bash Scripting - robbiehume/CS-Notes GitHub Wiki

• https://stackoverflow.com/questions/18219262/evaluating-variables-in-a-string-in-bash
• https://stackoverflow.com/questions/16553089/dynamic-variable-names-in-bash

shift: discards the first positional parameter (``) and moves all the remaining parameters one position to the left

command: bypass aliases and functions and run built-in commands explicitly

Exit if error occurs: set -e

• Handle the exit and output message:

  • set -e
    error_handler() {   # Function to handle errors
      echo "An error occurred on line $1. Exiting..."
      exit 1
    }
    trap 'error_handler $LINENO' ERR   # Trap errors and call error_handler with the line number

• See previous command return code: $?

Output:

• Can do echo -e or printf to interpret newline characters (\n)

Variable

• Can do: my name is $NAME_VAR or my name is ${NAME_VAR}

Increment number variable

• ((num++)) Built-in variables
• $@ //all command line arguments
• $# // number of command line arguments

Print array values

• echo ${arr[0]}

User input

• read -p "Enter your name: " NAME
  • Input is stored to NAME variable
• Multiple input:
  • read name1 name2
    • Stores first entered name as name1 and second as name2

If statement

• if [ "$NAME" == "Robbie" ]
  then
      echo "Your name is Robbie"
  elif [ "$NAME" == "Jack" ]
  then
      echo "Your name is Jack"
  else
      echo "Your name is NOT Robbie or Jack"
  fi
  

Integer Comparison

• Equal/NOT equal: -eq / -ne
• Greater (Less) than: -gt / -lt
• Greater (Less) than or equal to: -ge / -le

String comparison:

• Equal: = or ==
• NOT Equal: !=
• Check if string is null (zero length): -z

File Condition Flags

• For the following flags:
  • -e : true if file exists
  • -f : true if file exists and is a regular file
  • -d : true if file exists and is a directory
  • -s : true if the file has non-zero size

Case statement

• read - p 'Are you 21 or over? Y/N "  ANSWER
  case "$ANSWER" in 
      [yY] | [yY][eE][sS])
          echo 'You can have a beer :)'
          ;;
      [nN] | [nN][oO])
          echo 'Sorry, no drinking'
          ;;
      *)
          echo 'Please enter y/yes or n/no'
          ;;
  Esac
  

For loop

• FILES=$(ls *.txt)
  for FILE in $FILES
      do 
      echo "Renaming $FILE to new-$FILE"
      mv $FILE $NEW-$FILE
  done
  

While loop

• LINE=1
  while read -r CURRENT_LINE
      do
      echo "$LINE: $CURRENT_LINE"
      ((LINE++))
  done < "./file.txt"
  

Function

• function sayHello() {   # can also do it without the function keyword
      echo "Hello World, my name is $1"
  }
    
  sayHello "Robbie"
  

• stg_release=1.0
  env=stg
  declare -n env_release="${env}_release"
  echo $env_release ${!env_release}  // outputs: 1.0 stg_release
  

• acceptable_values=("value1" "value2" "value3")
  if [[ " ${acceptable_values[@]} " =~ " ${my_var} " ]]; then 
  ...