Find exec sh: Shell variable not getting passed to subshell

Question

Here is a simplified code that prints the name of Directory if it contains a Filename with same name as the parent directory and .md extension.

FIND(){
    find . -type d -exec sh -c '
        for d do
            [ -f "${d}/${d##*/}.md" ] && printf "%s\n" "$d"
        done' find-sh {} +
}


FIND

To generalize I want to send the Search term ${d}/${d##*/}.md as an argument to the FIND function, but unfortunately this does not outputs anything:

FIND(){
    local SearchTerm="${1}"
    find . -type d -exec sh -c '
        for d do
            [ -f "${SearchTerm}" ] && printf "%s\n" "$d"
        done' find-sh {} +
}

FIND '${d}/${d##*/}.md'

I am sure there is some issue with the quotation of the SearchTerm. Any hints?

---

I tried: FIND '\${d}/\${d##*/}.md' but has no output

Answer 1

"${SearchTerm}" needs to be outside of the sh -c ' to get extrapolated, like follows:

FIND(){
    local SearchTerm="${1}"
    find . -type d -exec sh -c '
        for d do
            [ -f "'"${SearchTerm}"'" ] && printf "%s\n" "$d"
        done' find-sh {} +
}

FIND '${d}/${d##*/}.md'

To explain a little more, we can run a small test :

SearchTerm='${d}/${d##*/}.md'
echo 'Before "$SearchTerm" After'
# output : Before "$SearchTerm" After
echo 'Before "'"$SearchTerm"'" After'
# output : Before "${d}/${d##*/}.md" After

'Before "$SearchTerm" After' is a single string without any parameter expansion because it's all in single quote.

'Before "'"$SearchTerm"'" After' is actually three strings joining together :

'Before "' # at the end, it's a double quote followed by a single quote

"$SearchTerm" # parameter expansion happens here.

'" After' # at the begining, it's a single quote followed by a double quote

Hope it's clearer.

Answer 2

The in-line script that you call is single-quoted (as it should be). This means that the sh -c shell will get a script where "${SearchTerm}" is unexpanded. Since that shell does not have a SearchTerm variable, its value will be empty.

Since you tagged your question with bash, you can pass the name of an exported function:

# Our find function.
# Takes the name of a test function that will be called
# with the pathname of a directory.
myfind () {
    local thetest="$1"

    # Run find, passing the name of the function into the in-line script.
    find . -type d -exec bash -c '
        testfunc=${1:-:}; shift
        for dirpath do
            "$testfunc" "$dirpath" && printf "%s\n" "$dirpath"
        done' bash "$thetest" {} +
}

# Our test function.
test_md_file () {
    [ -f "$1/${1##*/}.md" ]
}
export -f test_md_file

# Run the thing.
myfind test_md_file

The testfunc=${1:-:} in the code will assign $1 to testfunc if it's available and not empty, otherwise, it will use : as the test (a no-op utility that returns true).

Answer 3

Some alternative approaches to be able to run arbitrary shell code instead of just [ -f "$d/${d##*/}.md" ]:

Pass the shell code as the first argument and use eval to interpret it:

FIND(){
    find . -type d -exec sh -c '
        code=$1; shift
        for d do
            eval "$code" && printf "%s\n" "$d"
        done' find-sh "$1" {} +
}
FIND '[ -f "$d/${d##*/}.doc" ]'

Same with an environment variable

FIND(){
    CODE=$1 find . -type d -exec sh -c '
        for d do
            eval "$CODE" && printf "%s\n" "$d"
        done' find-sh "$1" {} +
}
FIND 'base=${d##*/}; [ -f "$d/$base.md" ] && [ -f "$d/$base.doc" ]'

Of if the shell code to interpret is always going to be [ -f "something" ]:

FIND(){
    FILE_TEST=$1 find . -type d -exec sh -c '
        for d do
            eval "[ -f \"$FILE_TEST\" ]" && printf "%s\n" "$d"
        done' find-sh "$1" {} +
}
FIND '$d/${d##*/}.doc'

Note that if using zsh, you can just use its e glob qualifier to further qualify the directories (/) with an arbitrary shell expression:

print -rC1 ./**/*(ND/e['[ -f $REPLY/$REPLY:t.md ]'])

Or with a function:

has_md() [ -f $REPLY/$REPLY:t.md ]
print -rC1 ./**/*(ND/+has_md)

(N for nullglob, D for dotglob to not ignore hidden files and dirs)