2, script basis

variable commands

1. Can not use reserved words in the program: such as if, for< br />2. Only numbers, letters and underscores can be used, and cannot start with a number.
3. See the name to know the meaning
4. Uniform naming rules: camel case nomenclature

 Suggested rules for naming in Shell:
1, uppercase variable names
2, lowercase local variables
3, lowercase function names
4, use English names, and reflect the actual effect

According to the effective scope of the variable and other criteria, divide the following variable types
Local variables: The effective scope is the current shell process; for other shell processes outside the current shell,
package Subshell processes including the current shell are invalid.
Environmental variables: the effective range is the current shell process and its subprocesses.
Local variables: the effective range is a code fragment in the current shell process, usually referring to functions
Location variable: $1, $2, ... to indicate that it is used to allow the script to call in the script code and pass the parameters passed to it through the command line
special variables: $?, $0, $* , [email protected], $#,$$
 
 $$ The pid of the shell itself, the current process ID
 $! The last background processes pid of the shell running

< br /> set 
 -e: If a command returns a non-zero exit status value (failure), exit. ?
?????? -n: No need to execute the script, just check the grammatical structure, and return all grammatical error messages. ??????
? -u: Treat unset variables as errors during replacement. ????
?? ?-v: When reading shell input lines, display them. ?????
? -x: When executing a command, display the commands and their parameters

Variable assignment: name=’value’
You can use the reference value
(1) It can be a direct string: name=”root”
(2) Variable reference: name=”$USER”
(3) Command reference: name=COMMAND
name=$(COMMAND)

Variable reference: ${name} or $name
" "Weak reference, the variable reference will be replaced with the variable value
' 'Strong Reference, the variable reference will not be replaced with the variable value, but keep the original string
 Display all the defined variables: set
 Delete variable: unset name

Read only Variables: can only be declared, but cannot be modified or deleted
 Declare read-only variables:
readonly name
declare -r name
 View read-only variables:
 readonly -p 

Variable scope

local 
export global
New subshell running script
source script execution, in this sehll

< p>Command line extension

$() or “

Declare variables
readonly name
declare -r name
-i integer
– x environment variable

readonly -p

Unset variable
unset var
unset -f function name

location variable

Exit code
bash custom exit status code
exit [n ]: Custom exit status code
Note: Once the exit command is encountered in the script, the script will terminate immediately; the termination exit status depends on the number after the exit command
Note: If the exit status code is not specified for the script, the entire The exit status code of the script depends on the status code of the last command executed in the script

Arithmetic operations
Only integer operations are supported

let
expr
ab=expr $a + $b
$[ ]
$(( ))

bc supports decimals
echo “5*7 .3” | bc

< p>Condition test

Condition test

True returns 0
False returns 1

Logical operation
and -a &&
or- o ||
Not!
a Use []
[-a ]

&& Use [[ ]]
[[ && ]]
[] && [ ]
Test command

test EXPRESSION
[EXPRESSION ]
[[ EXPRESSION ]]

1, use -n -z to determine whether the variable is empty
[] Need to add “” outside the variable [[ ]] No need

Note: There must be blank characters before and after EXPRESSION

Command combination

( command;command) Open subshell
{command; command;} Execute in this shell

< p>bash numerical test

-v VAR

whether variable VAR is set

numerical test
-gt is greater than -ge is greater than or equal to -eq Is it equal to -ne Is it not equal to -lt Is it less than -le Is it less than or equal to

shell variable string
${parameter}
${#parameter} character length
${parameter:offset} extract the substring from the offset to the end
${parameter :offset:le ngth} Extract a string of length length from the offset
${parameter#word} Delete the shortest matching word string from the beginning of ${parameter}
${parameter##word} From ${parameter }The longest matching word string is deleted from the beginning
${parameter%word} The shortest matching word string is deleted from the end of ${parameter}
${parameter%%word} From the end of ${parameter} Start to delete the longest matching word string
${parameter/pattern/string} string replace the first matching pattern
${parameter//pattern/string} all
character length
pipe echo ${char} |wc -L
expre length “${char}”

Special extended variable

${parameter:-word} If the parameter variable is empty or If the value is not assigned, it will return the word string and replace the variable value
${parameter:=word} If the parameter variable is empty or not assigned, it will be set to word and returned. Positional variables and special variables are not applicable
${ parameter:?word} If the parameter variable is empty or unassigned, word is output as standard error, otherwise the variable value is output
${parameter:+word} If the parameter variable is empty or unassigned, do nothing, otherwise word The value of return

Math operation

!/bin/bash

no1=4;
no2=5;
let result=no1+no2
echo $result
result=$[ no1 + no2 ]
result=$[ $no1 + 5 ]
You can also use (()), but use (( )), you need to add $ before the variable name:

Array

$ ass_array=([index1]=val1 [index2]=val2)
Use independent “index -Value” is assigned:
$ ass_array[index1]=val1
$ ass_array’ind ex2]=val2
Array index
$ echo ${!array_var[*]}
$ echo ${!array_var[@]
$ echo ${!fruits_value[*]}

while read n ;do
 echo "name is $n" 
 done 
 cat student_name.txt | while read name; do 
 useradd $name
 echo $name is created
 done

find

find /var -not (-user root -o -user lp -o -user gdm )

Cooperate with xargs

Compression

compress .Z
gzip .gz
bzip2 .bz2
xz .xz
zip .
tar
cpio

-j: bzip2, -z: gzip, -J: xz

Soft and hard symbolic links
df -i –print-type

What is the difference between a symbolic link and a hard link?
Symbolic links:
Soft links have their own file attributes and permissions, etc.;
Can create soft links to non-existent files or directories;
Soft links can cross file systems;
Soft links Can create files or directories;
When creating a soft link, the link count i_nlink will not increase;
Deleting a soft link does not affect the pointed file, but if the pointed original file is deleted, the related soft link It is called a dead link (that is, a dangling link, if the pointed path file is recreated, the dead link can be restored to a normal soft link).

Hard link:
Files have the same inode and data block;
Can only create existing files;
Can’t create hard links across file systems;
Cannot create directories, only files;
Deleting a hard-linked file does not affect other files with the same inode number.

There are two counters for each file: i_count and i_nlink, which are reference count and hard link count. The i_count in the structure inode is used to track the number of files accessed, and i_nlink is the number of hard links to the file viewed using the ls -l command above. In other words, i_count tracks the status of files in memory, while i_nlink is a disk counter. When the file is deleted, i_nlink is set to 0 first. These two counters of the file make it easy to upgrade the Linux system or program. The system or program can replace the new file with the same file name without shutting down (that is, the file i_count is not 0). The new file has its own inode and data block. The old file will be complete after the related process is closed. Delete

When the soft link is created with a relative path, the link will search for a file with the same file name in the current directory. If it cannot be found, the link will become invalid. The link file has its own node data.