UNIX for Dummies Questions & Answers

how can i make one script fail if some condition is not satisfied.i m writing if ..else logic in script.i need some standard command to do that ,,

Code:

if command; then
  : nothing
else
  rc=$?
  echo command failed: exit code $rc >&2
  exit $rc  # terminate script here and now
fi

# or
command || exit $?

# or
set -e
command

The inverted code in the if/then/else is required if you want to capture the exit code from the command. Otherwise you can simply write

Code:

if ! command; then
  echo command failed, exiting >&2
  exit 255
fi

The redirection to >&2 is customary; you want error messages on standard error, not standard output.

The shorthand with || is unobvious but quite logical (sic) once you get it.

The command set -e forces the script to exit immediately if any command fails.