为什么我的两个相同字符串的 bash 字符串比较总是错误的？
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Why is my bash string comparison of two identical strings always false?
提问by John Lyon
I'm trying to write a simple little script to query a 3g connection, and if the connection has dropped, instigate a reconnection.
我正在尝试编写一个简单的小脚本来查询 3g 连接，如果连接已断开，则发起重新连接。
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!
我的问题是检查命令的输出 - 两个看似相等的字符串没有被评估为相等。我确定这里某处有一个菜鸟错误！
#!/bin/bash
echo "Checking connection"
a="Not connected."
b=$(./sakis3g status --console)
if [[ "$a"!="$b" ]]; then
echo "Strings not equal:"
echo "$a"
echo "$b"
else
echo "Strings equal!!"
fi
The output when run:
运行时的输出：
user@mypc:~$ ./test_3g.sh
Checking connection
Strings not equal:
Not connected.
Not connected.
When running ./test_3g.sh | cat -A:
运行时./test_3g.sh | cat -A：
user@mypc:~$ ./test_3g.sh | cat -A
Checking connection$
Strings not equal:$
Not connected.$
Not connected.$
回答by sth
You have to put spaces around operators:
您必须在运算符周围放置空格：
if [[ "$a" != "$b" ]]; then ...
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...
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，则仅测试字符串返回 true ......
回答by ghostdog74
Use case/esac. If you don't have to mess with if/else's nitty gritty nuances
使用case/esac. 如果你不必if/else纠结于细节的细微差别
case "$a" in
"$b" ) echo "ok";;
*) echo "not ok";;
esac
回答by Slava Semushin
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:
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：
b=$(./sakis3g status --console 2>&1)
