如何在Linux或类Unix系统的shell脚本/bash中声明和使用布尔变量

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gaosmile 发布时间：2021-1-4 14:12

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需要定义一个名为failed的bash变量，并将值设置为False。当从cron作业调用我们的脚本时，特定的任务可能会失败，然后我需要将failed转换为True。基于$failed，我需要发送一封电子邮件警告我的cron作业失败了。那么，如何在Linux服务器上运行的shell脚本中声明和使用布尔变量呢?Bash中没有布尔值。但是，我们可以根据需要将shell变量的值定义为0(“False”)或1(“True”)。不过，Bash也支持布尔表达式条件。让我们看看如何在Bash中组合这两个概念来声明布尔变量，并在运行在Linux、macOS、FreeBSD或类unix系统上的shell脚本中使用它们。在bash中声明布尔变量语法如下，定义如下内容failed=0 # False
jobdone=1 # True
## 更具可读性的语法 ##
failed=false
jobdone=true现在，当$failed是数字（例如0或1）时，我们可以按以下方式检查它：if [ $failed -eq 1 ]
then
echo "Job failed"
else
echo "Job done"
fi

就这样。

如何在Shell脚本中声明和使用布尔变量（例如“ true”和“ false”）

当然，我们可以将它们定义为字符串，并使我们的代码更具可读性：

#!/bin/bash
# Declare it as string
failed="false"

if [ "$failed" == "true" ]
then
echo "Job failed"
else
echo "Job done"
fi

或者

# set it to true
email_sent=true
# ...do your stuff and flip email_sent to 'false' if needed ...
if [ "$email_sent" = true ]
then
echo 'Data logged and email sent too.'
else
echo 'ALERT: Operation failed.'
logger 'ALERT: Operation failed.'
fi

在bash下定义布尔变量的替代语法如下：

# Let us Declare Two Boolean Variables
# Set this one to true
jobstatus=true
# Check it
if [ "$jobstatus" = true ] ; then
echo 'Okay

'
else
echo 'Noop

'
fi
# Double bracket format syntax to test Boolean variables in bash
bool=false
if [[ "$bool" = true ]] ; then
echo 'Done.'
else
echo 'Failed.'
fi

让我们对其进行测试： å¾®ä¿¡å¾ç_20210104141214.png

shell脚本示例中的Bash布尔变量下面是一个示例脚本：

#!/bin/bash
# Purpose: Backup stuff from /data/apps
# Tested on : AWS EC2 with EFS and Ubuntu 20.04 Pro servers
# ---------------------------------------------------------
source "/apps/.keychain/$HOSTNAME-sh"
source "/apps/scripts/cli_app.sh"

# Set failed to 'False'
failed=0

D="/nfsefs/ec2mum/prodwwwroot"
log="/tmp/server.log.$$.txt"

# Log everything to our file
exec 3>&1 4>&2
trap 'exec 2>&4 1>&3' 0 1 2 3
exec 1>"${log}" 2>&1

# Backup all web servers
for s in www-0{1..8}
do
      /usr/bin/rsync -az --delete backupt@${s}:/data/apps/ ${D}/${s}/
      # set to 'true' when rsync failed and continue loop
      [ $? -ne 0 ] && failed=1
done

# See if rsync failed in loop
if [ $failed -eq 1 ]
then
      echo "$0 script error: rsync backup failed. See attached log file." | mail -A ${log} -s "$HOSTNAME - LXD backup failed" -r sysuser@my-corp-tld alert@somewhere-tld
      push_to_mobile "$0" "Dear sysadmin,

      Backup failed at $HOSTNAME at $(date).

      ---- log:start ---

      $(<${log})

      --- log:end --

      --
      Yours faithfully,
      $0"
fi
[ -f "${log}" ] && rm -f "${log}"

Bash布尔测试首先，定义一个日志文件：log="/tmp/rsnapshot.$$.txt"让我们运行rsnapshot命令：/usr/bin/rsnapshot "$1" 2>&1 | $HOME/bin/error-scanner.pl > "${log}"在$status中获取bash命令的退出状态，如下所示：status=$?接下来在我们的$log文件中搜索ERROR：alogs="$(egrep -w '^ERROR:|ERROR' $log)"如果$status不为零(命令失败)或$alogs不为空(由$HOME/bin/error-scanner.pl报告错误)，那么通过电子邮件/文本消息通知系统管理员或开发人员:

if [ $status -ne 0 ] ||  [ "$alogs" != "" ]; then
      sub="Backup job failed at $HOSTNAME"
      mail -A "$log" -s "$sub" -r sys@somewhere-tld sysadmin@gmail-tld <<< "$0 script ended with errors when we ran /usr/bin/rsnapshot \"$1\" $alogs"
      push_to_mobile "$0" "$sub

      $0 script ended with errors when we ran /usr/bin/rsnapshot \"$1\"

      $alogs

      See email for detailed log."

else
      sub="Backup successful at $HOSTNAME"
      #push_to_mobile "$0" "$sub. See email for detailed backup log." >/dev/null
      #mail -A "$log" -s "$sub" -r sys@somewhere-tld sysadmin@gmail-tld <<< "$0 /usr/bin/rsnapshot ran successfully\"$1\" $alogs"
fi