CHEATSHEET.md

README

My personal cheatsheet for simple examples

NOTE: Each script should be checked in bash, zsh and sh

Man pages

export MANPAGER=cat
man git

Arrays

Dealing with arrays

my_array=('hello', 'world')
echo "size:${#my_array[@]}"

my_array+=("again")
echo "size:${#my_array[@]}"

Loops

For Loops

A for loop with count

for index in $(seq 0 10 ); 
do
  # break from a for loop
  if [[ $index == 5 ]]; then break; fi

  # continue the loop
  if [[ $index == 1 ]]; then continue; fi

  echo "Sleep $index"
  sleep 1

done
# index exists outside of loop
echo "Exited loop $index"

Iterate over a list in a for loop

for index in hello world tomorrow is thursday; 
do
  echo "Sleep $index"
  sleep 1
done
# index exists outside of loop
echo "Exited loop $index"

files=(
  '../../output/fx/blending_difference.mp4'
  '../../output/fx/blending_mask_difference.mp4'
  '../../output/fx/blending_grainmerge.mp4' 
)
for file in "${files[@]}"; do
  echo "$file"
done

While Loops

A while loop

_continue=true
_count=0
while $_continue; 
do
  _count=$(( _count + 1 ))

  # continue the loop
  if [[ $_count == 2 ]]; then continue; fi

  # break from a while loop (uncomment one or other)
  #if [[ $_count == 5 ]] break
  if [[ $_count == 5 ]]; then break; fi

  echo "$_count"
done
# index exists outside of loop
echo "Exited loop $_count"

Loop over data structures

NOTE: When using readline loops you might need to pipe in < /dev/null to ensure the input characters from the list are not used as input. This happens with ffmpeg.

heredoc tempfile

TEMPFILE=$(mktemp)
cat <<- EOF > ${TEMPFILE}
4922049292,us-east-1,filename1.txt
3420302882,eu-west-1,filename2.txt
1334423422,ap-north-1,filename3.txt
EOF
while IFS=, read -r value1 value2 value3
do
    echo "$value1 $value2 $value3"
done < $TEMPFILE

heredoc pipe

while IFS=, read -r value1 value2 value3
do
    echo "$value1 $value2 $value3"
done << EOF
4922049292,us-east-1,filename1.txt
3420302882,eu-west-1,filename2.txt
1334423422,ap-north-1,filename3.txt
EOF

process redirection pipe

while IFS=, read -r value1 
do
    echo "FILE $value1"
done < <(ls -l)

Calculations

Calculations can be performed using $(( expression ))

for index in $(seq 0 10 ); 
do
  # use modulus
  echo $(( index % 2 * (10 + 1) ))

  # calculations in if blocks
  if [[ $(( index - 5 )) -ge 0 ]]; then
    echo "condition met $(( index - 5 ))"
  else
    echo "condition not met $(( index - 5 ))"
  fi
done

Compound Conditions

TEST1=true
TEST2=true

if [ $TEST1 == true ] && [ $TEST2 == true ]; then
  echo "Condition is true"
else
  echo "Condition is false"
fi

Resources