Routine BASH memory aid

Some common things I do in BASH

	#Do something if a file exists
	if [ -f $target_dir"/$target_file" ]
	then do something here
	fi
	#Calculate pi using bc
	pi=`echo "4*a(1)" | bc -l`
	#Convert degs to radians
	rad=`echo "$deg*($pi/180)" | bc -l`
	#Calculate tangent in degrees
	tandeg=$(echo "s($rad)/c($rad)" | bc -l)
	#List subdirectories in pwd
	subdir=$(ls -d */)
	#Clear screen
	printf "\033c"
	#Calling python to do a math operation rather than bc , e.g. a ceil:
	round_up=$(python -c "from math import ceil; print ceil($variable)")
	#Pass arguments to script from command line
	args=("$@")
	first_arg=${args[0]}
	second_arg=${args[1]}
	#Pass number lines in file to variable
	numrecords=$(wc -l $target_dir"/file.ext" | awk -F" " '{print $1}')
	#Delete every file which is not a *.ext file
	find $target_dir -maxdepth 1 -type f ! -iname "*.ext" | xargs -I '{}' rm '{}'
	#Pipe the first two columns of file to new file
	awk '{print $1 " " $2 " " (-$3) }' $xyzfile".dat" > $xyzfile"new.dat"
	#Max and min of a column (n) in file
	n=2
	lims=$(awk 'BEGIN{ max = -999999999 ; min = 9999999999 } $n > max {max = $n} $n < min {min = $n} END{ print min, max }' $file)
	#Find a replace in file (NaNs with Os)
	sed -i 's/NaN/0/g' file.ext
	#Select a directory using zenity and pass to variable
	export my_dir=`zenity --file-selection --title="Select a Directory" --directory`

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