Use xargs if IF statements is inside the program code

Question

There is a program like this:

while true
do
    echo 'Your pass: '
    read password
    if [ $password == 'qwerty' ]; then 
        echo 'Nice!'
        break
    fi
done

I can use xargs only if the program has arguments. But in this case ./program.sh password doesn't work. I have a list of password and I need to send them in loop to this program.

xargs -a list ./program.sh doesn't work.

Note that this outputs Nice! if the user enters qwerty, but also if they enter " qwerty " (leading or trailing space or tabs with the default value of $IFS) or q\w\e\r\t\y... See Understanding "IFS= read -r line", and also if they enter -d / -o y as your forgot to quote $password. — Stéphane Chazelas, Apr 19 '18 at 10:11
Besides the quoting issue raised by Stéphane (serious hole, btw, as it doesn't really matter what the password is), it would also be better if you do stty -echo before the read and stty echo after it, so the password is not displayed in the terminal. — JoL, Apr 19 '18 at 17:11

score 11 · Answer 1 · edited Apr 19 '18 at 10:14

11

There's no reason to use xargs here, you can simply do:

while IFS= read -r password
do
    if [ "$password" = 'qwerty' ]; then 
        echo 'Nice!'
        break
    fi
done

And then run it as:

./program.sh < list

If you really want xargs, you can do something like:

for password do
  case "$password" in
    'qwerty')
      echo 'Nice!'
      ;;
  esac
done

And then:

xargs -rd '\n' -a list ./program.sh

edited Apr 19 '18 at 10:14

Stéphane Chazelas

544,893

answered Apr 19 '18 at 09:42

terdon

242,166

@StéphaneChazelas thanks for the edit, I should have done most of those myself. Why change the "$@" though? Is $@ not portable? And why have them on the same line and without a ; before do? – terdon Apr 19 '18 at 10:23
1

for i do is the Bourne/POSIX<2016 syntax. for i\ndo is the most portable if you ever come across ancient BSDs. for i; do is now also standard with the 2016 edition of the standard. for i in "$@" has a number of issues with a number of shells in corner cases. It's discussed in some Q&A here I beleive. – Stéphane Chazelas Apr 19 '18 at 10:32
2

Note that neither -a, -r nor -d are standard xargs options. – Stéphane Chazelas Apr 19 '18 at 10:33
@StéphaneChazelas I'm having trouble finding that Q&A on the issues of for i in "$@"; do vs for i; do. It sounds very interesting, as I thought they'd have an identical effect. Would you do the favour of linking it for all of us? – JoL Apr 19 '18 at 20:32
1

@StéphaneChazelas Nevermind. I think I found it here. – JoL Apr 19 '18 at 20:44

Use xargs if IF statements is inside the program code

1 Answers1