ex43.sh

Shall高级编程

#!/bin/bash
# Exercising "eval" ...

y=`eval ls -l`  #  Similar to y=`ls -l`
echo $y         #+ but linefeeds removed because "echoed" variable is unquoted.
echo
echo "$y"       #  Linefeeds preserved when variable is quoted.

echo; echo

y=`eval df`     #  Similar to y=`df`
echo $y         #+ but linefeeds removed.

#  When LF's not preserved, it may make it easier to parse output,
#+ using utilities such as "awk".

echo
echo "==========================================================="
echo


# Now, showing how to do something useful with "eval" . . .
# (Thank you, E. Choroba!)

version=3.4     #  Can we split the version into major and minor
                #+ part in one command?
echo "version = $version"
eval major=${version/./;minor=}     #  Replaces '.' in version by ';minor='
                                    #  The substitution yields '3; minor=4'
                                    #+ so eval does minor=4, major=3
echo Major: $major, minor: $minor   #  Major: 3, minor: 4