Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
34.8. Security Issues
A brief warning about script security is appropriate. A shell script may contain a worm, trojan, or even a
virus. For that reason, never run as root a script (or permit it to be inserted into the system startup scripts in
/etc/rc.d) unless you have obtained said script from a trusted source or you have carefully analyzed it to
make certain it does nothing harmful.
Various researchers at Bell Labs and other sites, including M. Douglas McIlroy, Tom Duff, and Fred Cohen
have investigated the implications of shell script viruses. They conclude that it is all to easy for even a novice,
a "script kiddie", to write one. [65]
Here is yet another reason to learn scripting. Being able to look at and understand scripts may protect your
system from being hacked or damaged.
34.9. Portability Issues
This book deals specifically with Bash scripting on a GNU/Linux system. All the same, users of sh and ksh
will find much of value here.
As it happens, many of the various shells and scripting languages seem to be converging toward the POSIX
1003.2 standard. Invoking Bash with the −−posix option or inserting a set −o posix at the head of a script
causes Bash to conform very closely to this standard. Even lacking this measure, most Bash scripts will run
as−is under ksh, and vice−versa, since Chet Ramey has been busily porting ksh features to the latest versions
of Bash.
On a commercial UNIX machine, scripts using GNU−specific features of standard commands may not work.
This has become less of a problem in the last few years, as the GNU utilities have pretty much displaced their
proprietary counterparts even on "big−iron" UNIX. Caldera's release of the source to many of the original
UNIX utilities will only accelerate the trend.
Bash has certain features that the traditional Bourne shell lacks. Among these are:
• Certain extended invocation options
• Command substitution using $( ) notation
• Certain string manipulation operations
• Process substitution
• Bash−specific builtins
See the Bash F.A.Q. for a complete listing.
34.10. Shell Scripting Under Windows
Even users running that other OS can run UNIX−like shell scripts, and therefore benefit from many of the
lessons of this book. The Cygwin package from Cygnus and the MKS utilities from Mortice Kern Associates
add shell scripting capabilities to Windows.
Chapter 34. Miscellany 354