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** This entry contains information that is intended to be helpful, **
** accurate and act as a guide of HOW TO accomplish a given system **
** task. The provided explanations, techniques and procedures **
** have been reviewed for technical accuracy and applicability, **
** but have not been tested in every possible environment or **
** situation. **
** **
** Normal precautions should be taken in adopting these same **
** techniques and procedures, because as product and system **
** interfaces change, so would the usage of this information **
** change. **
*********************************************************************
SCENARIO/EXPLANATION:
SOLUTION:
1. Approaches
There are several approaches to invoke a shell script from a 'C'
program; each of these approaches is associated with one or more of
unique system calls and subroutines. Because of the changes in the
programming environment, the optimum solution to invoke a shell
script from a 'C' program varies from one application to another.
Consider the following available options while selecting an approach :
a. The process which executes a 'C' program, can be programmed to
create a child process that in turn executes the shell script. Upon
the completion of executing the shell script, the original process
continues to execute the rest of the program. However, a process can
also can be programmed to invoke a shell script from calling 'C'
program, without creating a child process. But the same process will
never return to the calling 'C' program again.
b. System calls and subroutines which are invoked from the 'C' program,
can pass parameters that point to null terminated character strings.
These strings are passed as the arguments to the shell script. System
calls and subroutines can also pass a parameter, which is an array
of pointers, pointing to null terminated character strings.
c. The shell script will inherit the same system environment as the
calling 'C' program.
2. The system call and the subroutines
The system calls and subroutines that are included in this article are
: 'system', 'execlp', and 'execvp'.
The 'system' subroutine invokes the 'fork' system call, to create a
child process that in turn uses 'exec' system call to run /bin/sh. Upon
the completion of this child process, the 'system' subroutine returns
the exit status of the shell to the calling program. For the purpose
of this article, the calling 'C' program is to invoke a shell script,
and return a value to the calling 'C' program after the execution.
The Syntax of this subroutine is :
#include
int system (string)
char *string;
where 'string' is a executable 'sh' command. for the purpose in
this article, it is the name of a shell script.
The 'execlp' and the 'execvp' system calls execute a new program in
the calling process. They do not create a new process, but overlap the
calling program with the called new program. Upon successful
completion, these system calls do not return to the calling program.
For the purpose of this article, the calling 'C' program invokes a
shell script, and executes this script by the original process.
The syntax for these system calls are :
include
int execlp (file, arg0, arg1, ...argn, 0)
char *file, *arg0, *arg1,...;
int execvp (file, argv)
char *file, *argv< >;
Where :
< > is the left block and the right block symbol in "*argv< >".
The 'file' is the name of a executable file.
The arg0, arg1 ... point to null terminated character string.
The argv is an array of pointers to null terminated string.
The following examples are programs, which invoke the related system
calls and subroutines to perform specific functions. These programs
have been tested on the RT/AIX platform only. A few changes might be
required before applying these programs to another AIX platform.
3. Examples
Example 1 :
In this program, the 'system' subroutine is to create a child process,
and execute a shell script in this child process.
# include
main()
{
printf("in C program, before system call\n");
if (system("script1 is in executing") == -1)
printf("system call failed\n");
printf("in C program, after system call\n");
}
The script file 'script1' might look like this :
# script1, print file name first
#
echo $0 $*
If the 'system' call succeeds, the output of this program will be :
in C program, before system call
script1 is in executing
in C program, after system call
Example 2 :
In this program, the 'execlp' system call is to execute a shell
script. The "in C program, after execlp call" should never be printed
if the 'execlp' system call succeeds. Note, the process will search
the script file using the 'PATH' environment variable.
# include
main(argc, argv)
int argc;
char **argv;
{
printf("in C program, before execlp call\n");
if (execlp("script1", "is", "in", "executing",0) < 0)
perror(argv<0>), exit(1);
printf("in C program, after execlp call\n");
}
The script file 'script1' might look like this :
# script1, print file name first
#
echo $0 $*
If 'execlp' call succeeds, the output will be :
in C program, before execlp call
script1 is in executing
Example 3 :
In this program, the 'execvp' system call is to execute shell script.
The "in C program, after execvp call" should never be printed if
the 'execvp' system call succeeds. Note the process will search the
script file using the 'PATH' environment variable.
/* Object file name is assigned to be "execvp.howto". */
# include
main(argc, argv)
int argc;
char **argv;
{
printf("in C program, before execvp call\n");
if (execvp("script1", argv) < 0)
perror(argv<0>), exit(1);
printf("in C program, after execvp call\n");
}
The script file 'script1' might look like this :
# script1, print file name first
#
echo $0 $*
Executing this program with the shell command "execvp.howto is in
executing". If 'execvp' call succeeds, the output will be :
in C program, before execvp call
script1 is in executing
REFERENCES:
RT PC AIX : Technical Reference, System Calls and
Subroutines.
RT PC AIX Language Guide and Reference.
IBM AIX C Language Reference, SC23-2058-0.
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This item was created from library item Q486222 4VLCH
Additional search words:
AIX AIXRT AIXRTL ALTERNATE COMPILERS HOWTO INDEX INVOCATION INVOKE
IX JUL91 PROGRAM SCRIPTS SHELL SOFTWARE 4VLCH
WWQA: ITEM: RTA000015300 ITEM: RTA000015300
Dated: 01/1996 Category: RISCO
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