Variable explanation of shell programming

Types of variables in bash

1. Local variable: The active scope is the current shell process and is not valid for Shell processes other than the current shell, including the current Shell's child shell process

Variable assignment: name= ' value '

Use reference value:

(1) Direct write character name= "root"

(2) variable reference name= "$USER"

(3) Command Reference name= ' command ', name=$ ()

Variable reference: ${name}, $name

Show all variables that have been defined: set

Delete variable: unset name

2. Environment variable: The active scope is the current shell process and its child process FFS

Variable declarations, assigning values:

Export NAME = "Value"

Declare-x name= "Value"

Variable reference: $name, ${name}

Show all environment variables:

Export

Env

printenv

Delete: unset name

3. Local variables: The effective range is the code snippet in the current shell process (usually referred to as a function)

4. Position variable: $1,$2,... To indicate that the parameters passed to it by the command line are called in the script code

5. Special variables:

$? Exit value of the last Exit program

$ A: The file name of the executed script

$*: Represents "$1c$2c$3c$4", where C is a delimited character, and the default is the SPACEBAR, which represents $ $ $ $4

[email protected]: On behalf of "$", "$", "$ $", "$4" meaning, each variable is independent, general memory [email protected]

$* and [email protected] have a difference when they're wrapped up in double quotes.

in the case of double quotation marks:

$*: Considers all parameters to be a field

[Email protected]: All parameters are separate fields

In the absence of double quotes, $* and [email protected] are the same

Example: Determining the number of rows to give a file

Linecount= "$ (wc-l |$1 |cut-d" "-f1)"

echo "has $linecount lines"

Instance:

Write a script test $* and [email protected] show the number of results

1.$* and [email protected] quoted

#!/bin/bashtest () {echo "$#"}echo ' the number of parameter in "[email protected]" is "$ (test" [email protected] ") echo ' the Number of parameter in "$*" is ' $ (test "$*")

Run results

[Email protected] shells]$ bash 2.sh a b c dthe number of parameter in "[email protected] ' is 4the number of parameter I N "$*" is 1

2..$* and [email protected] without quotation marks

#!/bin/bashtest () {echo "$#"}echo ' the number of parameter in "[email protected]" is "$ (test [email protected]) echo ' The N Umber of parameter in "$*" is ' $ (test $*)

Run results

[Email protected] shells]$ bash 2.sh a b c dthe number of parameter in "[email protected] ' is 4the number of parameter I N "$*" is 4

It can be seen from the test results that the quoted $* output is a field and the parameters of the quoted [email protected] output are separate fields

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Variable explanation of shell programming