BASH: how to pass a default argument if no arguments after the first were passed

Question

I have a bash script with a case statement in it:

case "$1" in 
    bash)
        docker exec -it $(docker-compose ps -q web) /bin/bash
        ;;
    shell)
        docker exec -it $(docker-compose ps -q web) python manage.py shell
        ;;
    test)
        docker exec -it $(docker-compose ps -q web) python manage.py test "${@:2}"
        ;;
esac

On the test command, I want to pass the default argument of apps, but only if the user didn't pass any arguments other than test to the bash script.

So, if the user runs the script like this:

./do test

it should run the command

docker exec -it $(docker-compose ps -q web) python manage.py test apps

However, if they run the script like this:

./do test billing accounts

it should run the command

docker exec -it $(docker-compose ps -q web) python manage.py test billing accounts

How can I test for the existence of arguments after the first argument?

[[ $# -ne 1 ]]... or, more correctly, (( $# != 1 ))

jasonwryan
– jasonwryan

2016-02-01 21:44:56 +00:00
Commented Feb 1, 2016 at 21:44 — jasonwryan
– jasonwryan, Commented Feb 1, 2016 at 21:44

Costas · Accepted Answer · 2016-02-02 08:34:59Z

8

I'd try to use bash variable substitution:

   test)
        shift
        docker exec -it $(docker-compose ps -q web) python manage.py test "${@-apps}"
        ;;

Other way is to check $* instead of $1:

case $* in 
    bash)
         ...
    test)
         docker exec -it $(docker-compose ps -q web) python manage.py test apps
         ;;
    test\ *)
         docker exec -it $(docker-compose ps -q web) python manage.py test "${@:2}"
         ;;

edited Feb 2, 2016 at 8:34

answered Feb 1, 2016 at 22:25

Costas

15.1k24 silver badges38 bronze badges

Did that edit mess it up? Should there be a : in "${@:-apps}"?

synic
– synic

2016-02-01 22:32:51 +00:00
Commented Feb 1, 2016 at 22:32
2

@synic - the : will default even for a null valued second argument, and so doesnt answer the question asked. Without the : it will default only if there is no second argument.

mikeserv
– mikeserv

2016-02-01 22:34:15 +00:00
Commented Feb 1, 2016 at 22:34
1

The test of $@ doesnt test for two arguments (and the white space would need escaping in the pattern anyway).

mikeserv
– mikeserv

2016-02-02 08:30:44 +00:00
Commented Feb 2, 2016 at 8:30
@mikeserv Do you mean to change to $*?

Costas
– Costas

2016-02-02 08:36:11 +00:00
Commented Feb 2, 2016 at 8:36
Not exactly. See the comments on BinaryZebra's answer. It still doesnt test for two arguments. It looks for a concatenation of argument(s) that starts with the pattern test\ . And so if the first argument starts w/ test\ it means nothing. Both of you guys should be using ? anyway. But it isnt a robist test regardless.

mikeserv
– mikeserv

2016-02-02 12:12:40 +00:00
Commented Feb 2, 2016 at 12:12

| Show 1 more comment

mikeserv · Accepted Answer · 2016-02-01 22:37:01Z

2

case $1:$# in
(test:1)
    docker $(this is bad) python test apps;;
(test:$#)
    docker $(still bad) python "$@";;
(bash:$#)
    docker ...
esac

answered Feb 1, 2016 at 22:37

mikeserv

59.4k10 gold badges123 silver badges244 bronze badges

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Charles Duffy · Accepted Answer · 2016-02-02 01:04:30Z

2

something like this would work well

if [ -z "$2" ]
  then
    echo "No argument supplied"
fi

edited Feb 2, 2016 at 1:04

Charles Duffy

1,92018 silver badges22 bronze badges

answered Feb 1, 2016 at 21:48

Justin Matthew Roberts

315 bronze badges

1

I'd rather say [ "$#" -ne "2" ] , as you could very well pass an empty 2nd argument : myscript foo "" bar baz # has 4 args, and the arg $2 is empty

Olivier Dulac
– Olivier Dulac

2016-02-02 15:52:43 +00:00
Commented Feb 2, 2016 at 15:52
@OliverDulac - i dont think [ 2 -ne "$#" ] is a very useful test here. It negates every number of args but 2.

mikeserv
– mikeserv

2016-02-02 17:53:23 +00:00
Commented Feb 2, 2016 at 17:53
@mikeserv: the poster tests only the 2nd arg, and say "no argument" (without s). If there can be more than 2, then if [ "$#" -lt 2 ]; then echo "Not enough arguments supplied: you need at least 2"; fi will be fine. The point of my comment being still the same: testing for the 2nd argument's length doesn't tell you how many arguments were supplied...

Olivier Dulac
– Olivier Dulac

2016-02-02 18:28:07 +00:00
Commented Feb 2, 2016 at 18:28
@OliverDulac - true. The absence of a second argument does, though. [ "${2+:}" ] is probably the most simple way to robustly test for 2 or more arguments.

mikeserv
– mikeserv

2016-02-02 19:07:16 +00:00
Commented Feb 2, 2016 at 19:07

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technosaurus · Accepted Answer · 2016-02-02 02:11:59Z

2

You can just reset your args however you want using set ...

case "$1" in 
    #...
    test)
        [ $# -lt 2 ] && set test apps
        docker exec -it $(docker-compose ps -q web) python manage.py $@
        ;;
esac

edited Feb 2, 2016 at 2:11

answered Feb 2, 2016 at 0:55

technosaurus

1413 bronze badges

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score 2 · Accepted Answer · 2016-02-05 16:05:42Z

2

If arguments do not have spaces or newlines.
Transform the arguments into an string: "$*", and use that:

f(){ docker exec -it $(docker-compose ps -q web) "$@"; }

case "$*" in
    bash)       f /bin/bash                    ;;
    shell)      f python manage.py shell       ;;
    test)       f python manage.py test apps   ;;
    test\ ?*)   f python manage.py "$@"        ;;
esac

Using a function to manage code (not a variable) and remove repetition.

edited Feb 5, 2016 at 16:05

answered Feb 2, 2016 at 3:15

user79743

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