Reverse lines

reverse.sh

#!/bin/sh

reverse_all() {
  input_file="$1"
  # shellcheck disable=SC2016
  sed '2,$G;h;$!d' "$input_file"
}

reverse_inner_group_by_prefix() {
  input_file="$1"
  prefix_sep="$2"

  # reverse lines in each group while preserving group order
  awk -F"$prefix_sep" '
  {
      # Use the prefix (up to the comma) as the key
      prefix = $1
      # Remove leading and trailing whitespace from the prefix
      prefix = gensub(/^[ \t]+|[ \t]+$/, "", "g", prefix)
      # Append the line to the corresponding prefix
      lines[prefix] = lines[prefix] "\n" $0
      # Track the first occurrence of each prefix
      if (!(prefix in seen)) {
          order[++count] = prefix
          seen[prefix] = 1
      }
  }
  END {
      # Loop over the prefixes in the order they were first seen
      for (i = 1; i <= count; i++) {
          key = order[i]
          # Print lines in reverse order for each group
          split(lines[key], arr, "\n")
          for (j = length(arr); j > 0; j--) {
              if (arr[j] != "") print arr[j]
          }
      }
  }' "$input_file"
}

$ cat file
g1 i1
g1 i2
g1 i3
g2 i1
g2 i2
g2 i3
$ reverse_all file
g2 i3
g2 i2
g2 i1
g1 i3
g1 i2
g1 i1
$ reverse_inner_group_by_prefix file ' '
g1 i3
g1 i2
g1 i1
g2 i3
g2 i2
g2 i1