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       execl, execlp, execle, execv, execvp - execute a file


       #include <unistd.h>

       extern char **environ;

       int execl(const char *path, const char *arg, ...);
       int execlp(const char *file, const char *arg, ...);
       int  execle(const  char  *path,  const  char  *arg  , ..., char * const
       int execv(const char *path, char *const argv[]);
       int execvp(const char *file, char *const argv[]);


       The exec() family of functions replaces the current process image  with
       a  new  process image.  The functions described in this manual page are
       front-ends for the  function  execve(2).   (See  the  manual  page  for
       execve()  for detailed information about the replacement of the current

       The initial argument for these functions is  the  pathname  of  a  file
       which is to be executed.

       The  const  char *arg and subsequent ellipses in the execl(), execlp(),
       and execle() functions can be thought of  as  arg0,  arg1,  ...,  argn.
       Together  they  describe  a  list  of  one  or  more  pointers to null-
       terminated strings that represent the argument list  available  to  the
       executed  program.   The first argument, by convention, should point to
       the file name associated with the file being  executed.   The  list  of
       arguments  must  be  terminated by a NULL pointer, and, since these are
       variadic functions, this pointer must be cast (char *) NULL.

       The execv() and execvp() functions provide  an  array  of  pointers  to
       null-terminated  strings  that represent the argument list available to
       the new program.  The first argument, by convention,  should  point  to
       the  file  name  associated with the file being executed.  The array of
       pointers must be terminated by a NULL pointer.

       The execle() function also specifies the environment  of  the  executed
       process  by  following  the  NULL  pointer  that terminates the list of
       arguments in the parameter list or the pointer to the argv  array  with
       an  additional  parameter.   This  additional  parameter is an array of
       pointers to null-terminated strings and must be terminated  by  a  NULL
       pointer.   The other functions take the environment for the new process
       image from the external variable environ in the current process.

       Some of these functions have special semantics.

       The functions execlp() and execvp() will duplicate the actions  of  the
       shell  in  searching  for an executable file if the specified file name
       does not contain a slash (/) character.  The search path  is  the  path
       specified  in  the  environment by the PATH variable.  If this variable
       isn’t specified, the  default  path  ‘‘:/bin:/usr/bin’’  is  used.   In
       addition, certain errors are treated specially.

       If  permission  is  denied  for a file (the attempted execve() returned
       EACCES), these functions will continue searching the rest of the search
       path.   If  no  other file is found, however, they will return with the
       global variable errno set to EACCES.

       If the header of  a  file  isn’t  recognized  (the  attempted  execve()
       returned ENOEXEC), these functions will execute the shell with the path
       of the file as its first argument.  (If this attempt fails, no  further
       searching is done.)


       If  any  of  the exec() functions returns, an error will have occurred.
       The return value is -1, and the global variable errno will  be  set  to
       indicate the error.




       All  of  these  functions  may fail and set errno for any of the errors
       specified for the library function execve(2).


       sh(1), execve(2), fork(2), ptrace(2), environ(5)


       On some other systems the default path (used when the environment  does
       not contain the variable PATH) has the current working directory listed
       after /bin and /usr/bin, as an anti-Trojan-horse  measure.  Linux  uses
       here the traditional "current directory first" default path.

       The   behavior  of  execlp()  and  execvp()  when  errors  occur  while
       attempting to execute the  file  is  historic  practice,  but  has  not
       traditionally  been  documented  and  is  not  specified  by  the POSIX
       standard. BSD (and possibly other systems) do an  automatic  sleep  and
       retry  if  ETXTBSY  is encountered. Linux treats it as a hard error and
       returns immediately.

       Traditionally, the functions execlp() and execvp() ignored  all  errors
       except  for  the  ones described above and ENOMEM and E2BIG, upon which
       they returned.  They now return  if  any  error  other  than  the  ones
       described above occurs.


       execl(),  execv(),  execle(),  execlp()  and  execvp()  conform to IEEE
       Std1003.1-88 (‘‘POSIX.1’’).