Re: [g.l.d.user] Re: A quick Q: how do I command something in large amount

[Arnt Karlsen <arnt@c2i.net>]

On Mon, 19 Sep 2011 11:15:08 +0700, Ivan wrote in message 
<[🔎] 86fwjtp3qr.fsf@gray.siamics.net>:

> The following message is a courtesy copy of an article
> that has been posted to comp.unix.shell as well.
> 
> >>>>> Arnt Karlsen <arnt@c2i.net> writes:
> >>>>> On Fri, 16 Sep 2011 00:17:47 +0700, Ivan wrote:
> 
> 	[Cross-posting to comp.unix.shell for no good reason at all.]
> 
>  >> Whitespace is not a problem as long as one remembers to
>  >> double-quote Shell $ubstitutions, like:
> 
>  >> for i in a b c ; do
> 
>  > ..or, e.g.: for i in $(ls /path/to/files/*.txt ) ; do
> 
> 	No, it isn't whitespace-tolerant, as the result of the
> unescaped $()-substitution is subject to IFS-splitting.  The correct
> form would be:
> 
>     for i in /path/to/files/*.txt ; do something ; done 
> 
> 	Consider, e. g.:
> 
> $ ls -1 
> a
> b c
> $ (for x in $(ls *) ; do echo ."$x". ; done) 
> .a.
> .b.
> .c.
> $ 
> 
> 	Moreover, ls(1) is an extra here; the command above requests
> the Shell to search for all the .txt filenames in /path/to/files/,
> 	then the filenames are passed to ls(1), which is supposed to
> 	pass them back to the shell unaltered.  There, ls(1) may
> 	essentially behave the same as, say, $ echo, or $ printf
> %s\\n.
> 
> 	This is somewhat akin to UUoC, as in:
> 
> $ cat < "$file" | grep something 
> 
> 	vs. simply:
> 
> $ grep something < "$file" 
> 
> 	Or it may not.  Consider that one of the .txt-filenames refers
> 	to a directory, like:
> 
> $ ls -1RF 
> .:
> a
> b c
> d/
> 
> ./d:
> x y
> $ (for x in $(ls *) ; do echo ."$x". ; done) 
> .a.
> .b.
> .c.
> .d:.
> .x.
> .y.
> $ 
> 
> 	Note that the above has ‘x’ set to ‘d:’, ‘x’ and ‘y’, neither
> of whose belong to the current directory.
> 
> 	(NB: the term “path” is also considered obsolete by GNU when
> 	speaking of “whole names”, as opposed to “search paths”, such
> 	as, e. g., "$PATH" or "$CDPATH".  In particular, GNU find(1)
> 	states that -wholename is preferred to -path.)
> 
>  >>   txt2pdf -input "${i}.txt" -out "${i}.pdf"
>  >> done
> 
>  > ..disclaimer: I've only used the alleged obsolete back-tick
>  > way. ;o)
> 
> 	The backticks are harder to both nest and mix with other Shell
> 	substitutions and escapes.  Consider, e. g.:
> 
> $ echo \\\" 
> \"
> $ 
> 
> 	To put the stdout of the command above into a Shell variable,
> we may use either the $()-substitution:
> 
> $ x=$(echo \\\") ; echo "$y" 
> \"
> $ 
> 
> 	or the backticks:
> 
> $ y=`echo \\\\\\"` ; echo "$y" 
> \"
> $ 
> 
> 	Now, can you explain why the backslashes above have to be
> 	doubled in the latter case?
> 

..unless that escapes my mind, I'd rather call them escaped. ;o)