How do I remove the header in the df command?

I normally use one of these options, if I have no reason to use awk:

df | sed 1d 

The 1d option to sed says delete the first line, then print everything else.

df | tail -n+2 

the -n+2 option to tail say start looking at line 2 and print everything until End-of-Input.

I suspect sed is faster than awk or tail, but I can't prove it.
EDIT If you want to use awk, this will print every line except the first:

df | awk '{if (FNR>1) print}' 

FNR is the File Record Number. It is the line number of the input. If it is greater than 1, print the input line.

Use tail -n +2 to remove the first line in df (or any other command) output:

df | tail -n +2 | sort -r -k3 -n 

Explanation: tail -n +N outputs starting with line N.

In a more general case, in order to remove the first lines of any output, you can use the tail -n +N to start with line N:

command | tail -n +2 | other_command 

Skip the first line, like this:

df | awk 'NR>1 {print $1 | "sort -r -k3 -n"}' 

Count the lines from the output of df with wc and then substract one line to output a headerless df with tail ...

LINES=$(df|wc -l) LINES=$((${LINES}-1)) df | tail -n ${LINES} 

OK - I see oneliner - Here is mine ...

DF_HEADERLESS=$(LINES=$(df|wc -l); LINES=$((${LINES}-1));df | tail -n ${LINES}) 

And for formated output lets printf loop over it...

printf "%s\t%s\t%s\t%s\t%s\t%s\n" ${DF_HEADERLESS} | awk '{print $1 | "sort -r -k3 -n"}' 

This might help with GNU df and GNU sort:

df -P | awk 'NR>1{$1=$1; print}' | sort -r -k3 -n | awk '{print $1}' 

With GNU df and GNU awk:

df -P | awk 'NR>1{array[$3]=$1} END{PROCINFO["sorted_in"]="@ind_num_desc"; for(i in array){print array[i]}}' 

Documentation: 8.1.6 Using Predefined Array Scanning Orders with gawk

Removing something from a command output can be done very simply, using grep -v, so in your case:

df | grep -v "Filesystem" | ... 

(You can do your awk at the ...)

When you're not sure about caps, small caps, you might add -i:

df | grep -i -v "FiLeSyStEm" | ... 

(The switching caps/small caps are meant as a clarification joke :-) )