how to delete letters from a string variable in bash?

Try

var=${var:${#var2}}

${var:${#var2}} expands to the characters in $var from index ${#var2} onwards. See Extracting parts of strings (BashFAQ/100 (How do I do string manipulation in bash?)).

Here's a fun one:

echo "${var/${var2//?/?}/}"

• ${var2//?/?} replaces every character in $var2 with the character "?"
• Then we use that string ("?????") as a pattern against $var, and we replace it with an empty string

You can remove one space at a time while the $var is longer:

#!/bin/bash
var="         |"
var2=hello
expect="    |"

while (( ${#var} > ${#var2} )) ; do
    var=${var#\ }
done

[[ $var = $expect ]] && echo Ok

or you can calculate the number of characters to remove and use the offset parameter expansion to skip them:

remove_length=$(( ${#var} - ${#var2} ))
var=${var:$remove_length}

Use parameter expansion:

#!/bin/bash

s1='      |'  # 6 spaces
s2='hello'    # 5 characters

(( to_remove=${#s2}+1 ))

echo "${s1:$to_remove}"  # 5 characters removed
# ' |'