Bash conditional expression not working

Question

$ echo $(adb shell getprop service.adb.root)
1
$ while [[ $(adb shell getprop service.adb.root) != "1" ]]; do echo -n .; done
.........^C

[[ "1" != "1" ]] should be false and the while loop should not run. But, it runs forever. What's the reason and the fix?

I am trying to write a loop that will wait until the adbd daemon on the connected Android device restarts as root.

Can you show output of: echo $(adb shell getprop service.adb.root) | cat -vte — anubhava
– anubhava, Commented Feb 12, 2014 at 21:29
The correct loop is as follows. When adb is restarting on the device (as root), the adb shell command will print errors. This loop discards the error messages. while [[ $(adb shell getprop service.adb.root 2> /dev/null) != $'1\r' ]]; do echo -n .; done — apricot
– apricot, Commented Feb 12, 2014 at 22:08

larsks · Accepted Answer · 2014-02-12 21:30:47Z

3

The result from the getprop command contains a carriage return (ASCII 0x0d). Since you're making a string -- not a numeric -- comparison, you're effectively running:

[[ "1" != "1\r" ]]

The simplist way to fix this is to explicitly strip the carriage return:

x=$(adb shell getprop  service.adb.root | tr -d '\015')

Now your comparison should work.

You can see exactly what getprop is returning by running:

$ adb shell getprop  service.adb.root | od -c
0000000   1  \r  \n
0000003

answered Feb 12, 2014 at 21:30

larsks

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1 Comment

apricot Over a year ago

I am accepting this answer since it explains the problem, but I like the simpler fix provided by anubhava.

anubhava · Accepted Answer · 2014-02-13 06:55:34Z

3

Since output of adb command has trailing \r you can use this while loop instead:

while [[ $(adb shell getprop service.adb.root) != $'1\r' ]]; do echo -n .; done

i.e. compare numbers not strings.

edited Feb 13, 2014 at 6:55

answered Feb 12, 2014 at 21:39

anubhava

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8 Comments

John Kugelman Over a year ago

Why? What's the reason?

apricot Over a year ago

I like your fix, but accepted the other answer because it explained the problem. My low rep does not allow me to vote for your answer.

anubhava Over a year ago

This is to make sure any string starting with "1" is matched as I wrote in my answer.

chepner Over a year ago

Even simpler, use (( $(adb ...) != 1 )); the arithmetic context will cause the trailing non-digits from the output of adb to be ignored.

glenn jackman Over a year ago

1* is overly broad: it will return true if the abd command returns "12". Since this is bash, you can check for a CR directly: [[ $out == $'1\r' ]]

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