About

Various testing on conditional events or variables (exit test, variable defaults).

Last Command Return Value

Make sure you have spaces inside the [] otherwise bash will spit the dummy!

Check for Success

if [ $? -eq 0 ]; then
  echo "Success"
else
  echo "Failure"

Check for Failure

if [ $? -ne 0 ]; then
  echo "Failure"
else
  echo "Success"

Conditionally Exit

Exit with Error in 1-Line

# Use braces
CARLA_TARBALL=`ls ${CARLA_ROOT}/Distd/*.tar.gz 2> /dev/null` || { echo "Carla tarball not found, exiting." ; exit 1; }

Conditionally Set Variables

Set if not already defined

: ${var=value}

This one is obscure....<TODO> explain!

Set if not defined OR set to the empty string

: ${var:=value}

Copy Selected Files

Recursively Copy Selected Files with Folder Structure

cp --parents `find -name .pydevproject` /target_directory/

Docker

Check if docker container with specified name exists

docker container inspect ecl >/dev/null 2>&1
if [ $? -ne 0 ]; then
  groot-rocker-workspace --colcon --name ecl --bind /mnt/mervin/workspaces/devel/ecl:/mnt/ecl --work-directory /mnt/ecl ubuntu:22.04
else
  docker container start -i ecl
fi

Random

Recursively Copy Selected Files with Folder Structure

# random number between 1 and 3
echo $((1 + $RANDOM % 3))

Install Packages

Install packages via sudo only if it is not already installed

install_package ()
{
  PACKAGE_NAME=$1
  # Does not work if the package has been installed and later removed
  # dpkg -s ${PACKAGE_NAME} > /dev/null
  dpkg-query -W -f='${Status}' ${PACKAGE_NAME} | grep -q -P '^install ok installed$'
  if [ $? -ne 0 ]; then
    sudo apt-get -q -y install ${PACKAGE_NAME} > /dev/null
  else
    pretty_print "  $(padded_message ${PACKAGE_NAME} "found")"
    return 0
  fi
  if [ $? -ne 0 ]; then
    pretty_error "  $(padded_message ${PACKAGE_NAME} "failed")"
    return 1
  fi
  pretty_warning "  $(padded_message ${PACKAGE_NAME} "installed")"
  return 0
}

# usage
install_package ansible

Technical Notes

Conditionals