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translating---geekpi
How to Trace Execution of Commands in Shell Script with Shell Tracing
============================================================
In this article of the [shell script debugging series][3], we will explain the third shell script debugging mode, that is shell tracing and look at some examples to demonstrate how it works, and how it can be used.
The previous part of this series clearly throws light upon the two other shell script debugging modes: verbose mode and syntax checking mode with easy-to-understand examples of how to enable shell script debugging in these modes.
1. [How To Enable Shell Script Debugging Mode in Linux – Part 1][1]
2. [How to Perform Syntax Checking Debugging Mode in Shell Scripts – Part 2][2]
Shell tracing simply means tracing the execution of the commands in a shell script. To switch on shell tracing, use the `-x` debugging option.
This directs the shell to display all commands and their arguments on the terminal as they are executed.
We will use the `sys_info.sh` shell script below, which briefly prints your system date and time, number of users logged in and the system uptime. However, it contains syntax errors that we need to find and correct.
```
#!/bin/bash
#script to print brief system info
ROOT_ID="0"
DATE=`date`
NO_USERS=`who | wc -l`
UPTIME=`uptime`
check_root(){
if [ "$UID" -ne "$ROOT_ID" ]; then
echo "You are not allowed to execute this program!"
exit 1;
}
print_sys_info(){
echo "System Time : $DATE"
echo "Number of users: $NO_USERS"
echo "System Uptime : $UPTIME
}
check_root
print_sys_info
exit 0
```
Save the file and make the script executable. The script can only be run by root, therefore employ the [sudo command][4] to run it as below:
```
$ chmod +x sys_info.sh
$ sudo bash -x sys_info.sh
```
[
![Shell Tracing - Show Error in Script](http://www.tecmint.com/wp-content/uploads/2016/12/Shell-Tracing-Errors.png)
][5]
Shell Tracing – Show Error in Script
From the output above, we can observe that, a command is first executed before its output is substituted as the value of a variable.
For example, the date was first executed and the its output was substituted as the value of the variable DATE.
We can perform syntax checking to only display the syntax errors as follows:
```
$ sudo bash -n sys_info.sh
```
[
![Syntax Checking in Script](http://www.tecmint.com/wp-content/uploads/2016/12/Syntax-Checking-in-Script.png)
][6]
Syntax Checking in Script
If we look at the shell script critically, we will realize that the `if statement` is missing a closing `fi` word. Therefore, let us add it and the new script should now look like below:
```
#!/bin/bash
#script to print brief system info
ROOT_ID="0"
DATE=`date`
NO_USERS=`who | wc -l`
UPTIME=`uptime`
check_root(){
if [ "$UID" -ne "$ROOT_ID" ]; then
echo "You are not allowed to execute this program!"
exit 1;
fi
}
print_sys_info(){
echo "System Time : $DATE"
echo "Number of users: $NO_USERS"
echo "System Uptime : $UPTIME
}
check_root
print_sys_info
exit 0
```
Save the file again and invoke it as root and do some syntax checking:
```
$ sudo bash -n sys_info.sh
```
[
![Perform Syntax Check in Shell Scripts](http://www.tecmint.com/wp-content/uploads/2016/12/Syntax-Check-in-Shell-Scripts.png)
][7]
Perform Syntax Check in Shell Scripts
The result of our syntax checking operation above still shows that there is one more bug in our script on line 21. So, we still have some syntax correction to do.
If we look through the script analytically one more time, the error on line 21 is due to a missing closing double quote `(”)` in the last [echo command][8] inside the `print_sys_info` function.
We will add the closing double quote in the echo command and save the file. The changed script is below:
```
#!/bin/bash
#script to print brief system info
ROOT_ID="0"
DATE=`date`
NO_USERS=`who | wc -l`
UPTIME=`uptime`
check_root(){
if [ "$UID" -ne "$ROOT_ID" ]; then
echo "You are not allowed to execute this program!"
exit 1;
fi
}
print_sys_info(){
echo "System Time : $DATE"
echo "Number of users: $NO_USERS"
echo "System Uptime : $UPTIME"
}
check_root
print_sys_info
exit 0
```
Now syntactically check the script one more time.
```
$ sudo bash -n sys_info.sh
```
The command above will not produce any output because our script is now syntactically correct. We can as well trace the execution of the script all for a second time and it should work well:
```
$ sudo bash -x sys_info.sh
```
[
![Trace Shell Script Execution](http://www.tecmint.com/wp-content/uploads/2016/12/Trace-Shell-Execution.png)
][9]
Trace Shell Script Execution
Now run the script.
```
$ sudo ./sys_info.sh
```
[
![Shell Script to Show Date, Time and Uptime](http://www.tecmint.com/wp-content/uploads/2016/12/Script-to-Show-Date-and-Uptime.png)
][10]
Shell Script to Show Date, Time and Uptime
### Importance of Shell Script Execution Tracing
Shell script tracing helps us identify syntax errors and more importantly, logical errors. Take for instance the `check_root` function in the `sys_info.sh` shell script, which is intended to determine if a user is root or not, since the script is only allowed to be executed by the superuser.
```
check_root(){
if [ "$UID" -ne "$ROOT_ID" ]; then
echo "You are not allowed to execute this program!"
exit 1;
fi
}
```
The magic here is controlled by the `if statement` expression `[ "$UID" -ne "$ROOT_ID" ]`, once we do not use the suitable numerical operator (`-ne` in this case, which means not equal ), we end up with a possible logical error.
Assuming that we used `-eq` ( means equal to), this would permit any system user as well as the root user to run the script, hence a logical error.
```
check_root(){
if [ "$UID" -eq "$ROOT_ID" ]; then
echo "You are not allowed to execute this program!"
exit 1;
fi
}
```
Note: As we looked at before at the start of this series, the set shell built-in command can activate debugging in a particular section of a shell script.
Therefore, the line below will help us find this logical error in the function by tracing its execution:
The script with a logical error:
```
#!/bin/bash
#script to print brief system info
ROOT_ID="0"
DATE=`date`
NO_USERS=`who | wc -l`
UPTIME=`uptime`
check_root(){
if [ "$UID" -eq "$ROOT_ID" ]; then
echo "You are not allowed to execute this program!"
exit 1;
fi
}
print_sys_info(){
echo "System Time : $DATE"
echo "Number of users: $NO_USERS"
echo "System Uptime : $UPTIME"
}
#turning on and off debugging of check_root function
set -x ; check_root; set +x ;
print_sys_info
exit 0
```
Save the file and invoke the script, we can see that a regular system user can run the script without sudo as in the output below. This is because the value of USER_ID is 100 which is not equal to root ROOT_ID which is 0.
```
$ ./sys_info.sh
```
[
![Run Shell Script Without Sudo](http://www.tecmint.com/wp-content/uploads/2016/12/Run-Shell-Script-Without-Sudo.png)
][11]
Run Shell Script Without Sudo
Well, that is it for now, we have come to the end of the [shell script debugging series][12], the response form below can be used to address any questions or feedback to us, concerning this guide or the whole 3-part series.
--------------------------------------------------------------------------------
作者简介：
![](http://1.gravatar.com/avatar/4e444ab611c7b8c7bcb76e58d2e82ae0?s=128&d=blank&r=g)
Aaron Kili is a Linux and F.O.S.S enthusiast, an upcoming Linux SysAdmin, web developer, and currently a content creator for TecMint who loves working with computers and strongly believes in sharing knowledge.
--------------------------------------------------------------------------------
via: http://www.tecmint.com/trace-shell-script-execution-in-linux/
作者：[Aaron Kili][a]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]:http://www.tecmint.com/author/aaronkili/
[1]:http://www.tecmint.com/enable-shell-debug-mode-linux/
[2]:http://www.tecmint.com/check-syntax-in-shell-script/
[3]:http://www.tecmint.com/enable-shell-debug-mode-linux/
[4]:http://www.tecmint.com/su-vs-sudo-and-how-to-configure-sudo-in-linux/
[5]:http://www.tecmint.com/wp-content/uploads/2016/12/Shell-Tracing-Errors.png
[6]:http://www.tecmint.com/wp-content/uploads/2016/12/Syntax-Checking-in-Script.png
[7]:http://www.tecmint.com/wp-content/uploads/2016/12/Syntax-Check-in-Shell-Scripts.png
[8]:http://www.tecmint.com/echo-command-in-linux/
[9]:http://www.tecmint.com/wp-content/uploads/2016/12/Trace-Shell-Execution.png
[10]:http://www.tecmint.com/wp-content/uploads/2016/12/Script-to-Show-Date-and-Uptime.png
[11]:http://www.tecmint.com/wp-content/uploads/2016/12/Run-Shell-Script-Without-Sudo.png
[12]:http://www.tecmint.com/enable-shell-debug-mode-linux/

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如何用 shell 跟踪跟踪命令在 shell 中的执行
============================================================
在本文的[ shell 脚本调试系列][3]中，我们将解释第三种 shell 脚本调试模式，即 shell 跟踪，并查看一些示例来演示它如何工作以及如何使用它。
本系列的前面部分清晰地阐明了另外两种 shell 脚本调试模式：verbose 模式和语法检查模式，以及如何在这些模式下启用 shell 脚本调试的易于理解的示例。
1. [如何在 Linux 中启用 shell 脚本调试模式 - 第1部分][1]
2. [如何在 shell 脚本中执行语法检查调试模式 - 第2部分][2]
shell 跟踪只是跟踪 shell 脚本中的命令的执行。要打开 shell 跟踪，请使用 `-x` 调试选项。
这让 shell 在终端上执行时显示所有命令及其参数。
我们将使用下面的 `sys_info.sh` shell 脚本，它会简要地打印你的系统日期和时间、登录的用户数和系统的运行时间。但是它包含我们需要查找和更正的语法错误。
```
#!/bin/bash
#script to print brief system info
ROOT_ID="0"
DATE=`date`
NO_USERS=`who | wc -l`
UPTIME=`uptime`
check_root(){
if [ "$UID" -ne "$ROOT_ID" ]; then
echo "You are not allowed to execute this program!"
exit 1;
}
print_sys_info(){
echo "System Time : $DATE"
echo "Number of users: $NO_USERS"
echo "System Uptime : $UPTIME
}
check_root
print_sys_info
exit 0
```
保存文件并执行脚本。脚本只能用 root 用户运行，因此如下使用[ sudo 命令][4]运行：
```
$ chmod +x sys_info.sh
$ sudo bash -x sys_info.sh
```
[
![Shell Tracing - Show Error in Script](http://www.tecmint.com/wp-content/uploads/2016/12/Shell-Tracing-Errors.png)
][5]
shell 跟踪 - 显示脚本中的错误
从上面的输出我们可以观察到，命令首先在其输出被替换为变量的值之前执行。
例如，首次执行 date，其输出被替换为变量 DATE 的值。
我们可以执行语法检查，只显示语法错误，如下所示：
```
$ sudo bash -n sys_info.sh
```
[
![Syntax Checking in Script](http://www.tecmint.com/wp-content/uploads/2016/12/Syntax-Checking-in-Script.png)
][6]
脚本中语法检查
如果我们批判地看这个 shell 脚本，我们就会发现 `if 语句`缺少了 `fi` 关键字。因此，让我们加上它，新的脚本应该看起来像这样：
```
#!/bin/bash
#script to print brief system info
ROOT_ID="0"
DATE=`date`
NO_USERS=`who | wc -l`
UPTIME=`uptime`
check_root(){
if [ "$UID" -ne "$ROOT_ID" ]; then
echo "You are not allowed to execute this program!"
exit 1;
fi
}
print_sys_info(){
echo "System Time : $DATE"
echo "Number of users: $NO_USERS"
echo "System Uptime : $UPTIME
}
check_root
print_sys_info
exit 0
```
再次保存文件并以 root 执行，同时做语法检查：
```
$ sudo bash -n sys_info.sh
```
[
![Perform Syntax Check in Shell Scripts](http://www.tecmint.com/wp-content/uploads/2016/12/Syntax-Check-in-Shell-Scripts.png)
][7]
在 shell 脚本中执行语法检查
上面的语法检查操作的结果仍然显示在脚本的第 21 行还有一个错误。所以，我们仍然要纠正一些语法。
如果我们再分析一次脚本，第 21 行的错误是由于在 `print_sys_info` 函数内最后一个 [echo 命令][8]中没有闭合双引号`（”）`。
我们将在 echo 命令中添加闭合双引号并保存文件。修改过的脚本如下：
```
#!/bin/bash
#script to print brief system info
ROOT_ID="0"
DATE=`date`
NO_USERS=`who | wc -l`
UPTIME=`uptime`
check_root(){
if [ "$UID" -ne "$ROOT_ID" ]; then
echo "You are not allowed to execute this program!"
exit 1;
fi
}
print_sys_info(){
echo "System Time : $DATE"
echo "Number of users: $NO_USERS"
echo "System Uptime : $UPTIME"
}
check_root
print_sys_info
exit 0
```
现在在一次检查语法。
```
$ sudo bash -n sys_info.sh
```
上面的命令不会产生任何输出，因为我们的脚本语法上正确。我们也可以再次跟踪脚本执行，它应该工作得很好：
```
$ sudo bash -x sys_info.sh
```
[
![Trace Shell Script Execution](http://www.tecmint.com/wp-content/uploads/2016/12/Trace-Shell-Execution.png)
][9]
跟踪 shell 脚本执行
现在运行脚本。
```
$ sudo ./sys_info.sh
```
[
![Shell Script to Show Date, Time and Uptime](http://www.tecmint.com/wp-content/uploads/2016/12/Script-to-Show-Date-and-Uptime.png)
][10]
用 shell 脚本显示日期、时间和运行时间
### shell 跟踪执行的重要性
shell 脚本跟踪帮助我们识别语法错误，更重要的是识别逻辑错误。例如，在`sys_info.sh` shell 脚本中的 `check_root` 函数，它用于确定用户是否为 root，因为脚本只允许由超级用户执行。
```
check_root(){
if [ "$UID" -ne "$ROOT_ID" ]; then
echo "You are not allowed to execute this program!"
exit 1;
fi
}
```
这里的魔法是由 `if 语句`表达式 `["$ UID" -ne "$ ROOT_ID"]` 控制的，一旦我们不使用合适的数字运算符（示例中为 `-ne`，这意味着不相等），我们最终会出现一个可能的逻辑错误。
假设我们使用 `-eq` （意思等于），这将允许任何系统用户以及 root 用户运行脚本，因此是一个逻辑错误。
```
check_root(){
if [ "$UID" -eq "$ROOT_ID" ]; then
echo "You are not allowed to execute this program!"
exit 1;
fi
}
```
注意：我们在本系列开头介绍的到 set 这个 shell 内置命令可以在shell脚本的特定部分激活调试。
因此，下面的行将帮助我们通过跟踪它在函数中的执行找到这个逻辑错误：
具有逻辑错误的脚本：
```
#!/bin/bash
#script to print brief system info
ROOT_ID="0"
DATE=`date`
NO_USERS=`who | wc -l`
UPTIME=`uptime`
check_root(){
if [ "$UID" -eq "$ROOT_ID" ]; then
echo "You are not allowed to execute this program!"
exit 1;
fi
}
print_sys_info(){
echo "System Time : $DATE"
echo "Number of users: $NO_USERS"
echo "System Uptime : $UPTIME"
}
#turning on and off debugging of check_root function
set -x ; check_root; set +x ;
print_sys_info
exit 0
```
保存文件并调用脚本，在输出中，我们可以看到一个普通的系统用户可以没有 sudo 运行脚本。 这是因为 USER_ID 的值为100，不等于 ROOT_ID 为 0 的 root。
```
$ ./sys_info.sh
```
[
![Run Shell Script Without Sudo](http://www.tecmint.com/wp-content/uploads/2016/12/Run-Shell-Script-Without-Sudo.png)
][11]
不用 sudo 运行 shell 脚本
那么，现在我们已经到了[ shell 脚本调试系列][12]的末尾，可以在下面的反馈栏里给我们关于本篇或者本系列提出问题或反馈。
--------------------------------------------------------------------------------
作者简介：
![](http://1.gravatar.com/avatar/4e444ab611c7b8c7bcb76e58d2e82ae0?s=128&d=blank&r=g)
Aaron Kili 是 Linux 和 F.O.S.S 爱好者，将来的 Linux SysAdmin、web开 发人员，目前是 TecMint 的内容创作者，他喜欢用电脑工作，并坚信分享知识。
--------------------------------------------------------------------------------
via: http://www.tecmint.com/trace-shell-script-execution-in-linux/
作者：[Aaron Kili][a]
译者：[geekpi](https://github.com/geekpi)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]:http://www.tecmint.com/author/aaronkili/
[1]:http://www.tecmint.com/enable-shell-debug-mode-linux/
[2]:http://www.tecmint.com/check-syntax-in-shell-script/
[3]:http://www.tecmint.com/enable-shell-debug-mode-linux/
[4]:http://www.tecmint.com/su-vs-sudo-and-how-to-configure-sudo-in-linux/
[5]:http://www.tecmint.com/wp-content/uploads/2016/12/Shell-Tracing-Errors.png
[6]:http://www.tecmint.com/wp-content/uploads/2016/12/Syntax-Checking-in-Script.png
[7]:http://www.tecmint.com/wp-content/uploads/2016/12/Syntax-Check-in-Shell-Scripts.png
[8]:http://www.tecmint.com/echo-command-in-linux/
[9]:http://www.tecmint.com/wp-content/uploads/2016/12/Trace-Shell-Execution.png
[10]:http://www.tecmint.com/wp-content/uploads/2016/12/Script-to-Show-Date-and-Uptime.png
[11]:http://www.tecmint.com/wp-content/uploads/2016/12/Run-Shell-Script-Without-Sudo.png
[12]:http://www.tecmint.com/enable-shell-debug-mode-linux/