bash.Variables

Bash parameter expansion is a powerful feature that allows you to manipulate and retrieve the value of variables in a variety of ways. Here’s a detailed explanation of some common and advanced forms of parameter expansion:

1. Default Values#

• ${VARIABLE:-default}: If VARIABLE is unset or null, it substitutes default. Otherwise, it returns the value of VARIABLE.

  myvar=
  echo "${myvar:-Hello}"  # Outputs "Hello" because myvar is empty

• ${VARIABLE:=default}: If VARIABLE is unset or null, it sets it to default and returns the value.

  echo "${myvar:=Hello}"  # myvar is set to "Hello"

• ${VARIABLE:+value}: If VARIABLE is set and not null, it returns value. Otherwise, it returns nothing.

  myvar="world"
  echo "${myvar:+Hello}"  # Outputs "Hello"

• ${VARIABLE:?message}: If VARIABLE is unset or null, it prints an error message (message) to stderr and exits the script. If no message is provided, a default message is shown.

  echo "${myvar:?myvar is not set}"  # Prints an error if myvar is not set

2. Substring Operations#

• ${VARIABLE:offset}: Extracts a substring starting from offset. If offset is negative, it starts from the end of the string.

  myvar="abcdef"
  echo "${myvar:2}"  # Outputs "cdef" (starts from index 2)

• ${VARIABLE:offset:length}: Extracts a substring starting from offset with the specified length.

  echo "${myvar:1:3}"  # Outputs "bcd" (3 characters starting from index 1)

3. String Length#

• ${#VARIABLE}: Returns the length of the string stored in VARIABLE.

  echo "${#myvar}"  # Outputs "6" (length of "abcdef")

4. Pattern Substitution#

• ${VARIABLE#pattern}: Removes the shortest match of pattern from the start of VARIABLE.

  myvar="file.txt"
  echo "${myvar#*.}"  # Outputs "txt" (removes "file.")

• ${VARIABLE##pattern}: Removes the longest match of pattern from the start.

  myvar="path/to/file.txt"
  echo "${myvar##*/}"  # Outputs "file.txt" (removes "path/to/")

• ${VARIABLE%pattern}: Removes the shortest match of pattern from the end of VARIABLE.

  echo "${myvar%.txt}"  # Outputs "file" (removes ".txt")

• ${VARIABLE%%pattern}: Removes the longest match of pattern from the end.

  myvar="path/to/file.txt"
  echo "${myvar%%/*}"  # Outputs "path" (removes everything after the first "/")

5. Search and Replace#

• ${VARIABLE/pattern/replacement}: Replaces the first match of pattern with replacement.

  myvar="apple, orange, apple"
  echo "${myvar/apple/grape}"  # Outputs "grape, orange, apple"

• ${VARIABLE//pattern/replacement}: Replaces all matches of pattern with replacement.

  echo "${myvar//apple/grape}"  # Outputs "grape, orange, grape"

• ${VARIABLE/#pattern/replacement}: If pattern matches the beginning of VARIABLE, replace it with replacement.

  echo "${myvar/#apple/grape}"  # Replaces only if "apple" is at the start

• ${VARIABLE/%pattern/replacement}: If pattern matches the end of VARIABLE, replace it with replacement.

  echo "${myvar/%apple/grape}"  # Replaces only if "apple" is at the end

6. Remove Variable Content#

• ${VARIABLE@operator}: An advanced feature that applies transformations based on the operator. Common ones are:

  • @Q: Quote the value (useful for shell-safe strings).
  • @A: Convert to an array.

  myvar="Hello"
  echo "${myvar@Q}"  # Outputs "'Hello'" (quoted)

Practical Examples#

1. Setting Default Values#

user="${USER:-defaultuser}"

2. Check if a Variable is Empty#

: "${myvar:?myvar is required but not set}"

3. String Manipulation#

• Remove file extension:

  filename="document.txt"
  echo "${filename%.txt}"

• Get the last part of a path:

  path="/usr/local/bin"
  echo "${path##\*/}" # Outputs "bin"