在C中将变量用于Shell脚本

Question

I have a shell script in C which is defined as

#define SHELLSCRIPT "\
sed 's/./& \
inserted text \
  /20' fileA.txt > fileB.txt \
"

When this shell script is run on terminal, it inserts the text inserted text in fileB.txt at offset 20. Now, I want this 20 fileA.txt and fileB.txt to be fetched from a variable.

How should I do that? I tried the following

#define SHELLSCRIPT "\
    sed 's/./& \
    inserted text \
      /$i' fileA.txt > fileB.txt \
    "

and in C before I run the above shell script, I run system("i=20"); but then I get this error below

sed: 1: "s/./& this comment has ...": bad flag in substitute command: '$'

How can I achieve this?

Answer 1

When you run system() , it starts up a fresh shell each time. So the shell that i=20 runs in is not the same shell that the sed command runs in.

Instead of $i in the script text, put %d there instead. Then you can use it as a format string to sprintf which can format the command into a separate variable.

#define SHELLSCRIPT "\
    sed 's/./& \
    inserted text \
      /%d' fileA.txt > fileB.txt \
    "

char command[500];
sprintf(command, SHELLSCRIPT, 20);
system(command);

Answer 2

how about you replace your script command

#define SHELLSCRIPT "\
    sed 's/./& \
    inserted text \
      /%d' %s > %s \
    "

and then you replace by your variables, before executing the command:

char cmd[100 +1];
sprintf(cmd, SHELLSCRIPT , 20, "file1", "file2");
system(cmd)