Automating diskpart commands in windows requires pointing to a .txt file that contains the sequence of commands you want to execute. I am writing a C program that needs to use this automated feature. I want to have it run completely independent of the working directory. How would I do this?
Either use a fully qualified name for the file, or, with a bit more work, pass the commands from the C program to diskpart through an anonymous pipe using popen.
Example added:
#include <stdio.h>
int main ( int argc, char *argv[] )
{
char buffer[MAXBUF];
FILE *fp = popen("gzip -dc data.gz","r");
while (fgets(buffer,MAXBUF,fp)) {
/* Process line of data, here just print it out… */
fputs(buffer,stdout);
}
printf ("Command exit status %d\n", pclose(fp));
}
Related
I'm trying to read command line arguments that have been redirected from a file. The command I'm using is ./a.out < test.txt
And the contents of test.txt is: Hello world.
But the output of my program below isn't printing Hello
world. Instead it is only showing ./a.out. Why is this?
int main(int argc, char* argv[], char* envp[]) {
for (int i = 1; i < argc; i++) {
printf("%s\n", argv[i]);
}
}
The shell intercepts the redirection commands before preparing the command line for the program:
myProg <infile -t >outfile
will pass to the program
myProg -t
with stdin and stdout already rerouted before the pogram starts. So the program never sees the rediretion.
There a lot of cases, besides simple derirection:
dir > myfile.txt
Especially you can pipe output from one program to another:
dir | more
It will send output if dir command to more command. Since program launch handled by OS shell, it handles a redirection too.
Because the language is defined that way. Suppose what you say is true —
All the user input will have to come from command line arguments, but text redirected from a file can satisfy input required in different functions. This can be achieved if the input appears as command line arguments.
Consider this program:
#include <stdio.h>
int is_dict(char *word)
{
/* code to look up a dictionary */
int result = 1;
return result;
}
int main(int argc, char *argv[])
{
if(argc == 2 && is_dict(argv[1]))
printf("%s found", argv[1]);
return 0;
}
If the program is written that way to accommodate it, then the input would have to come from the command line arguments. How would you take input when it is not redirected? It would require more program overhead to detect the missing inputs.
Moreover, imagine a text file containing a million words: it is unfeasible to expect each word to arrive as an argv[n].
There are other objections too. Suppose the program prints a series of prompts for responses. The user would have to know in advance what the prompts are, to supply the answers before the prompts appear.
Lastly, if the program is run from a GUI, then all the program's input will have to be edited into its properties before it is run.
I am using MS visual studio 2008, for C coding.
I know we can use
"int system(const char *command)" to execute commands.
Is there any other method to execute system commands in C program.
Also I need to store output of executed command in a variable.
system() function execute command and send output to stdout , is there any way to read from stdout and store in variable.
So my ultimate goal is to execute system command in C program for windows (using visual studio) and store output of that command in a variable.
Any suggestions ?
Standard C libraries give you only one way to execute external command in OS, so use int system(const char *command).
You can save output of this command to text file, and then read this file from you program.
For example:
#include <stdio.h>
#include <stdlib.h>
#define TMP_FILE_NAME "TMP_FOLDER_CONTENT.txt"
int main(int argc, char *argv[])
{
system("dir C:\* > "TMP_FILE_NAME);
FILE * fdir = fopen(TMP_FILE_NAME, "r");
char buff[100];
if (fdir)
{
while (1) {
if (fgets(buff, 100, fdir) == NULL) break;
printf("%s", buff);
}
}
fclose(fdir);
remove(TMP_FILE_NAME);
return 0;
}
Where dir is a program to be executed, C:\* - argument of the program, and > - redirection of standard output for that command after which filename TMP_FOLDER_CONTENT.txt will be substituted.
Also you can check returned value, as:
int errorcode = system("dir C:\* > "TMP_FILE_NAME);
printf("Command executed and returned a value %d\n", errorcode);
or taking into account command you use, change the logic of your program, e.g.:
int errorcode = system("dir C:\* > "TMP_FILE_NAME);
if( errorcode )
{
return errorcode;
}
UPDATE:
Alternatively, you could use pipes in C++, for example as shown in the answer to question How to execute a command and get output of command within C++ using POSIX?
you can do as #VolAnd said or also if you don't care about/don't want the output of the command to be in stdout and you also don't want anything else to be printed to stdout you can use freopen to set stdout to a file of your choice.
I have a script starting with a shebang stored in a string. I would like to execute this script without writing it in a temporary file.
I saw that execve takes a filename as argument. Is it possible to do the same with a script in memory.
A script is not directly executable, when executing a script, the kernel identify which interpreter to launch then pass the file name as an argument to the interpreter, in your case a shell.
Should you want to execute a script stored in a string, you might directly launch the shell of your choice and pass your string as its standard input through a pipe.
Here is way to do it using popen:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv[])
{
FILE *fp;
char *prefix="/bin/bash <<'%EOF%'\n";
char *script="#!/bin/bash\necho foo\ndate\n";
char *suffix="%EOF%\n";
char *command;
char buf[1024];
command=malloc(strlen(prefix)+strlen(script)+strlen(suffix)+1);
sprintf(command,"%s%s%s",prefix,script,suffix);
fp=popen(command, "r");
if(fp == NULL) {
perror("Error\n");
exit(1);
}
while(fgets(buf, sizeof(buf), fp) != NULL) {
printf("%s", buf);
}
pclose(fp);
return 0;
}
As you say that the script starts with a shebang string, you cannot directly pipe it into the standard input of a shell. But you can mimic what a shell would have done:
extract the shell command from the shebang line
start it with a pipe as standard input
pipe the remaining of the script into that shell command
A more generalist version would control whether the script string starts with a #!. If it does, use the above way, else just pipe the whole string into an instance of /bin/sh (or whatever shell you are used to)
What is the correct way to read and extract data from text files when you know that there will be many in a directory? I know that you can use fopen() to get the pointer to the file, and then do something like while(fgets(..) != null){} to read from the entire file, but then how could I read from another file? I want to loop through every file in the directory.
Sam, you can use opendir/readdir as in the following little function.
#include <stdio.h>
#include <dirent.h>
static void scan_dir(const char *dir)
{
struct dirent * entry;
DIR *d = opendir( dir );
if (d == 0) {
perror("opendir");
return;
}
while ((entry = readdir(d)) != 0) {
printf("%s\n", entry->d_name);
//read your file here
}
closedir(d);
}
int main(int argc, char ** argv)
{
scan_dir(argv[1]);
return 0;
}
This just opens a directory named on the command line and prints the names of all files it contains. But instead of printing the names, you can process the files as you like...
Typically a list of files is provided to your program on the command line, and thus are available in the array of pointers passed as the second parameter to main(). i.e. the invoking shell is used to find all the files in the directory, and then your program just iterates through argv[] to open and process (and close) each one.
See p. 162 in "The C Programming Language", Kernighan and Ritchie, 2nd edition, for an almost complete template for the code you could use. Substitute your own processing for the filecopy() function in that example.
If you really need to read a directory (or directories) directly from your program, then you'll want to read up on the opendir(3) and related functions in libc. Some systems also offer a library function called ftw(3) or fts(3) that can be quite handy too.
How do I transfer files from one folder to another, where both folders are present in oracle home directory?
int main(int argc, char *argv[]){
char *home, *tmp2;
home = getenv("ORACLE_HOME");
temp2 = getenv("ORACLE_HOME");
strcat (home,"A");
strcat (tmp2,"B");
//transfer files from home to tmp2
}
strcat doesn't seem to work. Here, I see tmp2 pointer doesn't get updated correctly.
Edit: OS is a UNIX based machine. Code edited.
I require a binary file which does this copying, with the intention that the real code cannot be viewed. Hence I didn't consider using shell script as an option. The files in A are encrypted and then copied to B, decrypted in B and run. As the files are in perl, I intend to use system command to run them in the same C code.
Using the system(3) command is probably a good idea since you get the convenience of a shell interpreter to expand filenames (via *) but avoids the hassle of computing the exact length of buffer needed to print the command by using a fixed length buffer and ensuring it cannot overflow:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define BUFSZ 0xFFF
int main(void)
{
char * ohome = getenv("ORACLE_HOME"), cmd[BUFSZ];
char * fmt="/bin/mv %s/%s/* %s/%s";
int written = snprintf(cmd, BUFSZ, fmt, ohome, "A", ohome, "B"), ret;
if ((written < 0) || (written >= (BUFSZ-1))) {
/* ERROR: print error or ORACLE_HOME env var too long for BUFSZ. */
}
if ((ret = system(cmd)) == 0) {
/* OK, move succeeded. */
}
return 0;
}
As commenter Paul Kuliniewicz points out, unexpected results may ensue if your ORACLE_HOME contains spaces or other special characters which may be interpreted by the subshell in the "system" command. Using one of the execl or execv family will let you build the arguments without worrying about the shell interpreter doing it's own interpretation but at the expense of using wildcards.
First of all as pointed out before, this "security" of yours is completely useless. It is trivial to intercept the files being copied (there are plenty of tools to monitor file system changes and such), but that is another story.
This is how you could do it, for the first part. To do the actual copying, you'd have to either use system() or read the whole file and then write it again, which is kind of long for this kind of quick copy.
int main(int argc, char *argv[]){
char *home, *tmp2;
home = strdup(getenv("ORACLE_HOME"));
tmp2 = strdup(getenv("ORACLE_HOME"));
home = realloc(home, strlen(home)+strlen("A")+1);
tmp2 = realloc(tmp2, strlen(tmp2)+strlen("B")+1);
strcat (home,"A");
strcat (tmp2,"B");
}
By the way, if you could stand just moving the file, it would be much easier, you could just do:
rename(home,tmp2);
Not realted to what you are asking, but a comment on your code:
You probably won't be able to strcat to the results of a getenv, because getenv might (in some environments) return a pointer to read-only memory. Instead, make a new buffer and strcpy the results of the getenv into it, and then strcat the rest of the file name.
The quick-n-dirty way to do the transferring is to use the cp shell command to do the copying, but invoke it using the system command instead of using a shell script.
Or, have your C program create a shell script to do the copying, run the shell script, and then delete it.