Type of shell
- View the shell program used by the user, the Seventh field in the/etc/passwd file (as if it were the last one, mainly the bash shell)
- There is also a default shell of/bin/sh, which acts as the default system shell for system shell scripts that need to be used at startup
- The system default shell is not necessarily consistent with the user (Ubuntu, its default shell is dash shell)
When the shell's parent-child relationship first uses ps-f, it shows two processes, one with an ID of 12546 running the bash shell, and another process (12587) corresponding to the Ps-f
You can continue to create child shells in the child shell. You can exit the child shell through exit. List of processes we can specify a sequence of commands to execute sequentially in one row (;).
Note: PWD current path But this is not a list of processes, for example, there are still only two processes, and no new child shells are generated. The following adds "()" to create a new process. To determine whether to create a child shell, you need to use the environment variable command echo $BASH _subshell (Note: SUBSHELL is a child shell), return 0 There is no child shell, more than 1 is a child shell, the number returned is the number of child shells. Child shells can be called child shell, such as: (PWD; (LS; cd/etc/;p wd; cd; echo $BASH _subshell)), so that two sub-Sheell are called.
Sleep command, usage: Sleep 10 pauses the session for 10 seconds. Adding the "&" character to the end of the command is to place the command into the background mode, which we can view through ps-f. You can also view all user processes in background mode by using jobs. The PID of the process can be viewed through jobs-l. Coproc: It generates a child shell in the background and executes the command in this child shell. Extended syntax ' {} ': A "space" must be added between the first "{" and the command, with a semicolon "after the command ends"; End with a space between the semicolon and the "}". The cost of generating a child shell is higher and slower. Type-a the implementation of each command, the which command displays only the external command file. Call the external command method to indicate the file. For example: PWD, you can use/BIN/PWD to implement external commands. History: View a list of recently used commands. 1000 records are usually recorded. "!!" You can swap out just execute the command and execute. All commands are saved in ". Bash_history", but you have to exit to save, we can force the save by History-a. We can call this command by "! Number".
Alias-p to view the current command alias, alias li= ' command ' to create your own alias. such as: Alias Ll= ' Ls-l ', next time can be directly through the "LL" to use.
Shell Preliminary understanding
