03-28-2009
The arguments absolutely must be separated because each argument will be a separate element in the argv array. Things like shells and the system() subroutine accept arguments separated by spaces, but they must isolate each argument prior to calling exec().
exec() tells the kernel to replace the current existing program with a new program. The kernel roughly:
1. saves a copy of the new argv array and a copy of the environment
2. frees any private memory area
3. disconnects the process from any shared memory area including a shared text segment and any shared libraries.
Note: At this point, any code following the exec call no longer exists in the process. The same is true of any code prior to the exec call and even the exec call itself. It's all gone.
4. reads in the exec'ed program and reconstructs a working program. The environment and the arguments are copied into the new program.
5. transfers control to the newly created program.
Note that this all happens in one process. The process id does not change. The program that the process is running changes. After an exec() a process might be smaller (use less memory) than it did prior to the exec().
You can't emulate stuff like this in userland. Only the kernel can do this sort of thing.
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LEARN ABOUT DEBIAN
dh-exec-subst
DH-EXEC-SUBST(1) dh-exec DH-EXEC-SUBST(1)
NAME
dh-exec-subst - Debhelper executable file substition scripts
SYNOPSIS
#! /usr/bin/dh-exec
src/libfoo-*.so.* debian/foo-plugins/usr/lib/foo/${DEB_HOST_MULTIARCH}/
DESCRIPTION
Being a sub-command of dh-exec(1), this program must not be ran directly, but through dh-exec, which automatically runs all available
sub-commands if run bare; or explicitly with dh-exec --with=subst.
It is a wrapper around the various other substitution helpers, and will pipe the input file through all the available substitution helpers.
It is up to these scripts to do the actual work.
When a helper fails to expand a variable within its input, it will leave it as-is, so that later in the pipeline it can perhaps be expanded
by another program.
SCRIPTS
dh-exec-subst-env
Substitutes any of the available, exported environment variables into its input. It does not do any kind of filtering: whatever is
available in the environment, will be available for substitution, however unsafe that may be.
dh-exec-subst-multiarch
Attempts to expand any of the variables known to dpkg-architecture(1), mostly useful for multi-arch support.
The command will query dpkg-architecture(1) directly, and does not rely on environment variables set (as the called program will
prefer already set environment variables anyway).
ENVIRONMENT
DH_EXEC_SCRIPTDIR
Indicates which directory the command-specific scripts should be sought for. If not specified, scripts will be searched for in
/usr/share/dh-exec/.
FILES
$DH_EXEC_SCRIPTDIR/dh-exec-subst-*
The various scripts for the higher-level program.
SEE ALSO
debhelper(1), dh-exec(1)
AUTHOR
dh-exec-subst is copyright (C) 2011-2012 by Gergely Nagy <algernon@madhouse-project.org>.
2012-05-03 DH-EXEC-SUBST(1)