What does $a and -e mean in sed? [closed]

Question

[desktop ~]$ cat sa
sanmesh
shammas
sujil
givin
nepo
bijal

[desktop ~]$ sed -i -e '1i Header' -e '$a Trailor' sa

[desktop ~]$ cat sa
Header
sanmesh
shammas
sujil
givin
nepo
bijal
Trailor

Answer 1

sed -i -e '1i Header' -e '$a Trailor' sa

-i:
Edits the file in place

-e script:
Add the script to the commands to be executed

'1i Header':
Match 1st line ('1') and insert ('i') 'Header'

'$a Trailor':
Match last line ('$') and append ('a') 'Trailor'