如何使用 bash 脚本和 for 循环回显文件行
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How to echo lines of file using bash script and for loop
提问by ChickenFur
I have a simple file called dbs.txt I want to echo the lines of that file to the screen using a for loop in bash.
我有一个名为 dbs.txt 的简单文件，我想使用 bash 中的 for 循环将该文件的行回显到屏幕上。
The file looks like this:
该文件如下所示：
db1
db2
db3
db4
The bash file is called test.sh it looks like this
bash 文件被称为 test.sh 它看起来像这样
for i in 'cat dbs.txt'; do
echo $i
done
wait
When I run the file by typing:
当我通过键入运行文件时：
bash test.sh
I get the terminal output:
我得到终端输出：
cat dbs.txt
instead of the hoped for
而不是所希望的
db1
db2
db3
db4
The following bash file works great:
以下 bash 文件效果很好：
cat dbs.txt | while read line
do
echo "$line"
done
Why doesn't the first script work?
为什么第一个脚本不起作用？
回答by Todor
You can use the shell builtin readinstead of cat. If you process just a single file and it's not huge, perhaps the following is easier and more portable than most solutions:
您可以使用内置的 shellread而不是cat. 如果您只处理一个文件并且它不是很大，那么以下方法可能比大多数解决方案更容易和更便携：
#!/bin/sh
while read line
do
printf "%s\n" "$line"
done < ""
I remember reading somewhere that printfis safer than echoin the sense that the options that echoaccepts may differ across platforms. So building a habit of using printfmay be worthwhile.
我记得在某处阅读printf比接受echo的选项echo可能因平台不同而更安全的地方。所以养成使用习惯printf可能是值得的。
For description of the readbuiltin, check the manual pages of your shell.
有关read内置的说明，请查看您的 shell 的手册页。
回答by Linus Gustav Larsson Thiel
You need to execute a sub-shell and capture the output, like this:
您需要执行一个子 shell 并捕获输出，如下所示：
for i in `cat dbs.txt`; do
echo $i
done
wait
Note the backticks ` instead of the single-quotes.
注意反引号 ` 而不是单引号。
In bash you can also use $(command):
在 bash 中，您还可以使用$(command)：
for i in $(cat dbs.txt); do
echo $i
done
wait
回答by Gilles Quenot
You need command substitutionshell feature. This require the POSIXexpression $().
您需要命令替换shell 功能。这需要POSIX表达式$()。
Please, don't use backticks as others said.
请不要像其他人所说的那样使用反引号。
The backquote (`) is used in the old-style command substitution, e.g.
反引号 ( `) 用于旧式命令替换，例如
foo=`command`
The
这
foo=$(command)
syntax is recommended instead. Backslash handling inside $() is less surprising, and $() is easier to nest. See
推荐使用语法。$() 中的反斜杠处理不那么令人惊讶，而且 $() 更容易嵌套。看
- http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html-> 2.6.3 Command Substitution
- http://mywiki.wooledge.org/BashFAQ/082
- http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html-> 2.6.3 命令替换
- http://mywiki.wooledge.org/BashFAQ/082
Despite of what Linus G Thiel said, $()works in sh, ash, zsh, dash, bash...
尽管 Linus G Thiel 说了些什么，但$()在sh, ash, zsh, dash, bash...
回答by Pedro del Sol
you need backticks rather than single quotes
你需要反引号而不是单引号
` vs '
'对'
