Bash variable types and bash Variables

Bash variable types and bash Variables

Local variable: it acts on the Current shell and is invalid for other shell processes other than the current shell and the Current shell Sub-processes.

Assign a value to a local variable

name='value'

Value can be a string or a variable. The referenced variable uses $ {name}, $ name, or ''.

Note: single quotes are strongly referenced, while double quotes are weak references.

[root@izwz9bqngpatc008nw00e7z tmp]# echo '$animal'$animal[root@izwz9bqngpatc008nw00e7z tmp]# echo "$animal"dog[root@izwz9bqngpatc008nw00e7z tmp]# echo "$animals.".[root@izwz9bqngpatc008nw00e7z tmp]# echo "${animal}s."dogs.

[root@izwz9bqngpatc008nw00e7z tmp]# myid = `id -u`-bash: myid: command not found[root@izwz9bqngpatc008nw00e7z tmp]# myid=`id -u`[root@izwz9bqngpatc008nw00e7z tmp]# echo '$myid'$myid[root@izwz9bqngpatc008nw00e7z tmp]# echo "$myid"0

[root@izwz9bqngpatc008nw00e7z tmp]# filename=`date +%F-%H-%M-%S`[root@izwz9bqngpatc008nw00e7z tmp]# echo "$filename"2018-03-19-18-41-21

Use set to display all Defined variables, and unset name to destroy the variables.

Local variable: takes effect in a code segment of the shell process.

Environment variable: takes effect in the current shell process and its sub-process.

The declaration method is

export name=valuedeclare -x name=value

[root@izwz9bqngpatc008nw00e7z tmp]# export TES='animal'[root@izwz9bqngpatc008nw00e7z tmp]# echo $TESanimal[root@izwz9bqngpatc008nw00e7z tmp]# declare -x myname='jeffd'[root@izwz9bqngpatc008nw00e7z tmp]# echo $mynamejeffd

Display All environment variables: export, env, and printenv.

Destroy environment variable: unset name.

Do not use bash built-in environment variables.

Read-Only variables:

readonly namedeclare -r name

Location variable: The parameter used for the script to call the parameter passed to it through the command line in the script code.

[root@izwz9bqngpatc008nw00e7z tmp]# cat test.sh #!/bin/bashlinecount="$(wc -l $1|cut -d' ' -f1)"echo "$1 has $linecount lines."[root@izwz9bqngpatc008nw00e7z tmp]# chmod +x test.sh [root@izwz9bqngpatc008nw00e7z tmp]# ./test.sh /etc/passwd/etc/passwd has 25 lines.

Special variable: $? The Return Value of the previous command execution status.

Program execution may return two types of values:
1. program execution results
2. Is the program status returned (0-255)
0 indicates that the execution is correct.
1-255 execution error (127, system reserved );

$ #: Get the total number of parameters in the Current shell Command Line
$ *: Get all parameters of the current shell "$1 $2 $3 ..., Controlled by IFS
$ @: All parameters of this program "$1" "$2" "$3" "…", Not controlled by IFS
$0 get the name of the currently executed shell script
$ N obtain the nth parameter value of the currently executed shell script, n = 1 .. 9
$ Obtain the current shell process ID (PID)
$! PID used to execute the previous command