SHELL

bash lowercase

Just use tr command to transform lowercase letters into uppercase as:
tr a-z A-Z < file.txt	#transforms letters into uppercase in a file
echo 'HELLO' | tr A-Z a-z	#Outputs: 'hello'

bash variable lowercase

# In Bash 4.0
a=DHCP
echo "${a,,}"
# Output:
dhcp

Bash - Convert a string from uppercase to lowercase

$ echo "$a" | tr '[:upper:]' '[:lower:]'
hi all