How can I sort sections of file alphabetically?

Question

I have a file of several sections, each section start with specific title but all of them ending with the same string, I want to sort the file sections according to the titles without sorting the content of each section (i.e. take the whole section as one block) there is also a blank line between each two section, to clarify the idea if the input is as

string5
z
y
x
string

string2
x
z
y
f
string

the desired output would be as

string2
x
z
y
f
string

string5
z
y
x
string

Answer 1

Using GNU sed and sort:

sed 's/^$/\x0/g' file | sort -z | tr '\0' '\n'

• Put null character in empty line
• sort using null character as delimiter ( -z)
• finally replace null delimiter with new line using tr.
• to remove empty lines in first and last line of the output, you may add | sed '1{/^$/d};${/^$/d}'

Output:

string2
x
z
y
f
string

string5
z
y
x
string

(maybe someone can help making \x0 work for non-GNU sed, related Question)

Answer 2

Using GNU awk in paragraph mode and sort the array's values in string mode then print:

awk -v RS= '{ seen[NR]=$0 }
END { PROCINFO["sorted_in"]="@val_str_asc";
      for (block in seen) {print sep seen[block]; sep=ORS}
}' infile

Answer 3

With perl:

perl -l -00 -e '
  chomp(@paragraphs = <>);
  print join "\n\n", sort @paragraphs' your-file