remove all lines taring with #
sed '/^#/d' $FILE
show everything in file after match
sed '1,/$MATCH/d' $FILE
remove first line in file
tail -n +2 $FILE
Warning
THIS WILL GIVE YOU AN EMPTY FILE!
tail -n +2 "$FILE" > "$FILE"
The reason is that the redirection (>) happens before tail is invoked by the shell.
Use:
tail -n +2 "$FILE" > "$FILE.tmp" && mv "$FILE.tmp" "$FILE"
The && will make sure that the file doesn't get overwritten when there is a problem.