Linux 为什么这个 bash expr 命令不起作用？
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Why doesn't this bash expr command work?
提问by Marin
I am trying to increment a variable in a bash script and it is not working. Here is my code:
我正在尝试在 bash 脚本中增加一个变量，但它不起作用。这是我的代码：
#! /bin/bash
COUNTER=0
while [ $COUNTER -lt 5 ]
do
echo "i will add this line to file mycreation">>./myfile
COUNTER = `expr $COUNTER + 1`
done
The quotes around the COUNTERassignment are backticks.
COUNTER作业周围的引号是反引号。
I tried replacing COUNTERwith $COUNTERlike this:
我试图取代COUNTER以$COUNTER这样的：
$COUNTER = `expr $COUNTER + 1`
But that did not solve the problem and gave me the following error:
但这并没有解决问题，并给了我以下错误：
line7: 0: command not found.
采纳答案by Shawn Chin
As @Coryrightly pointed out, there should not be spaces around the equal sign or else bash will confuse COUNTERfor a command.
正如@Cory正确指出的那样，等号周围不应有空格，否则 bash 会混淆COUNTER命令。
COUNTER=$(expr $COUNTER + 1)
going off-topic ...
跑题了……
That said, you could avoid having bash fork a subprocess by using the following alternatives:
也就是说，您可以通过使用以下替代方法来避免 bash fork 子进程：
Using the bash builtin 'let' command:
let COUNTER="COUNTER + 1"or, using bash c-style expression:
(( COUNTER++ ))
let COUNTER="COUNTER + 1"或者，使用 bash c 风格的表达式：
(( COUNTER++ ))
In fact, your while loop can be written as:
事实上，你的 while 循环可以写成：
for ((COUNTER=0; COUNTER <= 5 ; COUNTER++))
do
echo "i will add this line to file mycreation">>./myfile
done
Breaking down the error message
分解错误信息
When you were met with the error:
当您遇到错误时：
line 7: 0: command not found.
'-----' '--' '------------------'
| | |
location | Description of error.
culprit
my guess is what you had on line 7 was
我猜你在第 7 行的内容是
$COUNTER = `expr $COUNTER + 1`
-------- --------------------
| |
Evaluated to 0 |
Evaluated to 1
What bash ends up see is 0 = 1and since bash statements are generally in the form command arg1 arg1 ..., bash interprets it as run the command 0with arguments = 1. Thus the error message : 0: command not found.
bash 最终看到的是0 = 1，由于 bash 语句通常采用这种形式command arg1 arg1 ...，因此 bash 将其解释为运行0带参数的命令= 1。因此错误信息：0: command not found。
When you removed the spaces around the equal sign, what bash ends up interpreting is:
当您删除等号周围的空格时，bash 最终解释的是：
0=1
which means run command 0=1with no arguments, hence the error 0=1: command not found.
这意味着0=1不带参数运行命令，因此错误0=1: command not found.
Variable assignments should be in the form VAR_NAME=VALUE(without the $), so the syntax you should be using is:
变量赋值的形式应该是VAR_NAME=VALUE（不带$），所以你应该使用的语法是：
COUNTER=`expr $COUNTER + 1` # or any of the variants above
which bash evaluates and eventually interpret as:
which bash 评估并最终解释为：
COUNTER=2
回答by Cory Klein
Remove the spaces around the equals sign:
删除等号周围的空格：
COUNTER=`expr $COUNTER + 1`
回答by webbi
Another way.
其它的办法。
COUNTER=$(($COUNTER + 1))
回答by August Karlstrom
for i in {0..4}; do
echo "i will add this line to file mycreation" >> ./myfile
done
