BashFAQ/101

Common utility functions (warn, die)

(If you were looking for option processing, see BashFAQ/035.) Bash and sh don't offer a die builtin command like Perl does, but it's common to use a die function in scripts. You just have to write one yourself. Most people who write a die function like to keep it simple. There are two common varieties: one that only takes a message to print, and one that takes a message and an exit status value.

# Usage: die message ...
die() {
  printf '%s\n' "$*" >&2
  exit 1
}

# Usage: die exit_status message ...
die() {
  rc=$1
  shift
  printf '%s\n' "$*" >&2
  exit "$rc"
}

Some people like fancier functions. If you like fancier functions, here are some variants:

##
# warn: Print a message to stderr.
# Usage: warn "format" ["arguments"...]
#
warn() {
  local fmt=$1
  shift
  printf "script_name: $fmt\n" "$@" >&2
}

###
### The following three "die" functions
### depend on the above "warn" function.
###

##
# die (simple version): Print a message to stderr
# and exit with the exit status of the most recent
# command.
# Usage: some_command || die "message" ["arguments"...]
#
die () {
  local st=$?
  warn "$@"
  exit "$st"
}

##
# die (explicit status version): Print a message to
# stderr and exit with the exit status given.
# Usage: if blah; then die status_code "message" ["arguments"...]; fi
#
die() {
  local st=$1
  shift
  warn "$@"
  exit "$st"
}

##
# die (optional status version): Print a message to
# stderr and exit with either the given status or
# that of the most recent command.
# Usage: some_command || die [status code] "message" ["arguments"...]
#
die() {
  local st=$?
  if [[ $1 != *[^0-9]* ]]; then
    st=$1
    shift
  fi
  warn "$@"
  exit "$st"
}

##
# warn: Print a message to stderr.
# Usage: warn "message"
#
warn() {
  printf '%s\n' "$@" >&2
}

##
# die (optional status version): Print a message to
# stderr and exit with either the given status or
# that of the most recent command.
# Usage: some_command || die "message" [status code]
#        some_command && die "message" [status code]

die() {
  local st=$?
  case $2 in
    *[^0-9]*|'') :;;
    *) st=$2;;
  esac
  warn "$1"
  exit "$st"
} 

Since it's your script, you get to decide how fancy you want to make it.

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