Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
done
# The details, in particular the sleep N, are particular to my
#+ application, but the general pattern is:
while true
do
for job in {pattern}
do
{job already done or running} && continue
{mark job as running, do job, mark job as done}
continue 2
done
break # Or something like `sleep 600' to avoid termination.
done
# This way the script will stop only when there are no more jobs to do
#+ (including jobs that were added during runtime). Through the use
#+ of appropriate lockfiles it can be run on several machines
#+ concurrently without duplication of calculations [which run a couple
#+ of hours in my case, so I really want to avoid this]. Also, as search
#+ always starts again from the beginning, one can encode priorities in
#+ the file names. Of course, one could also do this without `continue 2',
#+ but then one would have to actually check whether or not some job
#+ was done (so that we should immediately look for the next job) or not
#+ (in which case we terminate or sleep for a long time before checking
#+ for a new job).
The continue N construct is difficult to understand and tricky to use in any
meaningful context. It is probably best avoided.
10.4. Testing and Branching
The case and select constructs are technically not loops, since they do not iterate the execution of a code
block. Like loops, however, they direct program flow according to conditions at the top or bottom of the
block.
Controlling program flow in a code block
case (in) / esac
The case construct is the shell equivalent of switch in C/C++. It permits branching to one of a number
of code blocks, depending on condition tests. It serves as a kind of shorthand for multiple if/then/else
statements and is an appropriate tool for creating menus.
case "$variable" in
"$condition1" )
command...
;;
"$condition2" )
command...
;;
Chapter 10. Loops and Branches 117