Technotes for future me

Shell scripting

Date Handling

Convert Date To Unix Timestamp

date -d "$date" +%s

Note that this only works for American style dates. European “25.06.2014” like dates are not supported. The simple solution is to convert them first to “2014-06-25” for example with

sed 's/\([0-9]*\)\.\([0-9]*\)\.([0-9]*\)/\3-\2-\1/'

Convert From Unix Timestamp

date -d "1970-01-01 1234567890 sec GMT"

Calculate Last Day of Month

Found here:

cal $(date "+%M %y") | grep -v ^$ | tail -1 | sed 's/^.* \([0-9]*\)$/\1/'

Lock Files

Using “flock”:

flock /tmp/myapp.lock <some command>
flock -w 10 /tmp/myapp.lock <some command>

Using “lockfile-*” commands:

lockfile-create /tmp/myapp.lock
lockfile-touch  /tmp/myapp.lock
lockfile-remove /tmp/myapp.lock

Parameter Handling


getopt is a standalone command, supporting GNU style long parameters and parameters mixed with options and can be used like this

PARAMS=`getopt -o a::bc: --long arga::,argb,argc: -n '' -- "$@"`
eval set -- "$PARAMS"

while true ; do
    case "$1" in
            case "$2" in
                "") ARG_A='some default value' ; shift 2 ;;
                *) ARG_A=$2 ; shift 2 ;;
            esac ;;
        -b|--argb) ARG_B=1 ; shift ;;
            case "$2" in
                "") shift 2 ;;
                *) ARG_C=$2 ; shift 2 ;;
            esac ;;
        --) shift ; break ;;
        *) echo "Unknown option!" ; exit 1 ;;


getopts is shell-builtin

while getopts ":ap:" opt; do
  case $opt in
      echo "Option -a ist set"
      echo "Parameter -p is given with value '$OPTARG'"
      echo "Unknown option: -$OPTARG"

shflags - portable getotps

If you ever need to port between different Unix derivates use shflags a Google library providing standard parameter handling. Example:

source shflags

DEFINE_string 'value' '0' 'an example value to pass with default value "0"' 'v'

FLAGS "$@" || exit $?
eval set -- "${FLAGS_ARGV}"

echo "${FLAGS_value}!"

Other Topics

Last updated on 31 Jan 2021
Published on 25 Dec 2019
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