BASH ERROR: syntax error: operand expected (error token is ") - bash

Question

I am new to bash scripting, and I'm having an issue with one of my scripts. I'm trying to compose a list of Drivers Under 25 after reading their birthdates in from a folder filled with XML files and calculating their ages. Once I have determined they are under 25, the filename of the driver's data is saved to a text file. The script is working up until a certain point and then it stops. The error I'm getting is:
gdate: extra operand ‘+%s’
Try 'gdate --help' for more information.
DriversUnder25.sh: line 24: ( 1471392000 - )/60/60/24 : syntax error: operand expected (error token is ")/60/60/24 ")
Here is my code:
#!/bin/bash
# define directory to search and current date
DIRECTORY="/*.xml"
CURRENT_DATE=$(date '+%Y%m%d')
# loop over files in a directory
for FILE in $DIRECTORY;
do
# grab user's birth date from XML file
BIRTH_DATE=$(sed -n '/Birthdate/{s/.*<Birthdate>//;s/<\/Birthdate.*//;p;}' $FILE)
# calculate the difference between the current date
# and the user's birth date (seconds)
DIFFERENCE=$(( ( $(gdate -ud $CURRENT_DATE +'%s') - $(gdate -ud $BIRTH_DATE +'%s') )/60/60/24 ))
# calculate the number of years between
# the current date and the user's birth date
YEARS=$(($DIFFERENCE / 365))
# if the user is under 25
if [ "$YEARS" -le 25 ]; then
# save file name only
FILENAME=`basename $FILE`
# output filename to text file
echo $FILENAME >> DriversUnder25.txt
fi
done
I'm not sure why it correctly outputs the first 10 filenames and then stops. Any ideas why this may be happening?

Answer 1

You need to quote the expansion of $BIRTH_DATE to prevent word splitting on the whitespace in the value. (It is good practice to quote all your parameter expansions, unless you have a good reason not to, for this very reason.)
DIFFERENCE=$(( ( $(gdate -ud "$CURRENT_DATE" +'%s') - $(gdate -ud "$BIRTH_DATE" +'%s') )/60/60/24 ))
(Based on your comment, this would probably at least allow gdate to give you a better error message.)

Answer 2

A best-practices implementation would look something like this:
directory=/ # patch as appropriate
current_date_unix=$(date +%s)
for file in "$directory"/*.xml; do
while IFS= read -r birth_date; do
birth_date_unix=$(gdate -ud "$birth_date" +'%s')
difference=$(( ( current_date_unix - birth_date_unix ) / 60 / 60 / 24 ))
years=$(( difference / 365 ))
if (( years < 25 )); then
echo "${file%.*}"
fi
done < <(xmlstarlet sel -t -m '//Birthdate' -v . -n <"$file")
done >DriversUnder25.txt
If this script needs to be usable my folks who don't have xmlstarlet installed, you can generate an XSLT template and then use xsltproc (which is available out-of-the-box on modern opertaing systems).
That is to say, if you run this once, and bundle its output with your script:
xmlstarlet sel -C -t -m '//Birthdate' -v . -n >get-birthdays.xslt
...then the script can be modified to replace xmlstarlet with:
xsltproc get-birthdays.xslt - <"$file"
Notes:
The XML input files are being read with an actual XML parser.
When expanding for file in "$directory"/*.xml, the expansion is quoted but the glob is not (thus allowing the script to operate on directories with spaces, glob characters, etc. in their names).
The output file is being opened once, for the loop, rather than once per line of output (reducing overhead unnecessarily opening and closing files).
Lower-case variable names are in use to comply with POSIX conventions (specifying that variables with meaning to the operating system and shell have all-upper-case names, and that the set of names with at least one lower-case character is reserved for application use; while the docs in question are with respect to environment variables, shell variables share a namespace, making the convention relevant).

Answer 3

The issue was that there were multiple drivers in some files, thus importing multiple birth dates into the same string. My solution is below:
#!/bin/bash
# define directory to search and current date
DIRECTORY="/*.xml"
CURRENT_DATE=$(date '+%Y%m%d')
# loop over files in a directory
for FILE in $DIRECTORY;
do
# set flag for output to false initially
FLAG=false
# grab user's birth date from XML file
BIRTH_DATE=$(sed -n '/Birthdate/{s/.*<Birthdate>//;s/<\/Birthdate.*//;p;}' $FILE)
# loop through birth dates in file (there can be multiple drivers)
for BIRTHDAY in $BIRTH_DATE;
do
# calculate the difference between the current date
# and the user's birth date (seconds)
DIFFERENCE=$(( ( $(gdate -ud $CURRENT_DATE +'%s') - $(gdate -ud $BIRTHDAY +'%s') )/60/60/24))
# calculate the number of years between
# the current date and the user's birth date
YEARS=$(($DIFFERENCE / 365))
# if the user is under 25
if [ "$YEARS" -le 25 ]; then
# save file name only
FILENAME=`basename $FILE`
# set flag to true (driver is under 25 years of age)
FLAG=true
fi
done
# if there is a driver under 25 in the file
if $FLAG == true; then
# output filename to text file
echo $FILENAME >> DriversUnder25.txt
fi
done