pdg@cs.uow.edu.au (Peter Gray) (06/04/91)
I have a question regarding sh and IFS.
From my reading of the man page for sh the following
script should not work.
#!/bin/sh
IFS=":"; export IFS
echo fred
ls fred jim
My understanding is the shell should be looking for
commands "echo fred" and "ls fred jim". But it works fine.
On the ls command fred and jim are treated as 2 arguments.
On the other hand
the shell builtins seem to use the IFS as documented.
EG
echo "fred:jim jack:mary" | (IFS=":"; read a b c; echo "$a\n$b\$c")
produces
fred
jim jack
mary
The SUNOS man page for sh says
After parameter and command substitution, the results of
substitution are scanned for internal field separator char-
acters (those found in IFS) and split into distinct argu-
ments where such characters are found. Explicit null argu-
ments ("" or '') are retained. Implicit null arguments
(those resulting from parameters that have no values) are
removed.
How does sh actually use IFS??? Why doesn't altering IFS alter
the interpretation of subsequent lines in a script?
pdg
Peter Gray Internet: pdg@draci.cs.uow.EDU.AU
Professional Officer UUCP: ...!munnari!draci.cs.uow.EDU.AU!pdg
Dept of Computer Science MHSnet: pdg@draci.cs.uow.oz.au
University of Wollongong Phone: +61 42 213770
N.S.W. 2500 Australia Fax : +61 42 213262 eric@mks.mks.com (Eric Gisin) (06/04/91)
In article <1991Jun4.074041.5300@cs.uow.edu.au> pdg@cs.uow.edu.au (Peter Gray) writes:
I have a question regarding sh and IFS.
From my reading of the man page for sh the following
script should not work.
#!/bin/sh
IFS=":"; export IFS
echo fred
ls fred jim
My understanding is the shell should be looking for
commands "echo fred" and "ls fred jim". But it works fine.
On the ls command fred and jim are treated as 2 arguments.
On the other hand
the shell builtins seem to use the IFS as documented.
---
Right on the first page of sh(1) it says [a command is a list of
words separated by spaces or tabs]. When the script is
read, commands are split into a list of words. This is necessary
so the keywords, assignments, and regular words can be determined.
IFS is used for further splitting when the script is executed.
For example:
cmd="ls fred jim"
IFS=" "
"$cmd" # one word, "" prevents splitting
$cmd # three words, runs ls
IFS=":"
$cmd # one word, does not run lsandy@xwkg.Icom.Com (Andrew H. Marrinson) (06/05/91)
pdg@cs.uow.edu.au (Peter Gray) writes: >I have a question regarding sh and IFS. > [...] >The SUNOS man page for sh says > After parameter and command substitution, the results of > substitution are scanned for internal field separator char- > acters (those found in IFS) and split into distinct argu- > ments where such characters are found. Explicit null argu- > ments ("" or '') are retained. Implicit null arguments > (those resulting from parameters that have no values) are > removed. Yes, so does the System V man page. However, earlier, under Commands it says: ``A simple command is a sequence of non-blank words separated by blanks.'' And, just above that, under Definitions, it says: ``A blank is a tab or a space.'' All of this takes place before parameter and command substitution. The IFS separators are a different mechanism, taking place after parameter and command substitution. >How does sh actually use IFS??? Why doesn't altering IFS alter >the interpretation of subsequent lines in a script? It uses it in addition to blank and tab substitution, and at a different point in processing the command line. In particular, it performs splitting with IFS after parameter substition, which explains why space and tab are in IFS, even though, from the above, it might seem redundant. For example, with the default IFS, consider this sequence of commands: foo='xxx yyy' echo $foo The echo here has two arguments, not one, because the splitting by IFS takes place after parameter substition. If IFS were changed to :, it would have just one argument, but then changing foo to xxx:yyy would cause echo $foo to have two arguments again. This was weird and somewhat surprising to me, but that it is how it is. I didn't understand what was going on until I went back and read the manual entry, but there it is, clear as mud! -- Andrew H. Marrinson Icom Systems, Inc. Wheeling, IL, USA (andy@icom.icom.com)
jfv@cbnewsk.att.com (j.f.van valkenburg) (06/05/91)
I didn't see if you exported the env variable IFS? This would account for the two results, the first one the IFS set but not exported and in the second it was local to the shell script. ------------------------ James F. Van Valkenburg a.k.a. "van" AT&T Attmail: !jfv jfv@cbnewsk.att.com Atlanta, GA. Voice 404-810-7920 =============================================================================== ---- Standard Disclaimers included -- Just another grunt at AT&T ---- ===============================================================================