shell--special position parameter variables and common built-in variables

Special positional parameter variables in 1.shell

$: Gets the file name of the currently executing shell script, including the script path if the execution script contains a path

$n: Gets the nth parameter of the currently executing shell script, n=1..9, which represents the file name of the script when n is 0, or if n is greater than 9, enclosed in curly braces

For example ${10}, the parameters are separated by a space

$#: Gets the total number of parameters that are followed by the currently executing shell script

$*: Gets the arguments of all parameters of the current shell script, without quotation marks and [email protected], if the $* with double quotes, such as "$*", it means that all parameters are treated as

As a single string, equivalent to "$ $ $"

[Email protected]: Get all parameters of the current shell script arguments, without quotation marks and $* the same, plus double quotes, such as "[email protected]", it means that all the parameters are treated as a different

Independent string, equivalent to "$" "$" "$" "..." This is the best way to pass multiple parameters to other programs because it preserves all inline

Any whitespace in each parameter.

Special state variables in the 2.shell process

$?: Gets the execution state return value that executes the last instruction (0 for success, not 0 for failure), commonly used

$$: Gets the process number (PID) of the currently executing shell script, not commonly used

$!: Gets the process number (PID) of the last process working in the background, not commonly used

$_: Gets the last parameter of the command or script executed before this, not commonly used

3.bash shell built-in variable command

The bash shell contains some built-in commands. These built-in commands are invisible in the table of contents and are provided by the shell itself. The common internal commands are:

Echo, eval, exec, export, read, shift.

Echo: Output information on the screen

Command format: Echo args

Parameters:

-N: Do not wrap output content

-E: Parsing escape characters

Escape characters:

\ n: Line break

\ r: Enter

\ t: Tab (TAB)

\b: Backspace

\v: Portrait tab

Eval:

Command format: eval args

Function: When the shell program executes to the Eval statement, the shell reads the parameter args, combines them into a new command, and then executes

Exec:

[Email protected] script]# vim exec.sh

[email protected] script]# cat exec.sh

#!/bin/bash

EXEC </script/1.txt

While Read line

Do

Echo $line

Done

Echo OK

[[email protected] script]# SEQ 5 > 1.txt

[email protected] script]# cat 1.txt

1

2

3

4

5

[Email protected] script]# sh exec.sh

1

2

3

4

5

Ok

This article is from "It look" blog, please be sure to keep this source http://jinlong.blog.51cto.com/3276088/1954810

shell--special position parameter variables and common built-in variables