why is echo different?
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From: Parmenides on 4 Dec 2009 09:40 On 12ÔÂ4ÈÕ, ÏÂÎç8ʱ09·Ö, Andrew McDermott <a.p.mcderm...(a)NOSPAM-rl.ac.uk> wrote: > Parmenides wrote: > > IFS=: > > echo $IFS # the result is space > > a space, or just a blank line? > > > echo "$IFS" # the result is : > > Is there any difference between $IFS and "$IFS" > > Also > $ aaa="a> b > > c > > d > > e" > > $ echo $aaa > a b c d e > $ echo "$aaa" > a > b > c > d > e > > IFS is the internal field separator. When you quote a string with double > quotes it is a single token. When you don't quote a string it is broken > into tokens using IFS to identify field separators. IFS normally contains > the space, tab and newline characters, so in my example 'echo $aaaa' > becomes 'echo "a" "b" "c" "d" "e"' (ie five arguments) while 'echo "$aaa"' > contains a single argument - a string on five lines. In your example $IFS > is either being interpreted as spaces (if indeed it is printing a space), > or as a string which contains a ':'. > > echo separates its arguments with a space when it prints them. > > Andrew I see, it means that shell interpretes the content of IFS in terms of its content(:), and the result is null. thanks a lot.
From: Seebs on 4 Dec 2009 13:33 On 2009-12-04, Parmenides <mobile.parmenides(a)gmail.com> wrote: > IFS=: > echo $IFS # the result is space > echo "$IFS" # the result is : > Is there any difference between $IFS and "$IFS" Well, without quotes, the expansion of $IFS is subject to field splitting, meaning that any characters in $IFS can be replaced by nulls which implicitly separate fields. So when you echo $IFS without quotes, it expands to a colon, and is then separated out into fields, where a field is anything other than colons, separated by any colons. Which means that it ends up being no arguments at all. So you're doing echo with no argument. -s -- Copyright 2009, all wrongs reversed. Peter Seebach / usenet-nospam(a)seebs.net http://www.seebs.net/log/ <-- lawsuits, religion, and funny pictures http://en.wikipedia.org/wiki/Fair_Game_(Scientology) <-- get educated!
From: Sven Mascheck on 2 Jan 2010 13:09
Geoff Clare wrote: > Stephane Chazelas wrote: > >> For some shells, IFS is a field separator (zsh, pdksh, ash), >> and for some, it's a field terminator (bash, AT&T ksh). >> POSIX [...] > [...] says the characters in IFS are used as field terminators. And adding to Stephane's results: - field separator in earlier posh and earlier ash - field terminator since posh-0.6.15, since early (d)ash-0.4.x, ~NetBSD3.1, and in mksh There are even more variations, probably only important to notice for such experiments: if the variable contains only characters from IFS, then one field collapses in mksh and recent posh, or all fields collapse in Bourne shell and bash-1.05. But such corner cases in shell tend to be a house of cards, anyway. |