Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
bash$ echo −e x\ty
xty
bash$ echo −e "x\ty"
x y
(Thank you, Wayne Pollock, for pointing this out.)
[15] "Word splitting", in this context, means dividing a character string into a number of separate and
discrete arguments.
[16] Be aware that suid binaries may open security holes and that the suid flag has no effect on shell scripts.
[17] On modern UNIX systems, the sticky bit is no longer used for files, only on directories.
[18] As S.C. points out, in a compound test, even quoting the string variable might not suffice. [ −n
"$string" −o "$a" = "$b" ] may cause an error with some versions of Bash if $string is
empty. The safe way is to append an extra character to possibly empty variables, [ "x$string" !=
x −o "x$a" = "x$b" ] (the "x's" cancel out).
[19] The pid of the currently running script is $$, of course.
[20] The words "argument" and "parameter" are often used interchangeably. In the context of this document,
they have the same precise meaning, that of a variable passed to a script or function.
[21] This applies to either command line arguments or parameters passed to a function.
[22] If $parameter is null in a non−interactive script, it will terminate with a 127 exit status (the Bash error
code code for "command not found").
[23] These are shell builtins, whereas other loop commands, such as while and case, are keywords.
[24] An exception to this is the time command, listed in the official Bash documentation as a keyword.
[25] A option is an argument that acts as a flag, switching script behaviors on or off. The argument
associated with a particular option indicates the behavior that the option (flag) switches on or off.
[26] The C source for a number of loadable builtins is typically found in the
/usr/share/doc/bash−?.??/functions directory.
Note that the −f option to enable is not portable to all systems.
[27] The same effect as autoload can be achieved with typeset −fu.
[28] These are files whose names begin with a dot, such as ~/.Xdefaults. Such filenames do not show
up in a normal ls listing, and they cannot be deleted by an accidental rm −rf *. Dotfiles are generally
used as setup and configuration files in a user's home directory.
[29] This is only true of the GNU version of tr, not the generic version often found on commercial UNIX
systems.
[30] A tar czvf archive_name.tar.gz * will include dotfiles in directories below the current working
directory. This is an undocumented GNU tar "feature".
[31] This is a symmetric block cipher, used to encrypt files on a single system or local network, as opposed
to the "public key" cipher class, of which pgp is a well−known example.
[32]
A daemon is a background process not attached to a terminal session. Daemons perform designated
services either at specified times or explicitly triggered by certain events.
The word "daemon" means ghost in Greek, and there is certainly something mysterious, almost
supernatural, about the way UNIX daemons silently wander about behind the scenes, carrying out their
appointed tasks.
Appendix J. Copyright 442