CODEWITHSUNDEEP
linuxbashloopsbreakuntil-loop
AKS
AKS

Reputation: 17326

Until loop - Why it's still working when the condition is true

Linux: Red Hat Enterprise Linux Server release 6.5 (Santiago)

BASH: GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)

One of the VM that I just provisioned is still waiting for the DNS to update it's node entries and one the node has a NAS which is mounted on at /project_support folder. It takes around 5-10 minutes for /project_support folder to be accessible, yea bad I know.

For my automation not break due to this issue and I didn't want to add a constant sleep NNN, I used until loop so that until the mount is mounted and /project_support folder is available, I wanted to sleep for 5 seconds and check for folder existence before proceeding further in the automation script/logic.

Why the first until loop is continuing and not exiting on the condition check when cd command to a given folder is successful aka exit status 0? The second until is working as expected.

For the 2nd until loop, even if I create the folder (from a second terminal, while until loop is running), it still doesn't break the until loop.

[root@nukaserver01 ~]# cd /project_support; echo $?
0
[root@nukaserver01 project_support]# pwd
/project_support

[root@nukaserver01 project_support]# until [ `cd /project_support` ]; do echo '-- sleep 5'; sleep 5; done
-- sleep 5
-- sleep 5
-- sleep 5
^C

[root@nukaserver01 project_support]# until [ `cd /project_support-I-DONT-EXIST-AT-ALL` ]; do echo '-- sleep 5'; sleep 5; done
-bash: cd: /project_support-I-DONT-EXIST-AT-ALL: No such file or directory
-- sleep 5
-bash: cd: /project_support-I-DONT-EXIST-AT-ALL: No such file or directory
-- sleep 5
^C
[root@nukaserver01 project_support]#

Why is this issue getting fixed when I do: ?

until [ `cd /project_support && pwd` ]; do ...; ...; done

Upvotes: 2

Views: 83

Answers (2)

John Kugelman
John Kugelman

Reputation: 361749

until [ `cd /project_support` ]

The backticks capture cd's output, which is an empty string whether or not the named directory exists (error messages are printed to stderr and aren't captured). The square brackets then test if that string is empty, which it always is.

Unnecessary square brackets is a common shell scripting malady. Get rid of them and the backticks.

until cd /project_support; do
    ...
done

Note that since cd is no longer being executed in a sub-shell it will actually take effect when it succeeds. To prevent that you can add an explicit sub-shell:

until (cd /project_support); do
    ...
done

Or if you're just waiting until the directory exists, check that directly:

until [[ -d /project_support ]]; do
    ...
done

Upvotes: 4

Paul Hodges
Paul Hodges

Reputation: 15348

You are returning and testing an empty string, which always evaluates as false.

until [ "" ] ; do sleep 1; date; done
Tue, Nov 06, 2018 12:03:17 PM
Tue, Nov 06, 2018 12:03:18 PM
Tue, Nov 06, 2018 12:03:19 PM
. . .

By adding the pwd it returns the directory name/path when the cd succeeds, which as a string eval's as true.

try this -

until [[ -d /project_support ]] # ...

that will test for the existence of a directory.

Upvotes: 2

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