bash 预期的整数表达式
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/7110723/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Integer expression expected
提问by LookIntoEast
I want to read my files line by line every 5 seconds. This time I just tried one-line bash command to do this. And bash command is:
我想每 5 秒一行一行地读取我的文件。这次我只是尝试了单行 bash 命令来执行此操作。bash 命令是：
let X=1;while [ $X -lt 20 ];do cat XXX.file |head -$X|tail -1;X=$X+1;sleep 5;done
However I got the error like:
但是我得到了这样的错误：
-bash: [: 1+1: integer expression expected
What's the problem? btw, why can't we do $X < 20? (Instead we have to do -lt, less than?)
有什么问题？顺便说一句，为什么我们不能做 $X < 20？（相反，我们必须做 -lt，小于？）
thx
谢谢
回答by Keith Thompson
Your assignment X=$X+1doesn't perform arithmetic. If $Xis 1, it sets it to the string "1+1". Change X=$X+1to let X=X+1or let X++.
您的作业X=$X+1不执行算术运算。如果$X是 1，则将其设置为 string "1+1"。更改X=$X+1为let X=X+1或let X++。
As for the use of -ltrather than <, that's just part of the syntax of [(i.e., the testcommand). It uses =and !=for stringequality and inequality -eq, -ne, -lt, -le, -gt, and -gefor numbers. As @Malvolio points out, the use of <would be inconvenient, since it's the input redirection operator.
至于使用-lt而不是<，那只是[（即test命令）语法的一部分。它使用=和!=表示字符串相等和不相等-eq, -ne, -lt, -le, -gt, 和-ge表示数字。正如@Malvolio 指出的那样，使用<会很不方便，因为它是输入重定向运算符。
(The test/ [command that's built into the bash shell does accept <and >, but not <=or >=, for strings. But the <or >character has to be quoted to avoid interpretation as an I/O redirection operator.)
（bash shell 中内置的test/[命令确实接受<and >，但不接受<=or >=，用于字符串。但必须引用<or>字符以避免解释为 I/O 重定向运算符。）
Or consider using the equivalent (( expr ))construct rather than the letcommand. For example, let X++can be written as ((X++)). At least bash, ksh, and zsh support this, though sh likely doesn't. I haven't checked the respective documentation, but I presume the shells' developers would want to make them compatible.
或者考虑使用等效的(( expr ))构造而不是let命令。例如，let X++可以写成((X++)). 至少 bash、ksh 和 zsh 支持这一点，尽管 sh 可能不支持。我没有检查相应的文档，但我认为 shell 的开发人员希望使它们兼容。
回答by Malvolio
Iwould use
我会用
X=`expr $X + 1`
but that's just me. And you cannot say $X < 20 because < is the input-redirect operator.
但这只是我。你不能说 $X < 20 因为 < 是输入重定向操作符。
回答by Luke Morgan
The sum X=$X+1should be X=$(expr $X + 1 ).
总和X=$X+1应该是X=$(expr $X + 1 )。
You can also use <for the comparison, but you have to write (("$X" < "20"))with the double parenthesis instead of [ $X -lt 20 ].
您也可以<用于比较，但您必须(("$X" < "20"))使用双括号而不是[ $X -lt 20 ].
