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/
Errors have occurred.
We won't tell you where or why.
Lazy programmers.
- Haiku Error Messages http://www.salonmagazine.com/21st/chal/1998/02/10chal2.html
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