shell programming basics (variables) - Linux

majun@instance-zqtg07w6:~$ echo $BASH
/bin/bash
majun@instance-zqtg07w6:~$ echo $BASH_VERSION
4.3.48(1)-release


majun@instance-zqtg07w6:~$ cd /etc/systemd/network/
majun@instance-zqtg07w6:/etc/systemd/network$ pwd
/etc/systemd/network
majun@instance-zqtg07w6:/etc/systemd/network$ ls
majun@instance-zqtg07w6:/etc/systemd/network$ cd ~
majun@instance-zqtg07w6:~$ CDPATH="/etc/systemd/"
majun@instance-zqtg07w6:~$ cd network
/etc/systemd/network

majun@instance-zqtg07w6:~$ echo $EUID
1001


majun@instance-zqtg07w6:~$ cat return.sh
#!/bin/bash
function func_01 {
        #return 100
        echo $FUNCNAME
}
func_01
echo $?

majun@instance-zqtg07w6:~$ bash return.sh
func_01
0

majun@instance-zqtg07w6:~$ bash return.sh
func_01
0
majun@instance-zqtg07w6:~$ echo $HISTCMD
390
majun@instance-zqtg07w6:~$ echo $HISTFILE
/home/majun/.bash_history
majun@instance-zqtg07w6:~$ echo $HISTFILESIZE
10000
majun@instance-zqtg07w6:~$ echo $HISTSIZE
1000


majun@instance-zqtg07w6:~$ echo $HOSTNAME
instance-zqtg07w6
majun@instance-zqtg07w6:~$ echo $HOSTTYPE
x86_64
majun@instance-zqtg07w6:~$ echo $MACHTYPE
x86_64-pc-linux-gnu
majun@instance-zqtg07w6:~$ echo $LANG
en_US.UTF-8
majun@instance-zqtg07w6:~$ echo $PWD
/home/majun
majun@instance-zqtg07w6:~$ echo $OLDPWD
/etc/systemd/network
majun@instance-zqtg07w6:~$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
majun@instance-zqtg07w6:~$



majun@instance-zqtg07w6:/root$ cd
majun@instance-zqtg07w6:~$ echo *
co.txt export.sh f.sh helloworld.sh read.sh return.sh shell shift_01.sh shift_02.sh txt
majun@instance-zqtg07w6:~$
majun@instance-zqtg07w6:~$ echo \*
*
majun@instance-zqtg07w6:~$ cmd='ls'
majun@instance-zqtg07w6:~$ echo $cmd
ls
majun@instance-zqtg07w6:~$ echo "$cmd"
ls
majun@instance-zqtg07w6:~$ echo '$cmd'
$cmd
majun@instance-zqtg07w6:~$ `ll`
ll: command not found
majun@instance-zqtg07w6:~$ `ls`
co.txt: command not found
majun@instance-zqtg07w6:~$ `whoami`
majun: command not found
majun@instance-zqtg07w6:~$ date
Tue Dec 31 10:19:23 CST 2019
majun@instance-zqtg07w6:~$ date
Tue Dec 31 10:19:25 CST 2019
majun@instance-zqtg07w6:~$ date
Tue Dec 31 10:19:26 CST 2019
majun@instance-zqtg07w6:~$ time=`date`
majun@instance-zqtg07w6:~$ echo $time
Tue Dec 31 10:19:47 CST 2019
majun@instance-zqtg07w6:~$ time=$(date)
majun@instance-zqtg07w6:~$ echo $time
Tue Dec 31 10:20:13 CST 2019
majun@instance-zqtg07w6:~$ LS=`ls -al`
majun@instance-zqtg07w6:~$ echo $LS
total 80 drwxrwxr-x 5 majun majun 4096 Dec 30 16:02 . drwxrwxrwx 5 root root 4096 Dec 25 21:02 .. -rw------- 1 majun majun 12533 Dec 31 10:20 .bash_history -rw-rw-r-- 1 majun majun 1864 Dec 31 10:21 co.txt -rw-rw-r-- 1 majun majun 22 Dec 30 12:10 export.sh -rw-r--r-- 1 root root 56 Dec 27 18:26 f.sh -rwxr-xr-x 1 root root 32 Dec 27 
majun@instance-zqtg07w6:~$ echo "$LS"
total 80
drwxrwxr-x 5 majun majun  4096 Dec 30 16:02 .
drwxrwxrwx 5 root  root   4096 Dec 25 21:02 ..
-rw------- 1 majun majun 12533 Dec 31 10:20 .bash_history
-rw-rw-r-- 1 majun majun  1864 Dec 31 10:21 co.txt
-rw-rw-r-- 1 majun majun    22 Dec 30 12:10 export.sh
-rw-r--r-- 1 root  root     56 Dec 27 18:26 f.sh

majun@instance-zqtg07w6:~$ $(ls)
co.txt: command not found
majun@instance-zqtg07w6:~$ echo $(ls)
co.txt export.sh f.sh helloworld.sh read.sh return.sh shell shift_01.sh shift_02.sh txt
majun@instance-zqtg07w6:~$ ls
co.txt     f.sh           read.sh    shell        shift_02.sh
export.sh  helloworld.sh  return.sh  shift_01.sh  txt
majun@instance-zqtg07w6:~$ ls|sed -n '1p'
co.txt
majun@instance-zqtg07w6:~$ ls|sed -n '2p'
export.sh
majun@instance-zqtg07w6:~$ ls|sed -n '3p'
f.sh

All scripts are in practice:
https://github.com/SaltNego/Learn_linux_bash