1、太复杂了，记不住的，有没有什么规则呀？
The == comparison operator behaves differently within a double-brackets test than within single brackets.
1 [[ $a == z* ]] # True if $a starts with an "z" (pattern matchin 2 [[ $a == "z*" ]] # True if $a is equal to z* (literal matching).
3
4 [ $a == z* ] # File globbing and word splitting take place.
5 [ "$a" == "z*" ] # True if $a is equal to z* (literal matching).
6
7 # Thanks, St?phane Chazelas
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2、测试数值用[ "$a" -ne "$b" ]，测试字符串用[ "$a" != "$b" ]。
3、Example 7-6. Testing whether a string is null通过几个例子反复说：测试字符串一定要用双引号把字符串变量括起来!从前面看，测试数值变量同样是把变量用双引号括起来的，这样，可以总结说，无论是数值变量和字符串变量都必须加双引号。
4、Example 7-7. zmore中测试文件类型的语句，不懂
if [ ${filename##*.} != "gz" ]
# Using bracket in variable substitution.
then
echo "File $1 is not a gzipped file!"
exit $NOTGZIP
fi
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5、复合比较：
1）、[[ condition1 && condition2 ]]相当于 [ "$exp1" -a "$exp2" ]
2）、在比较空字符串时，应该在有可能是空字符串的字符串前加上额外的字符，文中说：
As S.C. points out, in a compound test, even quoting the string variable
might not suffice. [ -n "$string" -o "$a" = "$b" ] may cause an error with
some versions of Bash if $string is empty. The safe way is to append an
extra character to possibly empty variables, [ "x$string" != x -o "x$a" =
"x$b" ] (the "x's" cancel out).
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英语疑问：
1、globbing
File globbing and word splitting take place.
2、Caution advised, however.
10 # Bash permits integer operations and comparisons on variables
11 #+ whose value consists of all-integer characters.
12 # Caution advised, however.
3、sealing wax, cabbages and kings
2 # str-test.sh: Testing null strings and unquoted strings,
3 #+ but not strings and sealing wax, not to mention cabbages and kings . . .
4、compound comparison
5、suffice:
As S.C. points out, in a compound test, even quoting the string variable
might not suffice.
6、the "x's" cancel out（上面5的2））