Less keyword commands used:

1. at: (automatic) represents the automatic execution of a task

Example:
```
at 10am mar 31 2015
at> echo "taxes due"
At> ^D (ctrl+D for storing the task)
```
atq: Show scheduled tasks
 atrm job-d: Delete a set task

2. crontab: Repeatedly perform a task

```
*   *    *   *   *  command to be excuted
-   -    -   -   -
|   |    |   |   |
|   |    |   |   +-------day of week(0 to 6)
|   |    |   +---- month (1 to 12)
|   |    +---------day of month (1 to 31)
|   +----- hour (0 to 23)
+---------min(0 to 59)
```
//Example:
```
crontab -e

5 * * * * echo "shell command crontab "  >> /Users/liudong/shell_learning
wq #save and quite crontab job
```Indicates that tasks in a command are executed every 5 minutes

3. head and tail commands: Show how many lines are before or after a text file.

seq 100 > numbers.txt #Generate 1-100 and write to numbers.txt
head -3 numbers.txt #Show first 3 lines
tail -5 numbers.txt #Show the last 5 lines
head -65 numbers.txt | tail -5 >log.txt # Display lines 61-65 and write to log.txt

4.diff: Used to compare two files with different contents.

diff file1 file2

5. cut: Extract the specified column from the text

-c: Specify which character to display
 -d: Specifies the separator between columns
 -f: Specify which columns to display

Example:

$ cut -d: -f1-5 /etc/password #Displays the contents of columns 1-5 of the file/etc/password, separated by:
output:
##
nobody:*:-2:-2:Unprivileged User
root:*:0:0:System Administrator
daemon:*:1:1:System Services
_uucp:*:4:4:Unix to Unix Copy Protocol
_taskgated:*:13:13:Task Gate Daemon
_networkd:*:24:24:Network Services
_installassistant:*:25:25:Install Assistant
_lp:*:26:26:Printing Services

$ date | cut -c1-5 #Show the first five characters of date output 
output:2019 year 

6. paste: File content merge display command (Note that merge display is the only thing, and the file content will not change.)

Example: Merge the contents of file file1 file2 file3
First look at the contents of the three files before merging

$ cat file                  #Content of file  
xiongdan 200  
lihaihui 233  
lymlrl 231  
$ cat file2              #Contents of the testfile file file  
liangyuanm  ss  
$ cat file3             #Contents of the testfile1 file  
huanggai 56  
zhixi 73 

Then execute the merge display command:

$ paste file1 file2 file3
output:
xiongdan 200  
lihaihui 233  
lymlrl 231  
liangyuanm  ss  
huanggai 56  
zhixi 73  

7. uniq: Remove adjacent duplicate lines (for display only, file content will not be changed).

Example:

$ cat test
aa
aa
cc
cc
bb
bb
yy
zz

$uniq test
aa
cc
bb
yy
zz

8, tr: Character replacement operation (only for display, does not change the actual contents of the file.)

Example:

tr '[a-z]' '[A-Z]' < filename #Convert lowercase to uppercase
tr '|' '~' < filename # Replace the'-'character in the file with'~'.

9. sort: Sort the text file contents (in behavioral units).

$ sort filename #Arabic order by first letter of each line
$ sort -d filename # Sort by dictionary (similar to above)
$ sort -r filename #Reverse sort
$ sort +num filename #Sort by the contents of column num

10, >>> < &> >&: IO related

$ echo "Welcome to Shell Scripting" > log.txt #The contents of log.txt are set to string contents
$ echo "Welcomet to shell scriptin" >> log.txt #The contents of the string will be added to the end of the file
$ ls > log.txt #Enter the output of the ls command into log.txt
$ gcc hello.c 2 > error_file #Compile hello.c and redirect the error content to error_file if an error occurs
$ find . -name "*.sh" > success_file 2> /dev/null # Enter the result of the find command into success_file and discard the error if an error occurs (/dev/null means discard any input to the character device)
$ find . -name "*.sh" &> log.txt #Redirect output results with errors to log.txt
$ find . -name "*.sh" > log.txt 2>&1 #Redirect results to log.txt and errors to output, both log.txt
$ echo "File needs an argument" 1>&2 #Redirect output to error stream

11. Wildcard correlation

12. Braces {}

$ touch file{1,2,3}
$ ls
file1 file2 file3
$ mkdir directory{1,2,3}{a,b,c}
$ ls
output:directory1a  directory1b ....

13. IO redirection related operators.

14. Vi-based related operations.

Example:

$ vi file3 #Open file3 via vim
#In command mode, type the command::set hlsearch #This highlights the string matched by the search.
:set ic #Ignore case when searching for matches
:set noic #Case sensitive when searching for matches.The mac system is case insensitive by default.

Search and Replace commands:

15. grep: Full-text retrieval command

Both grep content file_name content and file_name can use wildcards
g:globally RE:regular expression p: print out
Grep-n: Display line numbers at the same time
Grep-i: Fuzzy case
Grep-v: Show mismatched rows
Grep-l'Nuts'*: Lists the files containing Nuts in the current directory
Grep-c'Nuts'sample.txt: Displays the total number of rows matched
Grep-w'Nuts'sample.txt: Displays rows where the entire word matches the style

Metacharacter meaning:

16,?: The execution result of the last shell command.

0: on behalf of the previous statement executed successfully; non-0: on behalf of the previous statement executed unsuccessfully.
Example:

$ ls
$ echo $?
output:0
$ ls /root
$ echo $?
output: 1

17, ``: Command substitutes.

Characters between two ``` are treated as shell commands.
Example:

$ echo "Hello `whoami`"
output:Hello liudong
$ echo "Hello $(whoami)"

18, pwd: Current directory.

Example:

$pwd
output:/Users/liudong/shell_learning
$ dirname="$(basename $(pwd))" #Show only the current directory
$ echo dirname
outpu:shell_learning

19,;: Separator between two shell commands on the same line

20, (): Multiple commands are grouped together and their output can then be redirected uniformly to a pipe or file.

21. Logical Operator: & && ||

&: The shell command on behalf of this one will run in the background;
&&: The first statement must be executed successfully before the second statement can be executed.

$ ls /home/liudong && echo "Command executed successfully"
output: Command executed successfully
$ ls /root && echo "Command executed successfully"

||: The first statement must fail before the second statement can be executed.

$ ls /root || echo "Command execution failed"

22,[[ expression ]]

Is an unconditional expression judgment.

23. Single and double quotation marks.

$ person="liudong"
$ echo "$person" # liudong
$ echo '$person' # $person
$ echo $person #$person

24, $$: Displays the process id of the current shell

25.export: Used for variable scope control.

Variables declared by the parent process can be made visible to the child process by adding the keyword export, but keywords declared by the child process cannot be visible to the parent process.
Whenever another shell script is executed, a child process is generated.

$ person="liudong"
$ export person

26.read-only: Read-only variable.

$ readonly person="liudong"
$ unset person
output:bash: unset: person: cannot unset: readonly variable
 Another way to declare read-only variables:
$ Declare -r person="liudong"

27. Script parameter delivery

Set: When executing a script, parameters are not necessarily passed through the command line; they can be set through the set command

$ set USA Canada UK France
$ echo $1
$ echo $2
$ echo $3
$ echo $4

shift: parameter offset setting.

#!/bin/bash
echo "All arguments passed are as follow:"
echo "$*"
echo "shift by one position:"
shift
echo "value of positional parameter $1 after shift:"
echo $1
echo "shift by two positions:"
shift 2
echo "value of positional parameter $1 after two shifts:"
echo $1

inputs:sh shift.sh one two three four
outputs:
All arguments passed are as follow:
one two thress four \n
shift by one position:
value of positional parameter two after shift:
two
shift by two positions:
value of positional parameter four after two shifts:
four
So each shift offset is based on the current position.
Old parameter storage:

$set alan john dennis
$1 is alan $2 is john $3 is dennis
$ oldargs=$* #Save old parameters in the temporary variable oldargs
$ set $oldargs #Retrieve the old parameter.

28.while do case: conditional selection statement

#!/bin/bash
USAGE="usage: $0 -x -y"

while getopts :xy: opt_char
do
    case $opt_char in
    x)
     echo "option x was called."
     ;;
    y)
     echo "option y was called .Argument called is $OPTARG"
     ;;
    \?)
     echo "$OPTARG is not a valid option."
     echo "$USAGE"
     ;;
    esac
done

input:sh getopt.sh -x

output:option x was called.

input:sh getopt.sh -y ggggggggggggg

output:option y was called .Argument called is ggggggggggggg

input:sh getopt.sh -y gggggggggg -x

output:option y was called .Argument called is ggggggggggggg
option x was called.
When reading here, because x is followed by no OPTARG, the parameters after X will not be read, and case y is followed by OPTARG, so the parameters after -x will not be read, while case is followed by OPTARG, so the parameters after X will not be read, and case y is followed by OPTARG, so the parameters after -y will be readRead.

29. Default parameters.

Example:

#!/bin/bash
MY_PARAM=${1:default}
echo $MY_PARAM

Format:

${num:-value} # num: Indicates the default value for the number of times the parameter is set; value represents the value of the default parameter.Note that there is a -

Default parameters, of course, refer to the use of default parameters when no value is passed to a parameter at a specified location, and if a value is passed, the passed parameter is used.

30.array:

Example:

#!/bin/bash
FRUITS=(Mango Banana Apple)
echo ${FRUITS[*]}
echo $FRUITS[*]
echo $FRUITS[2]
FRUITS[3]=Orange
echo $FRUITS[*]

output:

Mango Banana Apple
Mango[]
Mango[2]
Mango[]

There are several ways to declare array variables:

 1 #!/bin/bash
  2 FRUITS=(Mango Banana Apple)
  3 echo ${FRUITS[*]}
  4 echo $FRUITS[*]
  5 echo $FRUITS[2]
  6 FRUITS[3]=Orange
  7 echo $FRUITS[*]
  8 declare -a fruit=('Mango' 'Banana' 'Apple' 'Orange' 'Papaya')
  9 declare -a array_name=(word1 word2 word3 word4) #Note: The left and right sides of the equal sign cannot have spaces.
 10 echo ${fruit[0]}
 11 echo ${fruit[1]}
 12 echo "all the fruits are ${fruit[*]}"
 13 echo "the number of elements in the array are ${#fruit[*]}"
 14 unset fruit

There is another way to initialize:
$countries=(USA [3]=UK [2]=Spain)