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Full Discussion: trap command
Top Forums UNIX for Dummies Questions & Answers trap command Post 302537011 by Corona688 on Wednesday 6th of July 2011 11:02:25 PM
Old 07-07-2011
Quote:
Originally Posted by Straitsfan
I'm sorry to be so dense corona, but can you tell me what is meant by external command, foreground process, etc. I'm not sure the book is clear on this. Smilie
An external command is one that's not built into the shell. It exists as an actual file somewhere, and when you run it, it creates a whole new process.
Code:
$ whereis -b cat
cat: /bin/cat
$ /bin/cat file.txt
contents of file
$ whereis read
read:
$ read VAR
qwertyuiop
$ echo $VAR
qwertyuiop
$

builtins, on the other hand, are commands the shell just automatically knows how to do. There's no 'read' program on the system, but 'read' works anyway, putting the qwertyuiop typed on the keyboard into the variable VAR.

So when you run an external command, it's run seperately, and it has control of the terminal until it's done. When you run an internal command, no programs are run, the shell just does what you tell it internally.

Another important builtin is one you probably already know -- 'cd'. Consider how external programs are run -- they have a separate process, their own environment. If cd wasn't a builtin it couldn't possibly work, because it'd create a new process, change that process' directory, quit, and return to your shell, leaving your shell unchanged.
 

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LEARN ABOUT OSF1

whereis

whereis(1)						      General Commands Manual							whereis(1)

NAME

       whereis - Locates files for programs

SYNOPSIS

       whereis [-bms] [-u] [-BMS directory... -f] program...

       The whereis command locates source, binary, and/or reference page files for the specified program program.

OPTIONS

       The -b, -m, and -s options can be used in combination to restrict the search.  Searches for an executable program.  Searches for the refer-
       ence page for a program.  Searches for the sources of a program.

       Use the -B, -M, and -S options to change or limit the places whereis searches.  Specifies the binary directories to search.  Specifies  the
       reference page directories to search.  Specifies the source directories to search.

       The  other options that can be used with whereis are as follows: Terminates the last directory list and signals the start of program names.
       Used with other options to indicate when there is no file for program in the default or specified directory or directories (source, binary,
       or reference page).

DESCRIPTION

       The  whereis  program  locates source, binary, and reference pages for the specified programs.  When looking for a match, the program names
       are stripped of leading pathname components and any trailing extensions of the form (for example, s.  resulting from the use of the  Source
       Code Control System see sccs(1), are also handled.

       The default directories searched by the whereis command are as follows: /etc /etc/nls /sbin /usr/bin /usr/lbin /usr/lbin/spell /usr/ccs/lib
       /usr/lib /usr/local /usr/hosts /usr/sbin

EXAMPLES

       To find files matching cat in the default source, binary, and reference directories, enter: whereis cat To search for reference	pages  for
       app13  in the directory /usr/local/man, enter: whereis -M /usr/local/man -f -f appl3 To find the programs in /usr/bin that do not have ref-
       erence pages in /usr/share/man/man1 with source files in /usr/src/cmd, enter: whereis -u -M /usr/share/man/man1 
	       -S /usr/src/cmd -f /usr/bin/* To find which files in the current directory either have no reference pages in the default  reference
       directories or have more than one, enter: whereis -m -u *

FILES

       Default binary directories.  Default binary directories.  Default reference directories.  Default source directories.

SEE ALSO

       Commands:  apropos(1), find(1), man(1), which(1), catman(8)

																	whereis(1)
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