bash bash脚本中的逻辑运算符问题
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Problem with logical operator in bash script
提问by John
I read in tldp.com that
我在 tldp.com 上读到
if [ $condition1 ] && [ $condition2 ]
Same as: if [ $condition1 -a $condition2 ]
Returns true if both condition1 and condition2 hold true..."
if [ $condition1 ] && [ $condition2 ]
同： if [ $condition1 -a $condition2 ]
如果条件 1 和条件 2 都为真，则返回真......”
but when I tried
但是当我尝试
if [ $a == 2 ] || [ $b == 4 ]
then
echo "a or b is correct"
else
echo "a and b are not correct"
fi
it gives error. I'm using bash.
它给出了错误。我正在使用 bash。
回答by nickjb
Your logic is ok but your comparison operators are incorrect, you should use the '-eq' for comparing integers and '==' for strings. See 'man test' for quick reference, though it's also documented in 'man bash'.
您的逻辑没问题，但您的比较运算符不正确，您应该使用 '-eq' 来比较整数，使用 '==' 来比较字符串。请参阅“man test”以获取快速参考，尽管它也记录在“man bash”中。
When using integer comparison it is always best to initialise variables to 0 as well otherwise if they remain unset you will get errors.
使用整数比较时，最好将变量也初始化为 0，否则如果它们保持未设置，您将得到错误。
As mentioned by c00k, use [[ rather than [ if using bash as it is a builtin so bash will not need to shell out to use the /usr/bin/[ command.
正如 c00k 所提到的，使用 [[ 而不是 [ 如果使用 bash 因为它是内置的所以 bash 不需要外壳来使用 /usr/bin/[ 命令。
i.e.
IE
a=0;b=0
# do something else with a or b
if [[ $a -eq 2 ]] || [[ $b -eq 4 ]]
then
echo "a or b is correct"
else
echo "a and b are not correct"
fi
回答by c00kiemon5ter
If you're using Bash, then drop the single [and use double ones [[.
For arithmetic operations, use ((.
如果您正在使用Bash，则删除单个[并使用双重[[。
对于算术运算，请使用((.
So you'd want to write this:
所以你想写这个：
if (( a == 2 )) || (( b == 4 )); then
echo "foo"
fi # etc
回答by bmk
Did you assign a value to aand b? If not you have to (otherwise you definitely should) quote your variables with double quotes:
你给a和赋值了b吗？如果不是，你必须（否则你绝对应该）用双引号引用你的变量：
if [ "$a" == "2" ] || [ "$b" == "4" ]; then
echo "a or b is correct";
else
echo "a and b are not correct";
fi
回答by Mubeen
Correct me if I'm wrong. && and || are bash comparison
如果我错了纠正我。&& 和 || 是 bash 比较
#!/usr/bin/ksh
set -x ###### debug mode on
while :
do
dt=`date '+%M'`
if ([ "$dt" -ge "15"] && [ "$dt" -le "17" ]) || ([ "$dt" -ge "25" ] && [ "$dt" -le "27" ])
then
echo "Time-->minutes between 15 to 17 OR 25 to 27"
else
echo "Time--> minutes out of range"
fi
sleep 300 ##### sleep for 5 minutes
done
------------- below lines are debug output
------------- 下面几行是调试输出
+ + date +%M
dt=17
+ [ 17 -ge 15 ]
+ [ 17 -le 17 ]
+ echo Time--> minutes between 15 to 17 OR 25 to 27
Time--> minutes between 15 to 17 OR 25 to 27
+ date
Sat Mar 3 15:17:23 AST 2012
+ sleep 300
>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ :
+ + date +%M
dt=22
+ [ 22 -ge 15 ]
+ [ 22 -le 17 ]
+ [ 22 -ge 25 ]
+ echo Time--> minutes out of range
Time--> minutes out of range
+ sleep 300
^C$ ######## breaking out of loop(terminating the execution).
$
$ env |grep -i shell
+ grep -i shell
+ env
SHELL=/bin/ksh
$ set +x ##### ending debug mode.
