varsety

varset.bash

#!/bin/bash

# flag for debug logging
DEBUG=true

# store the args passed to this script, just in case
ARGS=$@

# mark this build with an epoch. this is also the only place in the 
# file where it's necessary to use a subshell
EPOCH=$(date +%s)

########################################################################
##                            VARSETY                                 ##
########################################################################

# the name of the pipe used in place of subshells
P=$HOME/.varsety-$EPOCH

# removes the pipe file
function rm_varsety() {
  rm -f $P
}

# don't exit this script without cleaning up the pipe
trap rm_varsety SIGHUP SIGINT SIGKILL SIGTERM EXIT

# named pipe for assigning values to variables. this avoids the 
# creation of multiple subshells to run commands, thereby increasing
# the speed of this script by literally dozens of seconds
mkfifo $P

# var is a shortcut for assigning the output of a command to an
# unexported variable.
#
# the function expects at least two arguments: the name of the variable 
# and the command to execute from which to get output. the function 
# also can take n-1 arguments to account for the command's possible 
# arguments
#
# in the example below the echo statement will print "again" to the 
# console because the var function will overwrite the value of the
# already-defined hello variable
#
#     hello=world
#     var hello again
#     echo $hello
#
function var() {
    n=$1 && shift && c=$@
    if [[ $DEBUG = true ]]; then echo "varname=$n varcmd=$c "; fi
    eval "{ $c; } > $P & { read -r $n < $P; }"
}

# same as var but ony sets value if var name is yet undefined. 
#
# in the example below the echo statement will print "world" to the 
# console because the zvar function did not override the 
# already-defined hello variable
#
#     hello=world
#     zvar hello again
#     echo $hello
#
function zvar() {
    eval "if [[ -z \$$1 ]]; then var \$@; fi"
}

# VAR is a shortcut for assigning the output of a cmmand to an 
# exported variable
#
# for more information on this function please see the function var
function VAR() {
    var $@ 
    eval "export $1=\$$1"
}

# same as VAR but ony sets value if var name is yet undefined
#
# # for more information on this function please see the function zvar
function ZVAR() {
    eval "if [[ -z \$$1 ]]; then VAR \$@; export $1=\$$1; fi"
}