$ echo "I am the header" > header.txt
$ echo "I am the tail" > tail.txt
$ cat header.txt
I am the header
$ cat tail.txt 
I am the tail
$ for f in {1..10}; do echo "file name=$f.txt">$f.txt; done
$ cat {1..10}.txt
file name=1.txt
file name=2.txt
file name=3.txt
file name=4.txt
file name=5.txt
file name=6.txt
file name=7.txt
file name=8.txt
file name=9.txt
file name=10.txt

$ for f in {1..10}.txt; do cat header.txt "$f" tail.txt > "${f%.txt}.html"; done
$ cat {1..10}.html
I am the header
file name=1.txt
I am the tail
I am the header
file name=2.txt
I am the tail
...
I am the header
file name=10.txt
I am the tail
$

$ mkdir test
$ cd test
$ touch "one two three.txt" '3 two 1.txt' "1 2 3.txt" '9 10 11.txt'

$ for i in "*two*.txt"; do ls -l $i; done
-rw-r--r--  1 andrew  andrew  0 Apr 29 18:32 3 two 1.txt
-rw-r--r--  1 andrew  andrew  0 Apr 29 18:29 one two three.txt
# notice that the * glob expands inside the "*two*.txt"
# you can also use:  for i in *two*.txt; do ls -l "$i"; done

# or

$ string="one 2 three 4 five 6 seven ocho"
$ for i in $string; do echo $i; done
one
2
three
4
five
6
seven
ocho
# "$string" is broken on spaces (IFS) and the loop is executed 8 times
$ for i in "$string"; do echo $i; done
one 2 three 4 five 6 seven ocho
# $string is quoted so no break on IFS on the value of $string
for i in '$string'; do echo $i; done
$string
# single quotes, no variable expansion....

$ for i in [[:digit:]]*.txt; do ls -l $i; done
ls: 1: No such file or directory
ls: 2: No such file or directory
ls: 3.txt: No such file or directory
ls: 1.txt: No such file or directory
ls: 3: No such file or directory
ls: two: No such file or directory
ls: 10: No such file or directory
ls: 11.txt: No such file or directory
ls: 9: No such file or directory
# the unquoted regex with POSIX [:digit:] character class found the files but then the shell breaks the return on IFS...
$ for i in "[[:digit:]]*"; do ls -l $i; done
-rw-r--r--  1 andrew  andrew  0 Apr 29 18:59 1 2 3.txt
-rw-r--r--  1 andrew  andrew  0 Apr 29 18:59 3 two 1.txt
-rw-r--r--  1 andrew  andrew  0 Apr 29 18:59 9 10 11.txt
#alternatively:
$ for i in [[:digit:]]*; do ls -l "$i"; done
-rw-r--r--  1 andrew  andrew  0 Apr 29 18:59 1 2 3.txt
-rw-r--r--  1 andrew  andrew  0 Apr 29 18:59 3 two 1.txt
-rw-r--r--  1 andrew  andrew  0 Apr 29 18:59 9 10 11.txt
# BUT! quoting both does not work:
$ for i in "[[:digit:]]*"; do ls -l "$i"; done
ls: [[:digit:]]*: No such file or directory