Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
exit 0
$SECONDS
The number of seconds the script has been running.
#!/bin/bash
TIME_LIMIT=10
INTERVAL=1
echo
echo "Hit Control−C to exit before $TIME_LIMIT seconds."
echo
while [ "$SECONDS" −le "$TIME_LIMIT" ]
do
if [ "$SECONDS" −eq 1 ]
then
units=second
else
units=seconds
fi
echo "This script has been running $SECONDS $units."
# On a slow or overburdened machine, the script may skip a count
#+ every once in a while.
sleep $INTERVAL
done
echo −e "\a" # Beep!
exit 0
$SHELLOPTS
the list of enabled shell options, a readonly variable
bash$ echo $SHELLOPTS
braceexpand:hashall:histexpand:monitor:history:interactive−comments:emacs
$SHLVL
Shell level, how deeply Bash is nested. If, at the command line, $SHLVL is 1, then in a script it will
increment to 2.
$TMOUT
If the $TMOUT environmental variable is set to a non−zero value time, then the shell prompt will time
out after time seconds. This will cause a logout.
Unfortunately, this works only while waiting for input at the shell prompt console or
in an xterm. While it would be nice to speculate on the uses of this internal variable
for timed input, for example in combination with read, $TMOUT does not work in that
context and is virtually useless for shell scripting. (Reportedly the ksh version of a
timed read does work.)
Implementing timed input in a script is certainly possible, but may require complex machinations.
One method is to set up a timing loop to signal the script when it times out. This also requires a signal
handling routine to trap (see Example 30−5) the interrupt generated by the timing loop (whew!).
Chapter 9. Variables Revisited 70