Bash

• command or error

  <command> || {
      echo "Please install <package>"
      exit 1
  }
  

arrays Link to heading

• defining: fruits=("apple" "banana" "cherry")
• accessing by index: echo "${fruits[0]}"
• accessing all: echo "${fruits[@]}"
• length: echo "${#fruits[@]}"
• concatenating: echo "${fruits[*]}"
• appending: fruits+=("durian")
• appending multiple: fruits+=("durian" "elderberry")

fruits=("apple" "banana" "cherry")
echo "${fruits[0]}"
# apple

for f in "${fruits[*]}"; do
    echo "$f"
done
# apple banana cherry

more_fruits=("durian" "elderberry")
fruits+=("${more_fruits[@]}")

for f in "${fruits[@]}"; do
    echo "$f"
done
# apple
# banana
# cherry
# durian
# elderberry

if Link to heading

if [[ <condition> ]]; then
    <command>
elif [[ <condition> ]]; then
    <command>
else
    <command>
fi

Conditions Link to heading

Files/dirs Link to heading

• [[ -e <file> ]] file exists
• [[ -d <dir> ]] dir exists

Strings Link to heading

• [[ -z <string> ]] string is empty
• [[ -n <string> ]] string is not empty

Loops Link to heading

for Link to heading

fruits=("apple" "banana" "cherry")
for f in "${fruits[@]}"; do
    echo "$f"
done

sed Link to heading

• insert a line at the beginning of a file

  sed -i '1i\<lines>' "<file>"
  

• find and sed multiple files

  find "<dir>" -type f -exec sed -i "s|<search>|<replace>|" {} +
  

Substitution Link to heading

• ${<variable>} = variable substitution
  • same as $variable, but useful when doing string manipulation

  • text="hello"
    echo "${text:1:3}"
    # ell
    

• $(<command>) = command substitution

  • echo "Today is $(date +%Y-%m-%d)"
    ``