Cameron Simpson wrote:
> On Fri, Jan 05, 2001 at 07:03:12PM -0800, Brad Doster <[EMAIL PROTECTED]> wrote:
> | Thanks, Cameron. It's a cool idea, but I can't get the leading and trailing
> | spaces to stick within a variable. The following script and it's output
> | will show you what I mean:
> |
> | list="one two three four"
> | list=`echo $list | sed "s/^/ /"`
> | echo $list
> | list=`echo $list | sed "s/$/ /"`
> | echo $list
> [...]
> | Is there a way to get leading and trailing spaces (and multiple spaces
> | between words) to stick in variables, or this a "feature" of bash?
>
> Actually, the spaces would stick but you're throwing them away!
>
> Come, let us read from "man sh" (which is bash on most/all Linux systems):
>
> Command Substitution
> Command substitution allows the output of a command to
> replace the command name. There are two forms:
>
> $(command)
> or
> `command`
>
> Bash performs the expansion by executing command and
> replacing the command substitution with the standard out
> put of the command, with any trailing newlines deleted.
>
> (Other Bourne shells act the same, though not all support the $(...) variation.)
> So, only trailing newlines are stripped by the ``.
> However, your echo statement is doing space interpretation because the $list
> is unquoted. Observe the different behaviours here:
>
> bash$ foo=' a b c '
> bash$ echo $foo
> a b c
> bash$ echo "[$foo]"
> [ a b c ]
>
> (Those [] are just markers, not syntactically special - they're there to show
> the boundaries of what's in $foo.)
>
> $foo is "a b c ". So the first echo becomes the command
>
> echo a b c
>
> Note the two spaces after "echo" and one at the end of the line. Then the shell
> breaks that into words, making the command words:
>
> "echo" "a" "b" "c"
>
> (Those quotes are just to show the words - they're not `shell' quotes).
> Which prints
>
> a b c
>
> without leading or trailing spaces. The second echo statement has quoted the
> argument, so the [$foo] is not broken up on whitespace. The command words
> are
>
> "echo" "[ a b c ]"
>
> this time i.e. 2 strings, not four.
>
> Watch this:
>
> bash$ echo $foo ; echo "$foo"
> a b c
> a b c
>
> Perhaps that makes it clearer. So, you want to say:
>
> list=`echo "$list" | sed "s/ four / /"`
>
> and not:
>
> list=`echo $list | sed "s/ four / /"`
>
> Also, you want to use '' instead of "" in the sed argument. Otherwise your $
> in "s/ $//" can be misinterpreted. It is best to use '' unless you really
> need parameter substitution to occur. Safer.
>
> Also, you can do several things with one sed, which also faster:
>
> list=`echo "$list" | sed 's/^/ /
> s/$/ /
> s/ one / /
> s/ four //
> s/^ //
> s/ $//'`
>
> Concise and readable. It also neatly sidesteps a lot of the quoting
> issues because you do all the adding and then removing of spaces inside
> sed and never have to pass the intermediate versions of the line back
> out to $list.
>
> For complex things I sometimes put the sed script in a file (sed is a
> programming language with loops, blocks, tests and conditional
> branches), thus:
>
> bash$ ed sedf
> sedf: No such file or directory
> a
> s/^/ /
> s/$/ /
> s/ one / /
> s/ four //
> s/^ //
> s/ $//
> .
> w
> 156
> q
>
> and then go like this:
>
> list=`echo "$list" | sed -f sedf`
>
> Does this make stuff clearer?
> --
> Cameron Simpson, DoD#743 [EMAIL PROTECTED] http://www.zip.com.au/~cs/
>
Outstanding explaination, Cameron. Thanks
Bret
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