#!/bin/dash -vx

trim() {
  # remove leading whitespace characters
  __trim="${*#${*%%[![:space:]]*}}"
  # remove trailing whitespace characters
  printf "%s" "${__trim%${__trim##*[![:space:]]}}"
}

trim " alpha" | od -t x1a
+ trim  alpha
+ __trim=
+ printf %s 
+ od -t x1a
0000000
trim "beta " | od -t x1a
+ trim beta 
+ __trim=
+ printf %s 
+ od -t x1a
0000000
trim "
gamma	" | od -t x1a
+ trim 
gamma	
+ __trim=
+ printf %s 
+ od -t x1a
0000000

trim2() {
  # remove leading whitespace characters
  __trim="${*##[[:space:]]}"
  # remove trailing whitespace characters
  printf "%s" "${__trim%%[[:space:]]}"
}

trim2 " alpha" | od -t x1a
+ trim2  alpha
+ __trim= alpha
+ printf %s  alpha
+ od -t x1a
0000000  20  61  6c  70  68  61
         sp   a   l   p   h   a
0000006
trim2 "beta " | od -t x1a
+ trim2 beta 
+ __trim=beta 
+ printf %s beta 
+ od -t x1a
0000000  62  65  74  61  20
          b   e   t   a  sp
0000005
trim2 "
gamma	" | od -t x1a
+ trim2 
gamma	
+ __trim=
gamma	
+ printf %s 
gamma	
+ od -t x1a
0000000  0d  67  61  6d  6d  61  09
         cr   g   a   m   m   a  ht
0000007

trim3() {
  _space="$(echo -n '[\b\t\n\v\f\r ]')"
  # remove leading whitespace characters
  __trim="${*##$_space}"
  # remove trailing whitespace characters
  printf "%s" "${__trim%%$_space}"
}

trim3 " alpha" | od -t x1a
+ trim3  alpha
+ od -t x1a
+ echo -n [\b\t\n\v\f\r ]
+ _space=[	

 ]
+ __trim=alpha
+ printf %s alpha
0000000  61  6c  70  68  61
          a   l   p   h   a
0000005
trim3 "beta " | od -t x1a
+ trim3 beta 
+ od -t x1a
+ echo -n [\b\t\n\v\f\r ]
+ _space=[	

 ]
+ __trim=beta 
+ printf %s beta
0000000  62  65  74  61
          b   e   t   a
0000004
trim3 "
gamma	" | od -t x1a
+ trim3 
gamma	
+ od -t x1a
+ echo -n [\b\t\n\v\f\r ]
+ _space=[	

 ]
+ __trim=gamma	
+ printf %s gamma
0000000  67  61  6d  6d  61
          g   a   m   m   a
0000005