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Re: failglob handling


From: Chet Ramey
Subject: Re: failglob handling
Date: Fri, 3 Aug 2018 11:33:14 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 8/3/18 9:29 AM, Thomas Deutschmann wrote:
> Hi,
> 
> please see the following script:
> 
>> #!/bin/bash
>> shopt -s failglob
>> if [[ $? -ne 0 ]]; then
>>         echo 'shopt failed'
>>         exit 1
>> fi
>>
>> # Let's view current options just to be sure
>> shopt -p
>>
>> # The following glob should fail because /foo/bar does not exist ...
>> # Due to 'failglob' option, bash should stop here
>> echo /foo/bar/*
>>
>> # This code should never run if previous glob failed
>> echo Still alive
> 
> Is the assumption true that "Still alive" should *never* be printed
> when "/foo/bar" does not exist?

It might be an assumption, but it's not correct. The documentation says
that failglob affects the command:

"If  the  failglob  shell  option  is set, and no matches are
found, an error message is printed and the command is not executed."


> When you add "set -e" to the beginning, execution will stop immediately
> after the failing glob, i.e. no "Still alive" will be printed.

Because set -e will cause the shell to exit when a command fails.

> 
> However, see the following interesting difference between semicolons and
> newlines:
> 
>> $ echo "shopt -s failglob; echo /foo/bar/*; echo alive; " | bash
>> bash: line 1: no match: /foo/bar/*
>> $ echo "shopt -s failglob; echo /foo/bar/*; echo alive; " | sed 's:; :\n:g' 
>> | bash
>> bash: line 2: no match: /foo/bar/*
>> alive
>> $ echo "set -e; shopt -s failglob; echo /foo/bar/*; echo alive; " | sed 's:; 
>> :\n:g' | bash
>> bash: line 3: no match: /foo/bar/*

The failglob option affects the command. The `difference' between a line
and multiple lines is that bash always reads at least one complete line of
input before parsing and executing any of the commands on that line. As
a result:

shopt -s failglob; echo /foo/bar/*; echo alive

is a single compound command, and

shopt -s failglob
echo /foo/bar/*
echo alive

is three simple commands. So failglob causes the current command to fail,
which means the compound command (first case) or the simple command
(second case).

`failglob' has behaved this way since it was introduced in bash-3.0.

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    address@hidden    http://tiswww.cwru.edu/~chet/

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