DOC HOME SITE MAP MAN PAGES GNU INFO SEARCH PRINT BOOK
 
Automating frequent tasks

Directory-search order and the PATH variable

The PATH variable is a convenient mechanism for allowing organization and sharing of procedures. However, it must be used in a sensible fashion, or the result might be a great increase in system overhead.

The process of finding a command involves reading every directory included in every pathname that precedes the needed pathname in the current PATH variable. As an example, consider the effect of invoking nroff (that is, /usr/bin/nroff) when the value of PATH is :/bin:/usr/bin. The sequence of directories read is as follows:

   .
   /
   /bin
   /
   /usr
   /usr/bin
A long path list assigned to PATH can increase this number significantly.

The vast majority of command executions are of commands found in /bin and in /usr/bin. Careless PATH setup can lead to unnecessary searching. The following three examples are ordered from worst to best with respect to the efficiency of command searches:

   :/usr/john/bin:/usr/local/bin:/bin:/usr/bin
   :/bin:/usr/john/bin:/usr/local/bin:/usr/bin
   :/bin:/usr/bin:/usr/john/bin:/usr/local/bin
The first one above should be avoided. The others are acceptable and the choice among them is dictated by the rate of change in the set of commands kept in /bin and /usr/bin.

A procedure that is expensive because it invokes many short-lived commands can often be speeded up by setting the PATH variable inside the procedure so that the fewest possible directories are searched in an optimum order.


Next topic: Recommended ways to set up directories
Previous topic: Shortening directory searches

© 2005 The SCO Group, Inc. All rights reserved.
SCO OpenServer Release 6.0.0 -- 03 June 2005