bash - scripting

Now that all platforms (beside the apple ones) can be built on linux, I want a fabulous build-all script, that automagically creates a folder with all runtimes. Time to refresh my bash-basics.

And to say it from the start: Spaces do matter!!

• How to set a variable

    # GOOD:
    varname="fortuna"
    # WRONG:
    varname = "fortuna"

• Show argument count

  #!/bin/bash
  echo number of arguments: $#
  echo script itself: $0
  echo arg1:$1 arg2:$2 arg3:$3  ...

• basic conditionals for number comparison with if-elif-else-construct using some checks: -eq, -ne, -lt, -le, -gt, -ge

  #!/bin/bash
  
  #equals
  if [[ $# -eq 0 ]]; then
    echo "NO ARGS"
  elif [[ $# -eq 1 ]]; then
    echo "ONE ARGUMENT"
  else
    echo "$# Arguments"
  fi
  
  # not equals
  if [[ $# -ne 1 ]]; then
    echo "arg-count != 1"
  else
    echo "arg-count == 1"
  fi
  
  # check if a directory does NOT exist
  if [[ ! -d "linux" ]]; then
    echo "linux-directory does not exist"
  fi

  For string-comparison use: =,!=

• Some useful directory-magic:

  # get the directory from which the script is called
  CURRENT_DIR=$PWD
  
  # get the directory where the script is located (relative)
  REL_BASEDIR=$(dirname $0)
  
  # get the directory where the script is located (absolute)
  ABS_BASEDIR=$( cd $(dirname $0) ; pwd )
  
  # get the directory where the script is located (absolute with resolved symlinks)
  ABS_BASEDIR_RESOLVED=$( cd $(dirname $0) ; pwd -P )

• Check if something went wrong (e.g. with cmake).

  cmake -DCMAKE_BUILD_TYPE=Release .. || {
    echo "Something went wrong with CMAKE! Exiting..."
    exit 1;
  }
  
  echo "Everything went fine!"

• exit the script:

  # just exit with code 0
  exit
  
  # give it a special exit-code you can use on the shell
  exit 95

  There are some special exit-codes

• creating functions:

  #!/bin/bash
  function myEcho {
      echo $1
  }  
  myEcho Hello

• Prevent a command to output errors

  # if a.txt is not present do not output any error
  rm a.txt 2>/dev/null

Links:
* Bash Conditionals http://www.gnu.org/software/bash/manual/bash.html#Bash-Conditional-Expressions
* Testing exit values https://sanctum.geek.nz/arabesque/testing-exit-values-bash/
* Functions http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO-8.html