $ eval `date '+h=%H; m=%M'` ; echo $h $m | This is what I ended up doing > 21 38 | ... with the correct result > > In short: Why doesn't 'read' assign any values to variables h and m > when stdin comes in from a pipe? > > Thanks, > robert Read does assign the values, the problem is that read is running in a child shell which cannot affect the values of h and m in the parent shell. Try a here document: read h m <<-EOF date '+%H %M' EOF Process substitution should also work. Jerry
From: mop2 on 15 Jan 2010 19:41
On Fri, 15 Jan 2010 18:48:08 -0200, Robert Latest <boblatest(a)yahoo.com> wrote: > Hello people, > > the snippet from an interactive bash session below should illustrate my > "problem". Stuff on the right-hand side are my comments for the > respective line. I attempted assigning hours and minutes from date's > output to a couple of variables using 'read': > > > $ date '+%H +%M' | read h m ; echo $h $m | First try > | ... prints empty line > $ read h m ; echo $h $m | Huh? Can't 'read' read from stdin?? > 10 20 | This was typed in by hand > 10 20 | Sure enough: correct output > $ cat | read h m ; echo $h $m | Let's pipe stdin through cat > 11 22 | This was typed in > 10 20 | h and m still have the old values > $ eval `date '+h=%H; m=%M'` ; echo $h $m | This is what I ended up doing > 21 38 | ... with the correct result > > In short: Why doesn't 'read' assign any values to variables h and m > when stdin comes in from a pipe? As already said by others, your read is running in a subshell. Pass the read command to the current shell: read h m <<<`date '+%H %M'` As reference, a quick test with various shell: $ for s in /bin/*sh;do printf "$s \t:";$s -c 'printf "\t$h $m\t:\t";read h m <<<`date "+%H %M"`;echo $h $m';done /bin/ash :Syntax error: redirection unexpected /bin/bash : : 22 27 /bin/csh :Missing name for redirect. /bin/ksh : : 22 27 /bin/rksh : : 22 27 /bin/sh : : 22 27 /bin/tcsh :Missing name for redirect. /bin/zsh : : 22 27 $ |