Linux Basics: Bash built-in variables

Bash creates many built-in variables by default, which are described below.

1. BASH

Purpose: Displays the full path name of bash. Usually:/bin/bash or/usr/local/bin/bash.

Use case: Echo $BASH

2. Bash_version

Purpose: The version of Bash.

Use case: Echo $BASH

4 Execution Result: 4.1.2 (1)-release

3. FUNCNAME

Purpose: In the functional execution period, the content of this variable is the name of the function type.

Case:

function Show_name () {

Echo $FUNCNAME

echo "This is $ $, you are big, you're from $."

}

4 Execution Result: Displays the function name Show_name.

4. Histcmd

Purpose: After the current instruction is executed, it arranges the numbering in the history instruction (the historical command number assigned to the next instruction).

Use case: Echo $HISTCMD

4 Execution Result: 1051

5. Histcontrol

Purpose: Controls whether the instruction is stored in the history script file.

Usage: Histcontrol has 3 possible setpoint values:

Ignorespace: There are no characters at the beginning of the instruction, and it is not stored in the history script file.

Ignoredups: Consecutive repeated instructions, save only one.

Ignoreboth: Combines the functions of the former two.

6. Histfile

Purpose: Sets the path file name of the history script file. Usually in: Home directory/.bash_history

Use case: Echo $HISTFILE

4 Execution Result:/root/.bash_history

7. Histfilesize

Purpose: Sets the maximum number of rows for the history script file store instruction.

Use case: Echo $HISTFILESIZE

4 Execution Result: 1000

8. Histignore

Purpose: The instruction style is not stored in the history script file, separated by:.

Use case: Echo histgnore=ls:cd:type

9. HOME

Purpose: Sets the user's home directory location. Usually, the home directory of root is in/root, the home directory of the general user in the/home/account. The result of replacing ~ is the value of the home variable.

Ten. HOSTNAME

Purpose: Displays the host name.

Use case: Echo $HOSTNAME

4 execution results: www.contoso.com

HOSTTYPE.

Purpose: Displays the host form.

Use case: Echo $HOSTTYPE

4 Execution Result: x86_64

IFS

Purpose: Defines the field separator character. The default values are: space characters, tab characters, line breaks (newline).

Use Case 1

[email protected] lab]# cat ifs1.sh
#!/bin/bash
#

chk= "T1 t2 t3 T4"

For i in $CHK; Do
Echo $i
Done

4 Execution Results:

T1
T2
T3
T4

Because one of the contents of $ifs is a space character, the $chk can be split into 4 parts (called fields) and presented to the Echo $i respectively.

Use Case 2

PL is a line in the password file/etc/passwd and is now analyzed for each field in the line.

#!/bin/bash
#

Pl= "Root:x:0:0:root:/root:/bin/bash"

# Redefine IFS with ': ' As a delimiter

Ifs= ': '

# J for the first few columns

J=0

# to cycle through the fields

For i in $PL; Do
j=$ ((j+1))
echo the $j field is a $i
Done

4 Execution Results:

The 1th field is root
The 2nd field is X
The 3rd field is 0
The 4th field is 0
The 5th field is root
The 6th field is/root
The 7th field is/bin/bash

MACHTYPE.

Purpose: Describes the GNU format of the host form: cpu-company-System.

Use case: Echo $MACHTYPE

4 Execution Results:

X86_64-redhat-linux-gnu

MAIL

Purpose: Displays the current user's mail directory.

Use case: Echo $MAIL

4 Execution Results:

/var/spool/mail/root

MailCheck.

Purpose: Check the mail every time. Typically, the default is 60 seconds.

Use case: Echo $MAILCHECK

4 Execution Result: 60

. PATH

Purpose: The search path of the command.

Usage: If you want to add a search path yourself, the general practice is as follows:

Export path=/lab/tmp: $PATH

This allows you to add paths to the path/lab/tmp

PPID.

Purpose: The process number of the parent process.

Use case: Echo $PPID

RANDOM

Purpose: Random function.

The value of this variable, which randomly appears as an integer with a range of 0-32767. However, although it is random, it is not really random, because the random number is the same every time. To do this, before using the random variable, arbitrarily set a number to random, as the number of randomly seed, so that the random number will not be generated each time its order is the same.

Case:

random=$$ #使用bash Shell process number as a random number seed

Echo $RANDOM

REPLY.

Purpose: When select and read do not have a read variable set, the default variable name is reply.

shlvl.

Purpose: The number of sub-shell levels.

$1~ $n

Purpose: Position parameter.

Pass in a program or function parameter, the first parameter is $ $, the second parameter is $-a, and the other analogy.

Please note that if the position parameter n exceeds 9, it is represented by ${n}.

$ A represents the name of the executing program.

$*.

Purpose: Represents all positional parameters and is treated as a string.

For example: test.sh ABC 123 xyz, the content of $* is the string "ABC 123 XYZ"

[email protected]

Purpose: Represents all positional parameters, but [email protected] represents the serial of each positional parameter.

For example: test.sh ABC 123 xyz, the contents of [email protected] are 3 strings of "ABC", "123" and "XYZ".

$#.

Purpose: The number of positional parameters.

such as: test.sh ABC 123 xyz, there are 3 parameters, $ #的值为3.

25. $?

Purpose: The return value after the last command execution has ended. Typically 0 represents a successful execution, and a non-0 represents an error.

$$.

Purpose: The current bash Shell's process number.

$_.

Use:

A. When script executes, Bash's absolute path.

B. When the previous command was executed, the last positional parameter, such as the previous command, was: test.sh ABC 123 xyz, then the value of $_ is ' xyz '.

c. When checking a message, the value of $_ is the message file name.

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Linux Basics: Bash built-in variables