How to convert a String to all spaces in Bash?

Question

For formatting purposes, I want to replace all characters of a String with spaces.

STR="some string of unknown length"
SPC="                             " <--- this is what I want

With help of this page, the best I could come up with is this:

SPC=${STR//[a-Z]/ }

But this only replaces letters, not digits or any special characters. What's the placeholder for "any character" with this type of Bash string replacements?

Answer 1

You can use:

str="some string of unknown length"
spc="${str//[^[:space:]]/ }"

[^[:space:]] will match any non-space character and replace it by space.

Verify:

echo "$str" | wc -c
30
echo "$spc" | wc -c
30

PS: You can also do:

spc="${str//?/ }"

to get the string of same length containing only spaces. ? matches any character in glob.

Answer 2

This is an alternative (no regex/parsing at all) solution:

STR='some string of unknown length.' # Actually 30 chars
# Create an SCP var of the same length of STR filled with spaces (# updated implementation, thanks gniourf_gniourf)
printf -v SCP '%*s' ${#STR}

Test:

echo "${STR}"                 # 'some string of unknown length.'
echo "${SCP}"                 # '                              '
echo "Length STR -> ${#STR}"  # 'Length STR -> 30'
echo "Length SCP -> ${#SCP}"  # 'Length SCP -> 30'

How it works:

The printf command writes in variable SCP a string of at least the length equal to the number of chars of STR (${#STR}).

The tricks is in the s not replaced cause there is no further parameter after ${#STR}.

Usage example:

printf '%*s' 3       # '   '
printf '%*s' 3 .     # '  .'
printf '%*s' 3 123   # '123'
printf '%*s' 3 12345 # '12345'