Bash 中“if”语句的“and”运算符
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An "and" operator for an "if" statement in Bash
提问by HTF
I'm trying to create a simple Bash script to check if the website is down and for some reason the "and" operator doesn't work:
我正在尝试创建一个简单的 Bash 脚本来检查网站是否关闭以及由于某种原因“and”运算符不起作用：
#!/usr/bin/env bash
WEBSITE=domain.com
SUBJECT="$WEBSITE DOWN!"
EMAILID="[email protected]"
STATUS=$(curl -sI $WEBSITE | awk '/HTTP\/1.1/ { print }')
STRING=$(curl -s $WEBSITE | grep -o "string_to_search")
VALUE="string_to_search"
if [ $STATUS -ne 200 ] && [[ "$STRING" != "$VALUE" ]]; then
echo "Website: $WEBSITE is down, status code: '$STATUS' - $(date)" | mail -s "$SUBJECT" $EMAILID
fi
The "-a" operator also doesn't work:
“-a”运算符也不起作用：
if [ $STATUS -ne 200 ] -a [[ "$STRING" != "$VALUE" ]]
Could you also please advise when to use:
您能否还请建议何时使用：
- single and double square brackets
- parenthesis
- 单方括号和双方括号
- 插入语
?
?
回答by William Pursell
What you have should work, unless ${STATUS}is empty. It would probably be better to do:
你所拥有的应该工作，除非${STATUS}是空的。这样做可能会更好：
if ! [ "${STATUS}" -eq 200 ] 2> /dev/null && [ "${STRING}" != "${VALUE}" ]; then
or
或者
if [ "${STATUS}" != 200 ] && [ "${STRING}" != "${VALUE}" ]; then
It's hard to say, since you haven't shown us exactly what is going wrong with your script.
很难说，因为您还没有向我们展示您的脚本到底出了什么问题。
Personal opinion: never use [[. It suppresses important error messages and is not portable to different shells.
个人意见：从不使用[[. 它会抑制重要的错误消息，并且不能移植到不同的 shell。
回答by BrenanK
Try this:
尝试这个：
if [ $STATUS -ne 200 -a "$STRING" != "$VALUE" ]; then
回答by AnshBikram
Try this:
尝试这个：
if [ ${STATUS} -ne 100 -a "${STRING}" = "${VALUE}" ]
or
或者
if [ ${STATUS} -ne 100 ] && [ "${STRING}" = "${VALUE}" ]
回答by Jo So
Quote:
引用：
The "-a" operator also doesn't work:
if [ $STATUS -ne 200 ] -a [[ "$STRING" != "$VALUE" ]]
“-a”运算符也不起作用：
if [ $STATUS -ne 200 ] -a [[ "$STRING" != "$VALUE" ]]
For a more elaborate explanation: [and ]are not Bash reserved words. The ifkeyword introduces a conditional to be evaluated by a job (the conditional is true if the job's return value is 0or false otherwise).
更详细的解释：[并且]不是 Bash 保留字。该if关键字引入了要由作业评估的条件（如果作业的返回值为真，则条件为真，0否则为假）。
For trivial tests, there is the testprogram (man test).
对于琐碎的测试，有test程序 ( man test)。
As some find lines like if test -f filename; then foo bar; fi, etc. annoying, on most systems you find a program called [which is in fact only a symlink to the testprogram. When testis called as [, you have to add ]as the last positional argument.
由于有些人会发现诸如if test -f filename; then foo bar; fi等之类的行很烦人，因此在大多数系统上，您会发现一个名为的程序[，它实际上只是该test程序的符号链接。当test被称为 as 时[，您必须添加]作为最后一个位置参数。
So if test -f filenameis basically the same (in terms of processes spawned) as if [ -f filename ]. In both cases the testprogram will be started, and both processes should behave identically.
所以if test -f filename基本上与if [ -f filename ]. 在这两种情况下，test程序都将启动，并且两个进程的行为应该相同。
Here's your mistake: if [ $STATUS -ne 200 ] -a [[ "$STRING" != "$VALUE" ]]will parse to if+ some job, the job being everything except the ifitself. The job is only a simple command(Bash speak for something which results in a single process), which means the first word ([) is the command and the rest its positional arguments. There are remaining arguments after the first ].
这是你的错误：if [ $STATUS -ne 200 ] -a [[ "$STRING" != "$VALUE" ]]将解析为if+ 一些工作，工作是除if本身之外的一切。作业只是一个简单的命令（Bash 表示导致单个进程的东西），这意味着第一个单词 ( [) 是命令，其余是位置参数。在第一个之后还有剩余的参数]。
Also not, [[is indeed a Bash keyword, but in this case it's only parsed as a normal command argument, because it's not at the front of the command.
也不[[是，确实是 Bash 关键字，但在这种情况下，它仅被解析为普通命令参数，因为它不在命令的前面。
