如何在Bash中的带引号的字符串中使用环境变量

I've tried various forms of the following in a bash script:

我在bash脚本中尝试了以下各种形式:

#!/bin/bash
svn diff $@ --diff-cmd /usr/bin/diff -x "-y -w -p -W $COLUMNS"

But I can't get the syntax to correctly expand the COLUMNS environment variable.

但我无法获得正确扩展COLUMNS环境变量的语法。

I've tried various forms of the following:

我尝试了以下各种形式:

svn diff $@ --diff-cmd /usr/bin/diff -x '-y -w -p -W $COLUMNS'

and

svn diff $@ --diff-cmd /usr/bin/diff -x '-y -w -p -W ${COLUMNS}'

and

eval svn diff $@ --diff-cmd /usr/bin/diff -x "-y -w -p -W $COLUMNS"

Suggestions?

5 个解决方案

#1

If unsure, you might use the 'cols' request on the terminal, and forget COLUMNS:

如果不确定,您可以在终端上使用'cols'请求,并忘记COLUMNS:

COLS=$(tput cols)

#2

Just a quick note/summary for any who came here via Google looking for the answer to the general question asked in the title (as I was). Any of the following should work for getting access to shell variables inside quotes:

只需快速记录/摘要,任何通过谷歌来到这里寻找标题中提出的一般问题的答案(就像我一样)。以下任何一项都可以用于访问引号内的shell变量:

echo "$VARIABLE"
echo "${VARIABLE}"

Use of single quotes is the main issue. According to the Bash Reference Manual:

使用单引号是主要问题。根据Bash参考手册:

Enclosing characters in single quotes (') preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash. [...] Enclosing characters in double quotes (") preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when history expansion is enabled, !. The characters $ and ` retain their special meaning within double quotes (see Shell Expansions). The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or newline. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ! appearing in double quotes is escaped using a backslash. The backslash preceding the ! is not removed. The special parameters * and @ have special meaning when in double quotes (see Shell Parameter Expansion).

用单引号(')括起字符可以保留引号中每个字符的字面值。单引号之间可能不会出现单引号,即使前面有反斜杠也是如此。 [...]用双引号括起来的字符(“)保留引号内所有字符的字面值,但$,`,\除外,并且,当启用历史记录扩展时,!。字符$和`retain它们在双引号内的特殊含义(参见Shell Expansions)。反斜杠只有在跟随以下字符之一时才保留其特殊含义:$,`,“,\或newline。在双引号内,将删除后跟其中一个字符的反斜杠。没有特殊含义的字符前面的反斜杠保持不变。双引号可以在双引号内引用,前面加一个反斜杠。如果启用,将执行历史记录扩展,除非!出现在双引号中的是使用反斜杠转义的。之前的反斜杠!没有删除。在双引号中,特殊参数*和@具有特殊含义(请参见壳参数扩展)。

In the specific case asked in the question, $COLUMNS is a special variable which has nonstandard properties (see lhunath's answer above).

在问题中提到的特定情况下,$ COLUMNS是一个具有非标准属性的特殊变量(参见上面的lhunath答案)。

#3

Note that COLUMNS is:

请注意,COLUMNS是:

NOT an environment variable. It is an ordinary bash parameter that is set by bash itself.

不是环境变量。它是一个普通的bash参数,由bash本身设置。

Set automatically upon receipt of a SIGWINCH signal.

收到SIGWINCH信号后自动设置。

That second point usually means that your COLUMNS variable will only be set in your interactive shell, not in a bash script.

第二点通常意味着您的COLUMNS变量将仅在交互式shell中设置,而不是在bash脚本中设置。

If your script's stdin is connected to your terminal you can manually look up the width of your terminal by asking your terminal:

如果脚本的stdin已连接到终端,您可以通过询问终端来手动查找终端的宽度:

tput cols

And to use this in your SVN command:

并在SVN命令中使用它:

svn diff "$@" --diff-cmd /usr/bin/diff -x "-y -w -p -W $(tput cols)"

(Note: you should quote "$@" and stay away from eval ;-))

(注意:你应引用“$ @”并远离eval ;-))

#4

The following script works for me for multiple values of $COLUMNS. I wonder if you are not setting COLUMNS prior to this call?

以下脚本适用于$ COLUMNS的多个值。我想知道你是否在这次通话之前没有设置COLUMNS?

#!/bin/bash
COLUMNS=30
svn diff $@ --diff-cmd /usr/bin/diff -x "-y -w -p -W $COLUMNS"

Can you echo $COLUMNS inside your script to see if it set correctly?

你可以在脚本中回显$ COLUMNS以查看它是否设置正确吗?

#5

You are doing it right, so I guess something else is at fault (not export-ing COLUMNS ?).

你这样做是对的,所以我猜其他的东西是错的(不是出口COLUMNS?)。

A trick to debug these cases is to make a specialized command (a closure for programming language guys). Create a shell script named diff-columns doing:

调试这些情况的一个技巧是制作一个专门的命令(编程语言人员的闭包)。创建一个名为diff-columns的shell脚本:

exec /usr/bin/diff -x -y -w -p -W "$COLUMNS" "$@"

and just use

并且只是使用

svn diff "$@" --diff-cmd  diff-columns

This way your code is cleaner to read and more modular (top-down approach), and you can test the diff-columns code thouroughly separately (bottom-up approach).

通过这种方式,您的代码更易于阅读并且更加模块化(自上而下的方法),您可以单独测试diff-columns代码(自下而上的方法)。

#1