如何在 bash if/else 语句中使用文件 grep 比较？
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How do I use a file grep comparison inside a bash if/else statement?
提问by user77413
When our server comes up we need to check a file to see how the server is configured.
当我们的服务器启动时，我们需要检查一个文件以查看服务器是如何配置的。
We want to search for the following string inside our /etc/aws/hosts.conf file:
我们想在我们的 /etc/aws/hosts.conf 文件中搜索以下字符串：
MYSQL_ROLE=master
Then, we want to test whether that string exists and use an if/else statement to run one of two options depending on whether the string exists or not.
然后，我们要测试该字符串是否存在并使用 if/else 语句根据字符串是否存在运行两个选项之一。
What is the BASH syntax for the if statement?
if 语句的 BASH 语法是什么？
if [ ????? ]; then
#do one thing
else
#do another thing
fi
回答by
From grep --help, but also see man grep:
从grep --help，但也看到man grep：
Exit status is 0 if any line was selected, 1 otherwise; if any error occurs and -q was not given, the exit status is 2.
如果选择了任何行，退出状态为 0，否则为 1；如果发生任何错误并且未给出 -q，则退出状态为 2。
if grep --quiet MYSQL_ROLE=master /etc/aws/hosts.conf; then
echo exists
else
echo not found
fi
You may want to use a more specific regex, such as ^MYSQL_ROLE=master$, to avoid that string in comments, names that merely start with "master", etc.
您可能希望使用更具体的正则表达式，例如^MYSQL_ROLE=master$, 以避免在注释中使用该字符串，名称仅以“master”开头，等等。
This works because the iftakes a command and runs it, and uses the return value of that command to decide how to proceed, with zero meaning true and non-zero meaning false—the same as how other return codes are interpreted by the shell, and the opposite of a language like C.
这是有效的，因为if接受一个命令并运行它，并使用该命令的返回值来决定如何继续，零表示真，非零表示假 - 与 shell 解释其他返回码的方式相同，和像 C 这样的语言相反。
回答by Ignacio Vazquez-Abrams
iftakes a command and checks its return value. [is just a command.
if接受一个命令并检查它的返回值。[只是一个命令。
if grep -q ...
then
....
else
....
fi
回答by vladr
Note that, for PIPEbeing any command or sequence of commands, then:
请注意，对于PIPE任何命令或命令序列，则：
if PIPE ; then
# do one thing if PIPE returned with zero status ($?=0)
else
# do another thing if PIPE returned with non-zero status ($?!=0), e.g. error
fi
For the record, [ expr ]is a shell builtin?shorthand for test expr.
作为记录，[ expr ]是否内置了外壳？的简写test expr。
Since grepreturns with status 0 in case of a match, and non-zero status in case of no matches, you can use:
由于grep匹配时返回状态 0，不匹配时返回非零状态，因此您可以使用：
if grep -lq '^MYSQL_ROLE=master' ; then
# do one thing
else
# do another thing
fi
Note the use of -lwhich only cares about the file having at least one match (so that grepreturns as soon as it finds one match, without needlessly continuing to parse the input file.)
请注意，使用-lwhich 只关心至少有一个匹配项的文件（以便grep在找到匹配项后立即返回，而无需继续解析输入文件。）
?on some platforms [ expr ]is not a builtin, but an actual executable /bin/[(whose last argument will be ]), which is why [ expr ]should contain blanks around the square brackets, and why it must be followed by one of the command list separators(;, &&, ||, |, &, newline)
? 在某些平台上[ expr ]是不是内置的，但实际的可执行文件/bin/[（其最后一个参数会]），这就是为什么[ expr ]应该包含空格前后的方括号，以及为什么它必须遵循的命令列表分隔符的一个（;，&&，||，|，&，新队）
回答by ghostdog74
just use bash
只需使用 bash
while read -r line
do
case "$line" in
*MYSQL_ROLE=master*)
echo "do your stuff";;
*) echo "doesn't exist";;
esac
done <"/etc/aws/hosts.conf"
