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bug#26491: date and echo using with underscore
From: |
Assaf Gordon |
Subject: |
bug#26491: date and echo using with underscore |
Date: |
Fri, 14 Apr 2017 11:50:12 -0400 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 |
tag 26491 notabug
close 26491
stop
Hello Edmond,
On 04/14/2017 02:36 AM, Edmond Yuen wrote:
> echo $D_$i
As others have replied (and thanks to all who replied),
an underscore character is valid part of a shell variable,
thus the shell tries to use the content of the variable "D_" - which is
empty.
This is part of the POSIX standard for shell variable expansion.
Specifically here:
http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_06_02
2.6.2 Parameter Expansion
...
"If the parameter name or symbol is not enclosed in braces, the
expansion shall use the longest valid name (see the Base
Definitions volume of IEEE Std 1003.1-2001, Section 3.230, Name),
whether or not the symbol represented by that name exists."
And "Section 3.230, Name" says:
http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap03.html#tag_03_230
"In the shell command language, a word consisting solely of
underscores, digits, and alphabetics from the portable character
set."
This is not a bug in date or echo (or a bug at all),
and I'm closing this bug. Discussion can continue by replying to this
thread.
Lastly,
To detect and fail early when undefined variables are used,
you can optionally enable "-u" in your shell scripts:
#!/bin/sh
set -u
A=hello
echo A=$A
echo A_=$A_
And the shell will terminate the script with:
$ ./test.sh
A=hello
test.sh: 5: test.sh: A_: parameter not set
regards,
- assaf