为什么我的 bash 字符串比较两个相同字符串总是错误？
我正在尝试编写一个简单的小脚本来查询 3g 连接，如果连接断开，则发起重新连接。
我的问题是检查命令的输出 - 两个看似相等的字符串被评估为不相等。我确信这里某个地方存在菜鸟错误！
#!/bin/bash
echo "Checking connection"
a="Not connected."
b=$(./sakis3g status --console)
if [[ "$a"!="$b" ]]; then
echo "Strings not equal:"
echo "$a"
echo "$b"
else
echo "Strings equal!!"
fi
运行时的输出：
user@mypc:~$ ./test_3g.sh
Checking connection
Strings not equal:
Not connected.
Not connected.
运行 ./test_3g.sh | 时猫-A：
user@mypc:~$ ./test_3g.sh | cat -A
Checking connection$
Strings not equal:$
Not connected.$
Not connected.$
I'm trying to write a simple little script to query a 3g connection, and if the connection has dropped, instigate a reconnection.
My problem is in checking the output of the command - two seemingly equal strings are not evaluated as equal. I'm sure there's a noob error in here somewhere!
#!/bin/bash
echo "Checking connection"
a="Not connected."
b=$(./sakis3g status --console)
if [[ "$a"!="$b" ]]; then
echo "Strings not equal:"
echo "$a"
echo "$b"
else
echo "Strings equal!!"
fi
The output when run:
user@mypc:~$ ./test_3g.sh
Checking connection
Strings not equal:
Not connected.
Not connected.
When running ./test_3g.sh | cat -A:
user@mypc:~$ ./test_3g.sh | cat -A
Checking connection$
Strings not equal:$
Not connected.$
Not connected.$
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评论（3）
您必须在运算符周围放置空格：
如果没有空格，您最终会得到一个字符串，相当于
"$a!=$b"。如果该字符串非空，则仅测试一个字符串将返回 true...You have to put spaces around operators:
Without spaces you end up with a single string, equivalent to
"$a!=$b". And testing just a string returns true if that string is non-empty...使用
case/esac。如果您不必搞乱if/else的本质细微差别Use
case/esac. If you don't have to mess withif/else's nitty gritty nuances可能 sakis3g 程序将消息打印到 stderr 而不是 stdout。在这种情况下，您将消息与空字符串进行比较。尝试将 stderr 重定向到 stdout：
Probably sakis3g program print message to stderr instead of stdout. In this case you compare your message with empty string. Try to redirect stderr to stdout: