Linux Basics: Bash Shell built-in commands

The bash built-in command does not need to search the path provided in the environment variable $path, so it can be executed directly, so it is fast.

Here are a few simple, essential command usages.

1. Help

Purpose: Displays a list of all bash built-in commands, or the use of one of the built-in commands.

Usage:

4Help

Execution Result: Displays a list of all built-in commands.

4Help–s built-in commands

Execution Result: Displays the syntax format of the built-in command.

Case:

Help CD

Displays the detailed usage of the CD command.

Help–s printf

printf:printf [-v var] format [arguments]

Displays the syntax format of the printf command

2. Echo

Purpose: Used to display a line of text. The default is to wrap the lines automatically.

Usage:

4echo Hello World/echo "Hello world"/echo ' Hello World '

Execution Result: Displays the string Hello World.

4echo–n ' Hello world '

Execution Result: Display does not wrap.

4echo–e "I am the King \nof the world."

Option-E, which allows special characters in a string to be have, such as \ n, is interpreted as a newline character.

I am King
of the world.

3. printf

Purpose: Display parameter contents according to format

Syntax: printf "format string" parameter

A format character corresponds to the output of a parameter.

Suppose the variable str= "Hello World"

Usage:

4printf "%s\n" "$str" (%s string format)

Execution Result: The contents of the variable str are displayed, and \ n has a newline effect.

4 printf "%c\n" "$str" (%c character format)

Execution Result: Displays the first character h of the value of the STR variable.

4 printf "%s total%d characters \ n" "$str" one (%d integer format)

Execution result: Show Hello World in total of 11 characters.

4 printf "%f\n" (%f floating point format)

Execution result: Shows floating-point number 20.000000.

4 printf "%5s\n" Yes

Execution Result: Displays a 5-character-length string (right-aligned), and less than 5 characters are padded with whitespace.

4 printf "%-5s\n" Yes

Execution Result: Displays a 5-character string (left-aligned), and a portion less than 5 characters is padded with whitespace.

4 printf "%q\n" "$str"

Execution Result: Display hello\ world,%q will escape the special characters in the variable value with the \ character.

4 printf "%b" "abc\n123\nxyz\n"

Execution result:%b The escape character in the string (\ n, \ t, \v, \ ', \ \)

4.: (colon)

Purpose: Do nothing, return the true value (that is, pass back 0)

4 : >/path/to/file

Execution result: use: To create an empty file.

5.. (Half-width period)

Purpose: Executes the shell program in the current shell environment. Same as the source command

. /path/to/file

6. Alias

Purpose: Display, set program alias (aliases).

Usage:

4 alias

Execution result: Displays all current program aliases that have been set.

4 Alias New alias = ' combination of commands '

Execution result: Set a new alias for the program

Alias cp= ' Cp-f '

means that the CP is redefined with cp-f, and the execution of CP equals the execution of cp–f.

4 alias CP=CP

The CP alias is canceled, CP or the original CP.

7. Unalias

Purpose: Cancels the program alias.

Usage:

4 Unalias CP

Indicates that the alias setting of the CP is canceled.

8. Exit

Purpose: Leave the bash shell or harness script program.

Usage:

4 Exit 1

Indicates that you left the shell and returned a value of 1.

9. Logout

Purpose: Unregisters login Shell.

In the case where no strings are entered on the command line, if you press the "Ctrl+d" key combination, the function is equivalent to the logout instruction and can be logged off.

History

Purpose: Displays shell directives that have been executed in the past.

There are 3 variables related to history:

Histfile

4 Echo $HISTFILE

Execution Result:/root/.bash_history

A history script file that represents the root of the file, that is, the command that the root has executed is stored in this file.

Histfilesize

Specifies the number of rows for the history script file, and if the number of historical instructions exceeds the size of histfilesize, the order of the preceding instructions will be removed, but the sequence number of the respective history commands is unchanged.

Histsize

It is set in the shell of an interactive mode, with the number of historical instructions that can be remembered. Once the shell is finished, the history script file is only stored in the Histsize line number instruction.

One . FC

Purpose: Lists the commands that were recently executed after logging in to the host.

Usage:

4Fc–l

Read

Purpose: Reads a row of data from a standard input.

Usage: Read variable

Case:

Echo ' Please enter your English name? ‘

Read yname

Echo ' Your name is: '$yname

If you do not specify a variable to receive data after the read, the default variable name is reply

Echo ' Please enter your English name? ‘

Read

Echo ' Your name is: '$REPLY

The same can be achieved by using the READ–P approach:

Read–p ' Please enter your English name? ‘

Echo ' Your name is: '$REPLY

Option-P refers to the meaning of "hint message" followed by a string that prompts the user for input.

Read can fetch one row of data into an array at a time, using the following

Read-a Arr < < (Echo 123 45 97)

Option-A is to specify ARR as an array variable.

Extract array element value method:

echo ${arr[n]} N starting from 0

Shopt.

Purpose: Displays and sets the shell's behavior options to enhance the shell's ease of use.

Usage: shopt

Case:

4 shopt–p or Shopt

Execution result: Show all shell action options that can be set

4 shopt–s Mailwarn

Execution result: Enable message alert option

4 shopt–u Mailwarn

Execution result: Turn off message alert option

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Linux Basics: Bash Shell built-in commands