Why doesn't command substitution work? - bash

Question

I have a bash script that calls "curl" to post parameters. Some of these parameters have spaces in their values, so we need to surround it with quotes when passing it to curl. Otherwise, only the first part before the space is passed as a parameter.
I tried storing the command in a variable, using escaped quotes, but I cannot get it to work in command substitution except by using eval. I can also get it working by calling command substitution directly on the string (and not storing the command in a variable).
Is it possible to store the command in a variable and use command substitution on that variable? This is what I am trying - and failing - to do in attempt 1 in the code below. I don't understand why it doesn't work:
#!/bin/bash
yesterday=$(date +"%Y-%m-%d %H:%M:%S" -d "1 day ago") #2013-09-04 01:15:51
now=$(date +"%Y-%m-%d %H:%M:%S" ) #2013-09-05 01:15:51
echo -e "INPUTS: Today: $now, Yesterday: $yesterday"
#Attempt 1: Does not work
cmd="curl -s -u scott:tiger -d url=http://localhost:8080 -d \"start=$yesterday\" -d \"end=$now\" -d \"async=y\" http://192.168.1.46:8080/cmd/doSendMinimalToServer"
output=$($cmd)
echo "Output from executing cmd variable: $output"
#Result: The quotes get passed into the HTTP POST request. This is not good.
#Attempt 2: Using eval on the variable. Works.
output_eval=$(eval $cmd)
echo "Output from eval'ing variable: $output_eval"
#Result: This works, but I would prefer not to use eval
#Attempt 3: Using Command substitution directly on the string. Works.
output_direct=$(curl -s -u scott:tiger -d url=http://localhost:8080 -d "start=$yesterday" -d "end=$now" -d "async=y" http://192.168.1.46:8080/cmd/doSendMinimalToServer)
echo "Output from executing string: $output_direct"
#Result: This works. The HTTP POST parameters correctly have spaces in their values and no quotes.
I have also tried passing in the parameters as an array, unsuccessfully.

Answer 1

Store your command arguments in an array and run it like this:
#!/bin/bash
yesterday=$(date +"%Y-%m-%d %H:%M:%S" -d "1 day ago") # 2013-09-04 01:15:51
now=$(date +"%Y-%m-%d %H:%M:%S" ) #2013-09-05 01:15:51
echo -e "INPUTS: Today: $now, Yesterday: $yesterday"
cmd=(curl -s -u scott:tiger -d url=http://localhost:8080 -d "start=$yesterday" -d "end=$now" -d "async=y" "http://192.168.1.46:8080/cmd/doSendMinimalToServer")
output=$("${cmd[#]}")
echo "Output from executing cmd variable: $output"
Also I think you could use date '+%F %T' for simplicity.