.TH ZEXEC 1
.SH NAME
zexec \- decompress and execute a compressed binary
.SH SYNOPSIS
.B zexec
.I decompressor executable
.RI [ arg "...]"
.SH DESCRIPTION
.B Zexec
decompresses and executes a compressed binary.  It does this by calling
.I decompressor
to uncompress 
.I executable
to a file in
.B /tmp
and by calling the result with the other arguments given to
.BR zexec .
The temporary file is removed immediately after startup.
.PP
.B Zexec
checks if the first two bytes of the compressed file are the magic characters
.BR #! .
If so then the first line of the file is skipped before it is decompressed.
This allows a utility like
.BR install (1)
to create a compressed executable with a first line that looks like this:
.PP
.RS
#!/usr/bin/zexec /usr/bin/zcat
.RE
.PP
The kernel recognizes this as a request to run /usr/bin/zexec as an
interpreter for the file, just like it runs shell scripts that start with
#!/bin/sh.  The compressed executable can thus be used transparently, without
the user knowing that it is compressed.  The only clue is the slower startup
time.
.SH "SEE ALSO"
.BR install (1).
.SH BUGS
A setuid compressed executable works as required, but setuid #! scripts are
inheritely insecure.  It is trivial to misuse such a script to make a setuid
shell.
.SH AUTHOR
Kees J. Bot (kjb@cs.vu.nl)