为什么此bash expr命令不起作用? [英] Why doesn't this bash expr command work?

问题描述

我正在尝试在bash脚本中增加变量，但它不起作用.这是我的代码:

I am trying to increment a variable in a bash script and it is not working. Here is my code:

#! /bin/bash

COUNTER=0
while [ $COUNTER -lt 5 ]
do
    echo "i will add this line to file mycreation">>./myfile
    COUNTER = `expr $COUNTER + 1`
done

COUNTER赋值周围的引号是反引号.

The quotes around the COUNTER assignment are backticks.

我尝试用$COUNTER替换COUNTER，如下所示:

I tried replacing COUNTER with $COUNTER like this:

$COUNTER = `expr $COUNTER + 1`

但这并不能解决问题，并给了我以下错误:

But that did not solve the problem and gave me the following error:

line7: 0: command not found. 

推荐答案

正如 @Cory 正确指出的那样，等号前后不能有空格，否则bash会使COUNTER混淆.

As @Cory rightly pointed out, there should not be spaces around the equal sign or else bash will confuse COUNTER for a command.

COUNTER=$(expr $COUNTER + 1)

走出话题...

也就是说，通过使用以下替代方法，可以避免让bash派生一个子进程:

going off-topic ...

That said, you could avoid having bash fork a subprocess by using the following alternatives:

• 使用 bash内置的"let"命令:

let COUNTER="COUNTER + 1"

，或者使用bash c样式表达式:

or, using bash c-style expression:

(( COUNTER++ ))

实际上，您的while循环可以写为:

In fact, your while loop can be written as:

for ((COUNTER=0; COUNTER <= 5 ; COUNTER++))
do
    echo "i will add this line to file mycreation">>./myfile
done

分解错误消息

遇到错误时:

Breaking down the error message

When you were met with the error:

line 7:   0:    command not found.
'-----'  '--'  '------------------'
   |       |                 |
location   |            Description of error.
          culprit 

我的猜测是您在第7行上的状态

my guess is what you had on line 7 was

$COUNTER = `expr $COUNTER + 1`
--------   --------------------
    |                 |
Evaluated to 0        |
                  Evaluated to 1

bash最终看到的是0 = 1，并且由于bash语句通常采用command arg1 arg1 ...形式，因此bash将其解释为运行带有参数= 1的命令0.因此，错误消息:0: command not found.

What bash ends up see is 0 = 1 and since bash statements are generally in the form command arg1 arg1 ..., bash interprets it as run the command 0 with arguments = 1. Thus the error message : 0: command not found.

当您删除等号周围的空格时，bash最终解释为:

When you removed the spaces around the equal sign, what bash ends up interpreting is:

0=1

表示没有任何参数的运行命令0=1，因此错误为0=1: command not found.

which means run command 0=1 with no arguments, hence the error 0=1: command not found.

变量分配的格式应为VAR_NAME=VALUE(无$)，因此您应使用的语法为:

Variable assignments should be in the form VAR_NAME=VALUE (without the $), so the syntax you should be using is:

COUNTER=`expr $COUNTER + 1` # or any of the variants above

哪个bash评估并最终解释为:

which bash evaluates and eventually interpret as:

COUNTER=2

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