Testing Your Knowledge of Tests

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7.5. Testing Your Knowledge of Tests

The systemwide xinitrc file can be used to launch the X server. This file contains quite a number of if/then tests, as the following excerpt shows.

if [ -f $HOME/.Xclients ]; then exec $HOME/.Xclients

elif [ -f /etc/X11/xinit/Xclients ]; then exec /etc/X11/xinit/Xclients

else

#failsafe settings. Although we should never get here

#(we provide fallbacks in Xclients as well) it can't hurt. xclock -geometry 100x100-5+5 &

xterm -geometry 80x50-50+150 &

if [ -f /usr/bin/netscape -a -f /usr/share/doc/HTML/index.html ]; then netscape /usr/share/doc/HTML/index.html &

fi

fi

Explain the "test" constructs in the above excerpt, then examine the entire file, /etc/X11/xinit/xinitrc, and analyze the if/then test constructs there. You may need to refer ahead to the discussions of grep, sed, and regular expressions.

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Preliminary Exercises

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2.2. Preliminary Exercises

1.System administrators often write scripts to automate common tasks. Give several instances where such scripts would be useful.

2.Write a script that upon invocation shows the time and date, lists all logged-in users, and gives the system uptime. The script then saves this information to a logfile.

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Tests and Comparisons: Alternatives

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34.3. Tests and Comparisons: Alternatives

For tests, the [[ ]] construct may be more appropriate than [ ]. Likewise, arithmetic comparisons might benefit from the (( )) construct.

a=8
# All of the		comparisons below are equivalent.
test "$a"	-lt 16 && echo "yes, $a < 16"		# "and list"
/bin/test	"$a" -lt 16 && echo "yes, $a < 16"
[ "$a" -lt 16 ] && echo "yes, $a < 16"
[[ $a -lt	16	]] && echo "yes, $a < 16"	# Quoting variables within
(( a < 16	))	&& echo "yes, $a < 16"	# [[ ]] and (( )) not necessary.
city="New	York"
# Again, all		of the comparisons below are equivalent.
test "$city"		\< Paris && echo "Yes, Paris is greater than $city"		# Greater ASCII
order.
/bin/test	"$city" \< Paris && echo "Yes, Paris is greater than $city"
[ "$city"	\<	Paris ] && echo "Yes, Paris is greater than $city"
[[ $city < Paris ]] && echo "Yes, Paris is greater than $city"				# Need not quote
$city.
# Thank you,		S.C.

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