BASH replace a single letter only when it's alone not in a word

Question

I do not want to replace just any letters with words. I only want to replace letters that are stand alone with exact case sensitive matches.

I have a file that looks like this:

file>-string pile stage one on one/a>b
system > use while one is one stage/sump/f
get sole--> one by one dir/sub/main/c >b
multi string return one by one sub//c static a
ect...
ect...

I want to replace the letter that are not part of word.

a=ASTRING
b=BSTRING
c=CSTRING
f=FSTRING

The desired output would look like:

file>-string pile stage one on one/ASTRING>BSTRING
system > use while one is one stage/sump/FSTRING
get sole--> one by one dir/sub/main/CSTRING >BSTRING
multi string return one by one sub//CSTRING static ASTRING
ect...
ect...

Here is what I have been trying:

sed -i "s|a|ASTRING|g" file.txt

grep [a] file.txt | sed -i "s|a|ASTRING|g"

Repeated for b c f

That's not a bash question, that's an `sed` question, isn't it? There's a special symbol \b meaning "word boundary" in the regex. — Roland Weber, Jun 21 '19 at 20:41
Possible duplicate of [sed whole word search and replace](https://stackoverflow.com/questions/1032023/sed-whole-word-search-and-replace) — Roland Weber, Jun 21 '19 at 20:41

score 2 · Accepted Answer · answered Jun 21 '19 at 20:27

Use \b in the regular expression to match a word boundary.

sed -i 's/\ba\b/ASTRING/g' file.txt

BTW, you can't use -i if you're piping the input to sed, the filename has to be an argument to sed. If it gets its input from standard input, it doesn't know the filename so it can't update the file in place.

score 0 · Answer 2 · answered Jun 21 '19 at 20:37

0

nameref variables to the rescue.

a=ASTRING
b=BSTRING
c=CSTRING
f=FSTRING
for v in a b c f
do declare -n R=$v
   sed -Ei "s/\\b$v\\b/$R/g" file.txt
done

answered Jun 21 '19 at 20:37

Paul Hodges

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Why not just use an associative array? – Dennis Williamson Jun 21 '19 at 21:34
Considered it, but this was abstract enough that I wanted to keep the example simpler. – Paul Hodges Jul 01 '19 at 13:03

BASH replace a single letter only when it's alone not in a word

2 Answers2