I do not understand how execlp works in Linux

Question

I have spent the last 2 days trying to understand the execlp   system call  but yet here I am  Let me get straight to the issue   The man page of execlp declares the system call as int execlp const char  file  const char  arg        with the description  The const char arg and subsequent ellipses in the execl    execlp    and execle   functions can be thought of as arg0  arg1       argn   Yet I see the system call being called like this in our text book  execlp     bin sh             ls -l  bin               the       are for us to figure out as students   However this system call doesn  t even resemble anything like the declaration on the man page of the system call   I am super confused  Any help is appreciated

User · Answer

The limitation of execl is that when executing a shell command or any other script that is not in the current working directory, then we have to pass the full path of the command or the script. Example:

execl("/bin/ls", "ls", "-la", NULL);

The workaround to passing the full path of the executable is to use the function execlp, that searches for the file (1st argument of execlp) in those directories pointed by PATH:

execlp("ls", "ls", "-la", NULL);

User · Answer

this prototype     int execlp const char  file  const char  arg          Says that execlp   s a variable argument function  It takes 2 const char    The rest of the arguments  if any   are the additional arguments to hand over to program we want to run - also char   - all these are C strings  and the last argument must be a NULL pointer   So  the file argument is the path name of an executable file to be executed  arg is the string we want to appear as argv 0  in the executable  By convention  argv 0  is just the file name of the executable  normally it s set to the same as file   The     are now the additional arguments to give to the executable   Say you run this from a commandline shell     ls   That d be execlp  ls    ls    char   NULL   Or if you run    ls -l     That d be execlp  ls    ls    -l         char   NULL    So on to execlp   bin sh         ls -l  bin             Here you are going to the shell   bin sh   and you re giving the shell a command to execute  That command is  ls -l  bin      You can run that manually from a commandline shell      ls -l  bin      Now  how do you run a shell and tell it to execute a command   You open up the documentation man page for your shell and read it   What you want to run is      bin sh -c  ls -l  bin       This becomes    execlp   bin sh    bin sh    -c    ls -l  bin       char   NULL     Side note  The  bin    is doing pattern matching  this pattern matching is done by the shell  and it expands to all files under  bin  with 2 characters  If you simply did     execlp  ls   ls    -l     bin       char   NULL     Probably nothing would happen  unless there s a file actually named  bin     as there s no shell that interprets and expands  bin

[c] I do not understand how execlp() works in Linux

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