Linux Learning Notes < four >--bash features

Shell: The interface of the man-machine exchange, the related commands are processed and submitted to the kernel, and the kernel is then driven by the system call hardware execution.

Shell's classification:

Gui:gnome,kde,xfce

Cli:sh,csh,ksh,bash,tcsh,zsh

The concept of the program:

The program is a compiled binary executable file

Concept of the process:

A process is a copy of a program, a program execution instance, and in each process it appears that only the kernel and the current process exist on the current host

Features (or functions) of bash:

1. Command line history, command line completion

2. command-line editing

3. Command aliases

4. Command replacement

5. File name Wildcard

6. Piping, I/O redirection

7. Variables

8.shell programming

1. Command line history, command line completion

History: View command-line histories (records are temporarily saved in memory buffers)

-C: Clear command history

-D Number: Delete the command at the specified location

-W: Save command History to home directory history file (. bash_history) When exiting the login, the command line history in the memory buffer is written to. bash_history

The use of command history tips:

!n: Executes the nth command in the command history;

!-n: Executes the reciprocal nth command in the command history;

!! : Execution of the previous order;

!string: Executes the most recent command in the command history that begins with the specified string;

!$: References the last parameter of the previous command;

Press ESC to release and press. and ALT +.: Reference the last parameter of the previous command

Ctrl+r key: Search command line history and press ENTER to execute quickly

Historical command-related environment variables:

Histfilesize defines a command entry that is saved in a. bash_history file

Histsize defines the maximum number of entries that the history command can display

Histtimeformat defines the time format for the history command, which is typically configured histtimeformat= '%F%T '

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Command line completion:

Command completion: Searches for the executable file at the beginning of each path specified by the PATH environment with the string we give, and if more than one, press two tab to give the list, otherwise the unique command will be directly complete.

Path completion: Searches for each filename under the starting path we give, and tries to complement it.

2. command-line editing

Common key combinations:

Ctrl+p: equivalent to ↑ key

CTRL + N: Quite with ↓ key

Ctrl+b: equivalent to ← Key

Ctrl+f: Equivalent → Key

Ctrl+e: Jump to end of line

CTRL + A: Jump to the beginning

Ctrl+u: Delete all the contents of the left side of the cursor in a row

Ctrl+k: Deletes the entire contents of the cursor to the right (containing the characters specified by the cursor) in a row

3. Command aliases

Alias: type alias directly to display the aliases available under the current shell

Alias string= ' CMD [options] [arguments] '

Aliases defined in the shell are valid only for the current shell life cycle, and the valid range of aliases is only the current shell process

Ualias STRING: Deleting aliases

If string is originally a command, \string means ignoring the alias and executing the original string directly.

4. Command replacement

Command substitution is the output of using another command in one command, such as COMMAND1 ' COMMAND2 '

$ (command) ' command '

Quotes supported by bash:

': Anti-quote, for command substitution

"": double quotes, weak references, you can implement variable substitution, replace the variable with the value of the variable

': Single quote, strong reference, cannot implement variable substitution

5. File name wildcard (globbing)

*: Any character of any length

?: any single character

[]: matches any single character within the specified range

[A-z] [A-z] [A-za-z] [0-9a-za-z]

[[: Space:]]: white space characters

[[:p UNCT:]]: punctuation

[[: Lower:]]: lowercase letters

[[: Upper:]]: Uppercase

[[: Alpha:]]: Uppercase and lowercase letters

[[:d Igit:]]: Number

[[: Alnum:]]: Numbers and uppercase and lowercase letters

[^]: matches any single character outside the specified range

6.I/O Redirection, piping

Common input Devices: keyboard, mouse, hard disk, etc.

Common output devices: monitors, stereos, hard drives, etc.

System settings:

Default output device: Standard output, stdout,1

Default input device: standard input, stdin,0

Standard error Output: stderr,2

Standard input: Keyboard

Standard output and Error output: Display

I/O redirection:

Overwrite output, transfer output to specified other place

>>: Append output

SET-C: Prohibit overwrite redirection for already existing files;

Force overwrite output, use >|

Set-c: Turn off the above features

2>: Redirect Error output

2>>: Append method

&>: Redirect standard output or error output to the same file

<: Input redirection

Example: Cat>catfile<etc/profile

<<: End of input symbol

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Pipe: The output of the previous command, as input to the latter command

COMMAND1 | COMMAND2 | COMMAND3 ...

Example: cut-d:-f1 | head-1 | Id

7. Variables

Variable: A memory space that can be understood as a named

Memory: Addressable storage unit

Variable type: Determine the storage format and length of the data in advance

Character

Numeric type

Integral type

Floating point Type

Boolean type

Bash Variable categories:

Environment variable: scope is the current shell process driver process

Export Varname=value

Varname=value

Export VARNAME

Local variables: scope for the entire bash process

Set Varname=value

Local variables: Scope is the current code snippet

Local Varname=value

Positional variables: The parameters passed when the shell is programmed, the first parameter in the. sh file can be represented as $ $, and the second is $ ...

Special variables:

$?: Execution status of previous command return code (0-255)

0: Correct execution

1-255: Error execution, 1, 2, 127 system reserved

Variable operation

Undo Scalar: Unset VARNAME

View all variables in the current shell: set

To view environment variables in the current shell:

Printenv

Env

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Linux Learning Notes < four >--bash features