Miscellaneous syntax notes for bash 3–5

notes.bash

# Use command output as a string (command substitution)

  echo "Today is $(date +%F)."

# Use the contents of a file as a string (command substitution)

  # Good:
  var=$(<stuff.txt)
  # Bad:
  var=$(cat stuff.txt)
  # Ugly:
  read var < stuff.txt

# Use a string as a file (here strings)

  # Good:
  egrep "$pattern" <<< "$string"
  # Bad:
  echo "$string" | egrep "$pattern"
  # Worse:
  echo "$string" > file; egrep "$pattern" file

# Use command output as a file (process substitution)

  diff <(curl http://example.com) <(curl http://example.org)

# Accept both `moo "string"` and `moo < file`

  moo() { local input=${@:-$(</dev/stdin)}; cowsay "$input"; }

# Display command output while also capturing it in a variable

  stdin=$(tty) && today=$(date +%F | tee $stdin)

# Silently disable xtrace

  { set +x; } 2>/dev/null

# Reversible tilde expansion -- bash 3 & 4 compatible

  echo "${PWD/#$HOME/$'~'}"
  echo "${PWD/#$'~'/$HOME}"