Howto split a string on a multi-character delimiter in bash? - bash

Question

Why doesn't work the following bash code?
for i in $( echo "emmbbmmaaddsb" | split -t "mm" )
do
echo "$i"
done
expected output:
e
bb
aaddsb

Answer 1

The recommended tool for character subtitution is sed's command s/regexp/replacement/ for one regexp occurence or global s/regexp/replacement/g, you do not even need a loop or variables.
Pipe your echo output and try to substitute the characters mm witht the newline character \n:
echo "emmbbmmaaddsb" | sed 's/mm/\n/g'
The output is:
e
bb
aaddsb

Answer 2

Since you're expecting newlines, you can simply replace all instances of mm in your string with a newline. In pure native bash:
in='emmbbmmaaddsb'
sep='mm'
printf '%s\n' "${in//$sep/$'\n'}"
If you wanted to do such a replacement on a longer input stream, you might be better off using awk, as bash's built-in string manipulation doesn't scale well to more than a few kilobytes of content. The gsub_literal shell function (backending into awk) given in BashFAQ #21 is applicable:
# Taken from http://mywiki.wooledge.org/BashFAQ/021
# usage: gsub_literal STR REP
# replaces all instances of STR with REP. reads from stdin and writes to stdout.
gsub_literal() {
# STR cannot be empty
[[ $1 ]] || return
# string manip needed to escape '\'s, so awk doesn't expand '\n' and such
awk -v str="${1//\\/\\\\}" -v rep="${2//\\/\\\\}" '
# get the length of the search string
BEGIN {
len = length(str);
}
{
# empty the output string
out = "";
# continue looping while the search string is in the line
while (i = index($0, str)) {
# append everything up to the search string, and the replacement string
out = out substr($0, 1, i-1) rep;
# remove everything up to and including the first instance of the
# search string from the line
$0 = substr($0, i + len);
}
# append whatever is left
out = out $0;
print out;
}
'
}
...used, in this context, as:
gsub_literal "mm" $'\n' <your-input-file.txt >your-output-file.txt

Answer 3

A more general example, without replacing the multi-character delimiter with a single character delimiter is given below :
Using parameter expansions : (from the comment of #gniourf_gniourf)
#!/bin/bash
str="LearnABCtoABCSplitABCaABCString"
delimiter=ABC
s=$str$delimiter
array=();
while [[ $s ]]; do
array+=( "${s%%"$delimiter"*}" );
s=${s#*"$delimiter"};
done;
declare -p array
A more crude kind of way
#!/bin/bash
# main string
str="LearnABCtoABCSplitABCaABCString"
# delimiter string
delimiter="ABC"
#length of main string
strLen=${#str}
#length of delimiter string
dLen=${#delimiter}
#iterator for length of string
i=0
#length tracker for ongoing substring
wordLen=0
#starting position for ongoing substring
strP=0
array=()
while [ $i -lt $strLen ]; do
if [ $delimiter == ${str:$i:$dLen} ]; then
array+=(${str:strP:$wordLen})
strP=$(( i + dLen ))
wordLen=0
i=$(( i + dLen ))
fi
i=$(( i + 1 ))
wordLen=$(( wordLen + 1 ))
done
array+=(${str:strP:$wordLen})
declare -p array
Reference - Bash Tutorial - Bash Split String

Answer 4

With awk you can use the gsub to replace all regex matches.
As in your question, to replace all substrings of two or more 'm' chars with a new line, run:
echo "emmbbmmaaddsb" | awk '{ gsub(/mm+/, "\n" ); print; }'
e
bb
aaddsb
The ‘g’ in gsub() stands for “global,” which means replace everywhere.
You may also ask to print just N match, for example:
echo "emmbbmmaaddsb" | awk '{ gsub(/mm+/, " " ); print $2; }'
bb