Why does my shell script fail on second attempt?

Question

This script should accept a set of search terms and return a formatted URL to search google.

$ ./google_search.sh albert einstein
https://www.google.com/search?q=albert+einstein

It does that just fine, so I decided to add an option to search a specific site, or ignore that site, using a -s or -S flag.

$ ./google_search.sh -s wikipedia.org albert einstein
https://www.google.com/search?q=albert+einstein+site%3Awikipedia.org

This works the first time you run the script, but fails on every following attempt.

$ ./google_search.sh -s wikipedia.org albert einstein
https://www.google.com/search?q=albert+einstein
$ ./google_search.sh -s wikipedia.org albert einstein
https://www.google.com/search?q=albert+einstein

Opening a new Terminal window or restarting Terminal both clear this problem and allow one more attempt before failing.

The script:

#!/bin/bash

# original source of concatenate_args function by Tyilo:
# http://stackoverflow.com/questions/9354847/concatenate-inputs-in-bash-script
function concatenate_args
{
    string=""
    ignorenext=0
    for a in "$@" # Loop over arguments
    do
        if [[ "${a:0:1}" != "-" && $ignorenext = 0 ]] # Ignore flags (first character is -)
        then
            if [[ "$string" != "" ]]
            then
                string+="+" # Delimeter
            fi
            string+="$a"
        elif [[ $ignorenext = 1 ]]
        then
            ignorenext=0
        else
            ignorenext=1
        fi
    done
    echo "$string"
}

qry="$(concatenate_args "$@")"
glink="https://www.google.com/search?q="

site=""
while getopts :s:S: opt; do
    case $opt in
        s) site="+site%3A$OPTARG" ;; 
        S) site="+-site%3A$OPTARG" ;; 
    esac
done

url=$glink$qry$site

echo $url
# open -a Firefox $url

What needs to change to make this script more reliable?

Add the `-xv` option to the shebang line to see what's going on. — choroba, Dec 06 '12 at 15:46
That doesn't seem to do anything. I'm trying `#!/bin/bash -xv`. — Jack Stout, Dec 06 '12 at 16:00
This is behaving like your are sourcing the script instead of executing it. Are you sure you are using `./google_search.sh` and not `. google_search.sh`? Try `bash google_search.sh`. — Vaughn Cato, Dec 06 '12 at 16:07
@VaughnCato You're right, I'm using an alias: `alias google='. /Tools/google_search.sh'`. I didn't realize this would behave differently. Is there a way I can use an alias without it misbehaving? — Jack Stout, Dec 06 '12 at 17:19
If the script is executable, just drop the dot and use `alias google=/Tools/google_search.sh`. If not, use `alias google='bash /Tools/google_search.sh'` — Vaughn Cato, Dec 06 '12 at 17:23
Thanks Vaughn! I just re-wrote several lines in my .bash_profile. I might add a reminder comment as well. — Jack Stout, Dec 06 '12 at 17:28
glad you got it resolved, but for future reference, `bash -v` will tell you your version. — nullrevolution, Dec 06 '12 at 19:55

Vaughn Cato · Accepted Answer · 2012-12-06T17:36:38.873

This is behaving like your are sourcing the script instead of executing it. If you use a dot and a space before the script, it causes the script to be executed line by line inside your current shell instead of creating a new shell. This allows environment variables that are changed inside the script to leak into your current shell's environment, which can make one run of the script not behave the same as the next.

In this case, it appears to be the use of getopts. getopts updates an environment variable each time it is called so it can keep track of which argument is being examined. The second time you source the script, it thinks that all the arguments have already been examined, so your argument ends up being ignored.

You've just answered what I was going to ask next, "What happens differently when a script is 'sourced'?" I'll spend plenty of time learning how that should be used. Thanks, again! — Jack Stout, Dec 06 '12 at 17:38

score 1 · Answer 2 · answered Dec 07 '12 at 00:27

Your script, simplified:

#!/bin/bash
glink="https://www.google.com/search?q="
site=""

# if you're "source"ing, uncomment the following:
# OPTIND=1

while getopts :s:S: opt; do
    case $opt in
        s) site="+site:$OPTARG" ;; 
        S) site="+-site:$OPTARG" ;; 
        ?) echo "invalid option: -$OPTARG" >&2 ;;
    esac
done
shift $((OPTIND - 1))
# the positional parameters are now clear of "-s" and "-S" options

qry=$(IFS="+"; echo "$*")
url=$glink$qry$site
echo "$url"
# open -a Firefox "$url"

Why does my shell script fail on second attempt?

2 Answers2