How to make bash glob a string variable?

You can force another round of evaluation with eval, but that's not actually necessary. (And eval starts having serious problems the moment your file names contain special characters like $.) The problem isn't with globbing, but with the tilde expansion.

Globbing happens after variable expansion, if the variable is unquoted, as here^(*):

$ x="/tm*" ; echo $x
/tmp

So, in the same vein, this is similar to what you did, and works:

$ mkdir -p ~/public/foo/ ; touch ~/public/foo/x.launch
$ i="$HOME/public/*"; j="*.launch"; k="$i/$j"
$ echo $k
/home/foo/public/foo/x.launch

But with the tilde it doesn't:

$ i="~/public/*"; j="*.launch"; k="$i/$j"
$ echo $k
~/public/*/*.launch

This is clearly documented for Bash:

The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, ...

Tilde expansion happens before variable expansion so tildes inside variables are not expanded. The easy workaround is to use $HOME or the full path instead.

_{(* expanding globs from variables is usually not what you want)}

---

Another thing:

When you loop over the patterns, as here:

exclude="foo *bar"
for j in $exclude ; do
    ...

note that as $exclude is unquoted, it's both split, and also globbed at this point. So if the current directory contains something matching the pattern, it's expanded to that:

$ i="$HOME/public/foo"
$ exclude="*.launch"
$ touch $i/real.launch
$ for j in $exclude ; do           # split and glob, no match
    echo "$i"/$j ; done
/home/foo/public/foo/real.launch

$ touch ./hello.launch
$ for j in $exclude ; do           # split and glob, matches in current dir!
    echo "$i"/$j ; done
/home/foo/public/foo/hello.launch  # not the expected result

To work around this, use an array variable instead of a splitted string:

$ exclude=("*.launch")
$ exclude+=("something else")
$ for j in "${exclude[@]}" ; do echo "$i"/$j ; done
/home/foo/public/foo/real.launch
/home/foo/public/foo/something else

As an added bonus, array entries can also contain whitespace without issues with splitting.

---

Something similar could be done with find -path, if you don't mind what directory level the targeted files should be. E.g. to find any path ending in /e2e/*.js:

$ dirs="$HOME/public $HOME/private"
$ pattern="*/e2e/*.js"
$ find $dirs -path "$pattern"
/home/foo/public/one/two/three/e2e/asdf.js

We have to use $HOME instead of ~ for the same reason as before, and $dirs needs to be unquoted on the find command line so it gets split, but $pattern should be quoted so it isn't accidentally expanded by the shell.

(I think you could play with -maxdepth on GNU find to limit how deep the search goes, if you care, but that's a bit of a different issue.)

You can save it as an array instead of a string to use later in many cases and let the globbing happen when you define it. In your case, for example:

k=(~/code/public/*/*.launch)
for i in "${k[@]}"; do

or in the later example, you'll need to eval some of the strings

dirs=(~/code/private/* ~/code/public/*)
for i in "${dirs[@]}"; do
    for j in $exclude; do
        eval "for k in $i/$j; do tmutil addexclusion \"\$k\"; done"
    done
done

@ilkkachu answer solved the main globbing issue. Full credit to him.

V1

However, due to exclude containing entries both with and without wildcard(*), and also they may not exist in all, extra checking is needed after the globbing of $i/$j. I am sharing my findings here.

#!/bin/bash
exclude="
*.launch
.DS_Store
.classpath
.sass-cache
.settings
Thumbs.db
bower_components
build
connect.lock
coverage
dist
e2e/*.js
e2e/*.map
libpeerconnection.log
node_modules
npm-debug.log
testem.log
tmp
typings
"

dirs="
$HOME/code/private/*
$HOME/code/public/*
"

# loop $dirs
for i in $dirs; do
    for j in $exclude ; do
        for k in $i/$j; do
            echo -e "$k"
            if [ -f $k ] || [ -d $k ] ; then
                # Only execute command if dir/file exist
                echo -e "\t^^^ Above file/dir exist! ^^^"
            fi
        done
    done
done

Output Explaination

Following is the partial output to explain the situation.

/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/a.launch
    ^^^ Above file/dir exist! ^^^
/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/b.launch
    ^^^ Above file/dir exist! ^^^
/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/.DS_Store
    ^^^ Above file/dir exist! ^^^

The above are self explanatory.

/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/.classpath
/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/.sass-cache
/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/.settings
/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/Thumbs.db
/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/bower_components
/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/build
/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/connect.lock
/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/coverage
/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/dist

The above show up because the exclude entry($j) has no wildcard, $i/$j become a plain string concatenation. However the file/dir does not exist.

/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/e2e/*.js
/Volumes/HD2/JS/code/public/simple-api-example-ng2-express/e2e/*.map

The above show up as exclude entry($j) contain wildcard but has no file/directory match, the globbing of $i/$j just return the original string.

V2

V2 use single quote, eval and shopt -s nullglob to get clean result. No file/dir final checking require.

#!/bin/bash
exclude='
*.launch
.sass-cache
Thumbs.db
bower_components
build
connect.lock
coverage
dist
e2e/*.js
e2e/*.map
libpeerconnection.log
node_modules
npm-debug.log
testem.log
tmp
typings
'

dirs='
$HOME/code/private/*
$HOME/code/public/*
'

for i in $dirs; do
    for j in $exclude ; do
        shopt -s nullglob
        eval "k=$i/$j"
        for l in $k; do
            echo $l
        done
        shopt -u nullglob
    done
done