• C

Unix system command substitution in C

Hi Experts,

I wanted to write events (errors, etc) relating to a program to a logfile using the Unix command while in a C program e.g. :

         case 1:        /* Key error message for shared memory */
       system("date >> $MYLOG  | echo Error in obtaining key to shared memory!(DCSF) >> $MYLOG");
       system("logger -f $MYLOG ");

 Note that the $MYLOG is just to represent a logfile which in the actual case is the value of a C variable cLogFileName which is updated from a config file. (In this case, cLogFileName equals a value "../data/logfile01.dat" ). My question is how can I substitute the value of a C variable into the Unix shell command so that the Unix command appends to the physical file (in this case ../data/logfile01.dat) ??

Thanks in advance.

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cjjcliffordConnect With a Mentor Commented:
you can build the command string for the system() call using snprintf(), which formats a string using the same formatters as printf...

char cmdbuf[512];
// can also use sprintf(), but snprintf() is safer...
snprintf( cmdbuf, sizeof( cmdbuf ), "logger -f %s", cLogFileName );
system( cmdbuf );

However, you might want to write a function that will open() the correct file, and write the information directly to the end of the file, rather than trying to wrap a shell command to echo information into a file...

look at fopen( filename, "a" ), fprintf() and fclose() (or even open(), write(), close() if you want to use the system calls rather than the wrapped File IO API)
Hi sbzainal,

use syslog()

If you still insist on your method, use sprintf

char command[128];

sprintf (command, "date >> %s blah blah", my_c_variable);

snprintf is a safer version of sprintf that you can use for added security

forgot, if you want to control environment variables for child programs, you can use the setenv() call:

setenv( "MYLOG", cLogFileName, 1 );

Then, a script, or program, is launched through system(), that uses this environment variable, should get the value set here (only for children, cannot pass environment to parents!)
I would AVOID using system() for this kind of thing.  It generates at least two tasks, one the user shell, the other a copy of "echo".  That's okay if these tasks are going to do a lot of work for you, but these do very little.  These calls are going to be about 99% overhead.  

How about using syslog() or good old fopen( "...", "a+" ) ??

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