Longest common prefix of two strings in bash

If you have an option to install a python package, you can use this python utility

# install pythonp
pythonp -m pip install pythonp

echo -e "$string1\n$string2" | pythonp 'l1,l2=lines
res=itertools.takewhile(lambda a: a[0]==a[1], zip(l1,l2)); "".join(r[0] for r in res)'

If using other languages, how about python:

cmnstr() { python -c "from difflib import SequenceMatcher
s1, s2 = ('''$1''', '''$2''')
m = SequenceMatcher(None,s1,s2).find_longest_match(0,len(s1),0,len(s2))
if m.a == 0: print(s1[m.a: m.a+m.size])"
}
$ cmnstr x y
$ cmnstr asdfas asd
asd

(h/t to @RickardSjogren's answer to stack overflow 18715688)

Another python-based answer, this one based on the os.path module's native commonprefix function

#!/bin/bash
cat mystream | python -c $'import sys, os; sys.stdout.write(os.path.commonprefix(sys.stdin.readlines()) + b\'\\n\')'

Longform, that's

import sys
import os
sys.stdout.write(
    os.path.commonprefix(sys.stdin.readlines()) + b'\n'
)

/!\ Note: the entire text of the stream will be loaded into memory as python string objects before being crunched with this method

If not buffering the entire stream in memory is a requirement, we can use the communicative property and to the prefix commonality check between every input pair

$!/bin/bash
cat mystream | python -c $'import sys\nimport os\nfor line in sys.stdin:\n\tif not os.path.isfile(line.strip()):\n\t\tcontinue\n\tsys.stdout.write(line)\n') | pythoin sys.stdin:\n\tprefix=os.path.commonprefix([line] + ([prefix] if prefix else []))\nsys.stdout.write(prefix)''

Long form

import sys
import os
prefix = None
for line in sys.stdin:
    prefix=os.path.commonprefix(
        [line] + ([prefix] if prev else [])
    )
sys.stdout.write(prefix)

Both of these methods should be binary-safe, as in they don't need input/output data to be ascii or utf-8 encoded, if you run into encoding errors, python 3 renamed sys.stdin to sys.stdin.buffer and sys.stdout to sys.stdout.buffer, which will not automatically decode/encode input/output streams on use

I've generalized @ack's answer to accommodate embedded newlines.

I'll use the following array of strings as a test case:

a=(
  $'/a\n/b/\nc  d\n/\n\ne/f'
  $'/a\n/b/\nc  d\n/\ne/f'
  $'/a\n/b/\nc  d\n/\ne\n/f'
  $'/a\n/b/\nc  d\n/\nef'
)

By inspection we can see that the longest common prefix is

$'/a\n/b/\nc  d\n/\n'

We can compute this and save the result into a variable with the following:

longest_common_prefix=$(
  printf '%s\0' "${a[@]}" \
  | sed -zE '$!{N;s/^(.*).*\x00\1.*$/\1\x00\1/;D;}' \
  | tr \\0 x # replace trailing NUL with a dummy character ①
)
longest_common_prefix=${longest_common_prefix%x} # Remove the dummy character
echo "${longest_common_prefix@Q}" # ②

Result:

$'/a\n/b/\nc  d\n/\n'

as expected. ✔️

I applied this technique in the context of path specs here: https://unix.stackexchange.com/a/639813

^{① To preserve any trailing newlines in this command substitution, we used the usual technique of appending a dummy character that is chopped off afterwards. We combined the removal of the trailing NUL with the addition of the dummy character (we chose x) in one step using tr \\0 x.}

^{② The ${parameter@Q} expansion results in "a string that is the value of parameter quoted in a format that can be reused as input." – bash reference manual. Requires bash 4.4+ (discussion). Otherwise, you can inspect the result using one of the following:}