The content of this article is about how to use the built-in Shell commands during the operation and maintenance process. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

:Command

The general format is: :
This is actually an empty command. It is also used to meet the requirement that a command must appear.

if who | grep jack > /dev/null ;then
    :else
    echo "jack is not logged in"fi

　:The command will return an exit status of 0

.Command

General format. file
　.The command will cause the Shell file to read and execute the specified file, just as if the command in the file was entered directly. Note: file does not have to be executable, as long as it is readable. Additionally, the shell uses the path variable to find files.

.progdefs

When executing commands in progdefs, file is not executed in a sub-shell. After the commands in file are executed, the variables set and modified are still valid. .

alias command

General format:

alias name=string 　　[name=string...]<br>

The alias command assigns string to the alias name. When name is used as a command, the shell replaces it with a string and then performs a command line substitution. (= There can be no spaces before and after)
The following format:

alias name <br>

will print out the alias of name. alias without parameters will list all aliases. alias will Returns an exit status code of 0, unless the name given has no alias.

bg command

General format:

bg job_id

If job control is enabled, the job identified by job_id will be set Enter the background. If no parameters are given, the most recently suspended job will be placed into the

break command

General format:

break

The break command will immediately terminate the innermost for, while, and until loops. The program then continues execution from the command after the loop.
If the format is used:

break n

The inner nth level loop is automatically terminated.

case command

General format:

case value in
         pat<sub>1</sub>) command
               command
               ···               command;;
         pat<sub>2</sub>) command
               command
               ···               command;;
         pat<sub>3</sub>) command
               command
               ···               command;;
    esac

The value will be consecutive with pat₁, * can match anything, usually the last pattern that appears in the case as a default, or as a catch-all condition.
In the matching pattern, you can use Shell's metacharacters (*,?,[...]); the character | can specify the "logical OR" relationship between the two patterns, for example:
pat₁ |pat₂ Represents pat₁ or pat₂

General format:

cd directory

This command will cause the Shell to use directory as the current directory. If the directory is ignored, the Shell will set the directory saved in the HOME variable as the current directory. If the Shell variable CDPATH is empty, directory must be a complete directory path or a relative path to the current directory.
If CDPATH is not empty and directory is not a full path, the Shell will search for the directory containing directory in a series of colon-separated directories saved in CDPATH.
Example:

cd documents/memos      //切换到documents的memos目录cd                      //切换到HOME目录

The cd command will set the Shell variable PWD to the new current directory and OLDPWD to the previous directory

Execute this command in a for, while or until loop to skip the statement after continue in the loop, and then execute the next loop>. If you use the format:

continue n

, the remaining statements in the innermost n-th level of loop will be skipped, and then the next loop will be executed.

General format:

echo args

This command will write args to the standard output. Each word in args is separated by a space character. A newline character will be added at the end. If args is omitted, the result is to jump one line down.

General format:

eval args

This command will cause the Shell to evaluate args and then perform the evaluation. As a result, this actually enables a secondary scan of the command line. Example:

x='abc def'y='$x'echo $yxeval echo $yabc def

General format:

exec command args

This command will execute the specified command. And takes args as parameters. Unlike other commands, command will replace the current process (that is, it will not create a new process). After executing command, it will not return to call the exec program. If only I/O redirection is specified, the input or output of the shell will be changed.
Example:

exec /bin/sh             使用sh替换当前进程

exit命令

　　一般格式：exit n
　　该命令会立即终止当前Shell程序，该程序的退出状态码是整数n的值,如果没有使用n，则使用exit命令之前那条命令的退出状态码。
　　注意，直接在登录Shell中执行exit，其效果相当于登出系统。

export命令

　　一般格式：

export variables

　　该命令可以导出指定的变量，也就是说，这些变量的值会被传入到子Shell

false命令

　　一般格式：

false

　　 该命令会返回非0的退出状态码

fc命令

　　一般格式：

fc -e editor -lnr first lastfc -s old=new first

　　该命令可用于编辑命令历史记录中的命令。可以指定从first到last的命令范围，first和last可以是命令编号或字符串，其中负数被视为相对于当前命令编号的偏移量，而字符串指定了以该字符串起始的最近的那条命令。命令会被读入编辑器，然后在退出编辑器时执行，如果没有指定编辑器，就用Shell变量的FCEDIT的值，如果没有设置FEDCIT，就使用ed。
　　-1选项可以列出范围在first和last之间的命令（也就是说不调用编辑器）。如果选择了-n选项，这些命令前面不会出现命令编号。
　　-r选项会颠倒命令出现的次序。
　　如果没有指定last,则默认使用first的值。如果也没有指定first，默认编辑上一条命令，列出前16条命令。
　　-s选项会执行选中的命令，无需事先编辑。

fg命令

　　一般格式：

fg job_id

　　如果启用了作业控制，由job_id指定的作业被带回前台，如果没有指定参数，将最近挂起的作业或者最近置入后台的作业带回前台。

jobs命令

　　该命令会打印当前活跃的作业的列表，如果指定了-l选项，还会列出没想作业的详细信息（包括其进程和ID）如果指定了-p选型，则只会列出进程ID
　　如果提供了可选的作业的id，只列出指定作业的详细信息。

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