Builtin 'read' data not saved
Hello all, I'm at my wit's end. This script is not working: = #!/bin/bash B=none echo "2" | while read -r A; do B="$A" echo 1.B=$B done echo 2.B=$B == The output, from GNU bash, version 4.2.25(1)-release (x86_64-pc-linux-gnu), bash-4.2-2ubuntu2.1.deb, is this: 1.B=2 2.B=none I'm baffled; it's like the while loop is in a parallel universe. I tried 'export B=none' in line 2, with no effect. Help? -- f...@ch.pkts.ca
Re: Builtin 'read' data not saved
Here's some more oddities: =failing.sh: #!/bin/bash R="1|2" IFS='|' read -r A B <<< $R echo A=$A, B=$B Expected: "A=1, B=2" Actual: "A=1 2, B=" fail2.sh: #!/bin/bash R="1|2" while IFS='|' read -r A B; do echo 1:A=$A, B=$B done <<< $R echo 2:A=$A, B=$B Expected: 1:A=1, B=2 2:A=1, B=2 Actual: 1:A=1, B=2 2:A=, B= GNU bash, version 4.2.25(1)-release (x86_64-pc-linux-gnu) bash-4.2-2ubuntu2.1.deb Cheers! -- f...@ch.pkts.ca Thanks!
Re: Builtin 'read' data not saved
On Thu, 2 Jan 2014 11:35:11 -0800, "P Fudd" wrote: > Here's some more oddities: > > =failing.sh: > #!/bin/bash > R="1|2" > IFS='|' read -r A B <<< $R > echo A=$A, B=$B > > Expected: "A=1, B=2" > Actual: "A=1 2, B=" > > > fail2.sh: > #!/bin/bash > R="1|2" > while IFS='|' read -r A B; do > echo 1:A=$A, B=$B > done <<< $R > echo 2:A=$A, B=$B > > Expected: > 1:A=1, B=2 > 2:A=1, B=2 > Actual: > 1:A=1, B=2 > 2:A=, B= > > > GNU bash, version 4.2.25(1)-release (x86_64-pc-linux-gnu) > bash-4.2-2ubuntu2.1.deb This is a FAQ, see http://mywiki.wooledge.org/BashFAQ/024 for a full discussion. -- D.
Re: Builtin 'read' data not saved
On Thu, 2 Jan 2014 11:35:11 -0800, "P Fudd" wrote: > Here's some more oddities: Ok, the link I sent you is more about the issue you describe in your first message. See below for more on the new ones in this message. > =failing.sh: > #!/bin/bash > R="1|2" > IFS='|' read -r A B <<< $R > echo A=$A, B=$B > > Expected: "A=1, B=2" > Actual: "A=1 2, B=" Here $R is expanded and wordsplitted using IFS before read is executed, which means that $R does *not* contain any pipe symbol at the time "read" is executed, so $A gets the whole thing. > fail2.sh: > #!/bin/bash > R="1|2" > while IFS='|' read -r A B; do > echo 1:A=$A, B=$B > done <<< $R > echo 2:A=$A, B=$B > > Expected: > 1:A=1, B=2 > 2:A=1, B=2 > Actual: > 1:A=1, B=2 > 2:A=, B= This is slightly different, as when $R is expanded, IFS is still the default, so it's expanded to "1|2" as expected, and the alternate IFS is only in effect during the while/read loop. As expected, after the first loop iteration $A is 1 and $B is 2. However read is executed once more, and that second execution reads nothing, thus obviously setting both A and B empty. Run with "set -x" and you'll see it yourself. -- D.
Re: Builtin 'read' data not saved
On 1/2/14, 2:19 PM, P Fudd wrote: > Hello all, > > I'm at my wit's end. This script is not working: This comes up regularly. The last time was less than a month ago: http://lists.gnu.org/archive/html/bug-bash/2013-12/msg00066.html If you have the Bash FAQ, this is covered in question E4. Greg Wooledge's FAQ also considers the topic: http://mywiki.wooledge.org/BashFAQ/024 > I'm baffled; it's like the while loop is in a parallel universe. That is, in effect, what a pipeline is: separate processes run in parallel with chained input and output. Chet -- ``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: Builtin 'read' data not saved
On 1/2/14, 2:35 PM, P Fudd wrote: > Here's some more oddities: > > =failing.sh: > #!/bin/bash > R="1|2" > IFS='|' read -r A B <<< $R > echo A=$A, B=$B > > Expected: "A=1, B=2" > Actual: "A=1 2, B=" This is a bug in bash-4.2. The expansion of $R is not supposed to be subject to the value of IFS. You can read a discussion of the issue here: http://lists.gnu.org/archive/html/bug-bash/2013-01/msg00041.html > fail2.sh: > #!/bin/bash > R="1|2" > while IFS='|' read -r A B; do > echo 1:A=$A, B=$B > done <<< $R > echo 2:A=$A, B=$B A and B will be unset when the second `read' hits EOF and returns failure, breaking the loop. Chet -- ``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/
