Ubuntu system after I install the adb is undetectable. What should I do, for a great god answer QAQ-CodePudding

Just started to learn android development, and then use the

 $sudo apt install adb

This code installed adb, but after using "the adb devices" command displays

 
Keith @ Keith - optiplex - 7070: ~ $adb devices 

The Command 'adb is not found, but can be installed with: 

Sudo apt install adb

And then I will according to clew, perform the "sudo apt" install the adb command, according to

 Keith @ Keith - optiplex - 7070: ~ $sudo apt install adb 
Is reading package list... Complete the 
Are analyzing software package dependency tree 
Is reading status information... Complete the 
The adb is already the latest version (8. 1.0 + r23-5-18.04), 
0 0 the upgrade package, the new installed a package, 0 to uninstall packages, there are five packages are not upgrade,

But after performing "the adb devices" is still

 Keith @ Keith - optiplex - 7070: ~ $sudo apt install adb 
Is reading package list... Complete the 
Are analyzing software package dependency tree 
Is reading status information... Complete the 
The adb is already the latest version (8. 1.0 + r23-5-18.04), 
0 0 the upgrade package, the new installed a package, 0 to uninstall packages, there are five packages are not upgrade, 
Keith @ Keith - optiplex - 7070: ~ $adb devices 

The Command 'adb is not found, but can be installed with: 

Sudo apt install adb

Then I used the "sudo apt - get" remove the adb command to unload
Above uninstall before installation, operation or old
Has since recourse QAQ, hope to have a great god can know the QAQ, thank you very much!!!!!!