shell结构化命令if-then-else语句

if command
then 
       command
else
        command
fi

[root@localhost 20190105]# vi test.sh
filename=test1
if [ -r $filename ]            //输出test1可读则输出信息
then
echo $filename' is readable !'
fi
[root@localhost 20190105]# sh test.sh
test1 is readable !

[root@localhost 20190105]# vi number.sh
number=200
if [ $number -eq 100 ]                 //如果number等于100则输出“The number is equal 100 ！”提示
then
       echo 'The number is equal 100 !'
else                            //否则输出“The number is not equal 100 ！”提示
       echo 'The number is not equal 100 !'
fi
[root@localhost 20190105]# sh number.sh
The number is not equal 100 !

[root@localhost 20190105]# vi number1.sh
number=25
if [ $number -lt 10 ]              //如果number小于10
then
       echo 'The number < 10 !'
elif [ $number -ge 10 -a $number -lt 20 ] //如果number大于等于10且小于20
then
       echo '10 =< The number < 20 !'
elif [ $number -ge 20 -a $number -lt 30 ] //如果number大于等于20且小于30
then
       echo '20 =< The number < 30 !'
else                         //除上述3种情况以外的其他情况
       echo '30 <= The number !'
fi
[root@localhost 20190105]# sh number1.sh
20 =< The number < 30 !

$ cp test3.sh test4.sh
$
$ nano test4.sh
$
$ cat test4.sh
#!/bin/bash
# testing the else section
#
testuser=NoSuchUser
#
if grep $testuser /etc/passwd
then
    echo "The scropt files in the home directory of $testuser are:"
    ls /home/$testuser/*.sh
    echo
else
    echo "The user $testuser does not exist on this system."
    echo
fi
echo "We are outside the if statement"
$
$ ./test4.sh
The user NoSuchUser does not exist on this system.

We are outside the if statement