Linux 在 bash 脚本中使用条件来检查字符串参数
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use conditional in bash script to check string argument
提问by user1535776
I am trying to write my shell script thing.shso that upon making it an executable and running it with the single letter ``A" like so:
我正在尝试编写我的 shell 脚本，thing.sh以便在使其成为可执行文件并使用单个字母“A”运行它时，如下所示：
$ ./thing.sh A
I get the output
我得到输出
A
If argument 1 is not A, I want the output
如果参数 1 不是 A，我想要输出
Not A
Here is my code so far :
到目前为止，这是我的代码：
#!/bin/bash
if [ "" -eq "A"]
then echo "A"
else echo "Not A"
fi
which returns, no matter what I enter,
无论我输入什么，它都会返回，
./thing.sh: line 3: [:missing `]'
Not A
I am trying what I hoped would check something with one or several letters and compare it against the letter A; could someone tell me what I am missing to get this to work? Thank you
我正在尝试我希望用一个或几个字母检查某些内容并将其与字母 A 进行比较的方法；有人可以告诉我我缺少什么才能让它发挥作用吗？谢谢
采纳答案by Gilles Quenot
What about the shorter :
更短的呢：
#!/bin/bash
[[ == A ]] && echo "A" || echo "not A"
?
?
And a beginner version (identical logic) :
和初学者版本（相同的逻辑）：
#!/bin/bash
if [[ == A ]]; then
echo "A"
else
echo "not A"
fi
Like Scott said, you have a syntax error (missing space).
就像斯科特说的，你有一个语法错误（缺少空格）。
explanations
解释
- I use boolean logic here.
[[ $1 == A ]]is executed, and then if its true,echo "A"is executed, and if it's false,echo "not A"is executed, See http://mywiki.wooledge.org/BashGuide/TestsAndConditionals [[is a bash keyword similar to (but more powerful than) the[command. See http://mywiki.wooledge.org/BashFAQ/031and http://mywiki.wooledge.org/BashGuide/TestsAndConditionalsUnless you're writing for POSIX sh, I recommend[[.
- 我在这里使用布尔逻辑。
[[ $1 == A ]]被执行，然后如果为真，echo "A"则执行，如果为假，echo "not A"则执行，参见http://mywiki.wooledge.org/BashGuide/TestsAndConditionals [[是一个 bash 关键字，类似于（但比[命令更强大）。请参阅http://mywiki.wooledge.org/BashFAQ/031和http://mywiki.wooledge.org/BashGuide/TestsAndConditionals除非您为 POSIX sh 编写代码，否则我推荐[[.
回答by Scott
Try putting a space between "A"and ].
尝试在"A"和之间放置一个空格]。
回答by Fred Thomsen
You need a space after "A"
“A”后需要一个空格
if [ "" -eq "A" ]
回答by Geoff Gustafson
Change the first line to:
将第一行更改为：
if [ "" == "A" ]
The -eq operator is for integers. And as someone else mentioned, the space does matter before the ']'.
-eq 运算符用于整数。正如其他人提到的，空格在 ']' 之前确实很重要。
