Calling a shell script from a C++ program with parameters

Question

I am trying a call a shell script from a cpp program and passing some variables to the script. The script simply copies a file from one directory to another. I want to pass the name of the file, the source directory and the destination directory to shell script from the cpp program. I get the error omitting directory "/" when I try. Please how can I fix this

C++ Code:

std::string spath="/home/henry/work/gcu/build/lib/hardware_common/";

std::string dpath="/home/henry/work/gcu/dll/";

std::string filename="libhardware_common.a";

std::system("/home/henry/work/gcu/build/binaries/bin/copy.sh spath dpath filename");

Shell Script code:

SPATH=${spath}

DPATH=${dpath}

FILE=${filename}

cp ${SPATH}/${FILE} ${DPATH}/${FILE}

spath and dpath and not substituted in your C++ code. They will be the literals spath and dpath. Maybe you should use a std::ostringstream and insert them into a command to use with std::system. You should probably show more of your shell code, too. I think there are some things wrong with it but more contrext is probably needed. — jww
– jww, Commented Jul 19, 2018 at 12:34
[teach-me] You have some misconceptions about strings and variable names in C and in shell. "dpath" string in C does not magically become "/home/henry/work/gcu/dll/" when you pass it to system(). Nor can shell access command line arguments by their C variable names - look into $1, $2and so on in shell. — user3458
– user3458, Commented Jul 19, 2018 at 12:38

dibery · Accepted Answer · 2018-07-19 13:01:42Z

2

Your C++ code and the shell script are not in the same scope. In other words, variables in C++ will not be visible in your script, and when passed to the script, the variables will be renamed as $1, $2, and so on.

To fix it, you can change your code to the following:

std::string spath = "/home/henry/work/gcu/build/lib/hardware_common/";

std::string dpath = "/home/henry/work/gcu/dll/";

std::string filename = "libhardware_common.a";

std::string shell = "/home/henry/work/gcu/build/binaries/bin/copy.sh"

std::system(shell + " " + spath + " " + dpath + " " + filename);

In this way, the spath will be substituted by the value of it, which is then passed to your script.

In your script, you can use:

cp $1/$2 $3/$2

or if you prefer:

SPATH=$1

DPATH=$2

FILE=$3

cp ${SPATH}/${FILE} ${DPATH}/${FILE}

The script will never know the variable name in the C++ code. When the script is called, the parameters will be replaced by $1, $2...

answered Jul 19, 2018 at 13:01

dibery

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2 Comments

user3458 Over a year ago

Just a caveat: if any of the spath, dpath and filename have any spaces in them, the code in the answer will break. More elaborate approach would be needed then.

Nemo Over a year ago

@Arkadiy: Or asterisk, or dollar sign, or backticks... It is worth noting that this sort of thing is pretty rare in production code, because it is a likely source of security holes. (At the very least, whoever is reviewing your code will need to spend some time proving that it's not.)

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