echo cmd collapsing spaces in value

chrism01 · 05-31-2013, 06:27 AM

Observe

Code:

s='x   asd                      x'

printf "$s\n" $s
x   asd                      x

printf "%s\n" $s
x
asd
x

echo $s
x asd x

echo "$s"
x   asd                      x

Can anyone explain why 1st echo is collapsing the spaces?
Also, why 2nd printf inserts newlines?

GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)

Guttorm · 05-31-2013, 07:18 AM

Hi

The first echo collapses the spaces because the shell executes one echo command with many arguments/parameters. The shell parses it as command, and it can be followed by many arguments/paramters and they're separated by one or more arguments. So the echo command gets 3 parameters and knows nothing about the whitespace. For example:

Code:

ls  /home                  /bin     /etc

The number of whitespace characters here doesn't matter. The ls command gets 3 parameters.

The other is the behaviour of printf when you give it more arguments than you specify in the format string. Then it outputs the rest of the arguments with the format string. Same thing as this example:

Code:

printf "The argument is %s.\n" first second third

chrism01 · 05-31-2013, 08:30 AM

Thinking about it, that makes sense; don't know why I didn't remember that before.
Guess I rarely have used it that way with 'data' that has important spacing ...
Thx

grail · 05-31-2013, 09:25 AM

Actually the thing to remember is that unquoted variables undergo word splitting. I feel the ls example is actually a little ambiguous because if you placed quotes before /home and after /etc you would get an error about no such file or directory.

If we look at the examples in order:

Code:

s='x   asd                      x'

# here printf is actually printing the expanded string within the quotes and as there is no format criteria (like %s) the $s outside the quotes is actually being ignored
printf "$s\n" $s
x   asd                      x

# here we have a format identifier, so $s undergoes word splitting due to no quotes hence printf has now been gived 3 strings to print
printf "%s\n" $s
# So this is expanded to: printf "%s\n" x asd x
x
asd
x

# here we also undergo word splitting and so as Guttorm explained, echo is now receiving three arguments similar to printf above
echo $s
# expands to: echo x asd x
x asd x

# lastly, the variable is quoted, hence white space is preserved and echo is passed the string in its original form
echo "$s"
x   asd                      x

chrism01 · 06-03-2013, 03:59 AM

Yeah exactly, I was just having a brain-fade.

Was working on other stuff and using echo (later printf) to just check my other code and was just taking them for granted.
If I'd stopped to think clearly I'd have got that.
It was an 'of course!' moment when I saw Guttorm's answer and your later answer also occurred to me at that time.