如何在getopts上使用$ {OPTARG}？

Question

I have the following code:

while getopts ":p:t:n:" o; do
case "${o}" in
    p)
        p=${OPTARG}
        numep=$p
        mkdir $numep
        ;;
    t)
        t=${OPTARG}
        tip=$t
        if [ $tip == "c" ]; then
            touch $numep/$numep.c
            touch $numep/$numep.h
        elif [ $tip == "c++" ]; then
            touch $numep/$numep.cpp
            touch $numep/$numep.h
        fi
            ;;
        n)
            n=${OPTARG}
            nrFis=$n
            if [ $tip == "c" ]; then
                for i in $(seq $n); do
                    touch $numep/src/$numep$i.c
                    touch $numep/inc/$numep$i.h
                done
            elif [ $tip == "c++" ]; then
                for i in $(seq $n); do
                    touch $numep/src/$numep$i.cpp
                    touch $numep/inc/$numep$i.h
                done
            fi
            ;;
        *)
            err-help
            ;;
        esac
    done

err-help is a previously defined function that delivers instructions for a correct call.

A correct call of the script looks like this: ./script.sh -p project_name -t project_type -n source_files_number

What am I trying to do: create a directory with a specific number of source files for ac or a c++ project.

My questions here: How do I have to use the ${OPTARG} ? What exactly does the p=${OPTARG} thing? Will the p, t, n variables fill with the right content? How am I doing this right, please? :(

Answer 1

You are using getopts correctly, but you are making assumptions about what order you're going to see the options. More on this later.

Let's look at a couple of excerpts from help getopts

OPTSTRING contains the option letters to be recognized; if a letter is followed by a colon, the option is expected to have an argument , which should be separated from it by white space.

... When an option requires an argument, getopts places that argument into the shell variable OPTARG .

This is the magic of getopts. Since you added p: into your optstring, when $0 = "p", the variable $OPTARG will contain whatever you provided to -p , in this case the string "project_name".

Now for some code review:

case "${o}" in
    p)
        p=${OPTARG}
        numep=$p
        mkdir $numep
        ;;

You only need to use braces if you are using arrays, or you need so disambiguate the variable name from the surrounding text ( $o_x vs ${o}_x ) It would be tidier to write

case "$o" in ...

You don't need to create a special variable with the same name as the option. You never use $p . You can just write

numep="$OPTARG"

Always quote your variables. Always. Unless you specifically know when to leave them unquoted. If I provide -p "some dir with spaces" , then mkdir $numep will create 4 directories. You need to write

mkdir "$numep"

If the $numep directory already exists, you'll get an error message. If you don't want that, use mkdir -p "$numep" (see man mkdir )

Now, do address my first concern: there's nothing wrong with invoking your script and providing the options in any order

./script.sh -t project_type -n source_files_number -p project_name

Your option parsing assumes, in the t and n branches, that you've already set $numep, so you're assuming that the user provided -p first . If -t comes first, then $numep is empty, and you will attempt to create the file /.h . Even if you have permission to write to the root directory, this is not what you want to do. Parse all your options before you start doing stuff.

I would write this:

while getopts ":p:t:n:" o; do
    case "$o" in
        p)  numep="$OPTARG" ;;
        t)  case "$OPTARG" in
                c)  ext="c" ;;
                "c++")  ext="cpp" ;;
                *)  echo "Error: use 'c' or 'c++' for -t option" >&2
                    exit 1
                    ;;
            esac
            ;;
        n)  nrFis="$OPTARG" ;;
        *)  err-help ;;
    esac
done

mkdir -p "$numep"
touch "$numep/$numep.$ext"
touch "$numep/$numep.h"
for ((i=1; i<=$nrFis; i++)); do
    touch "$numep/src/$numep$i.$ext"
    touch "$numep/inc/$numep$i.h"
done