Echoing a string in bash with brackets surrounding it returns 'b'

Question

$ a='"apple","ball","cat"'
$ a='['$a
$ echo $a
["apple","ball","cat"
$ a=$a']'
$ echo $a
b

I'm stumped hard by the result b while I expect to see ["apple,"ball","cat"]. What am I missing here?

This is from bash shell on Mac. Also see it on CentOS 7, while not on Fedora. Can someone please explain?

Some of your lines begin with a dollar sign ($). Is that the Bash prompt or are you typing that in? — cherdt, Nov 14 '17 at 22:23
I'm assuming the dollar signs are Bash prompts, in which case I am not able to reproduce the behavior you've described. — cherdt, Nov 14 '17 at 22:24
@choroba, this is what I see a=[$a] a=[$a]

a='["apple","ball","cat"]' but when I do echo $a

echo b

b — user3866672, Nov 14 '17 at 22:37
Can't reproduce with GNU bash, version 4.3.48, I get the expected ["apple","ball","cat"]. — dessert, Nov 14 '17 at 23:21

score 27 · Answer 1 · answered Nov 14 '17 at 23:29

27

There is a file with the name b in the current directory.

[...]

is a pattern matching expression. It matches every file of which the name consists of a single letter between [ and ].

This is similar to having * in a variable value and using the variable without quotes.

answered Nov 14 '17 at 23:29

Hauke Laging

90,279

2

+1 Good catch. The corollary here is, I suppose, that variables should be quoted when used (echo "$a" writes the literal value of $a). – Chris Davies Nov 14 '17 at 23:38
6

For loads more information about that: Security implications of forgetting to quote a variable in bash/POSIX shells – glenn jackman Nov 15 '17 at 00:15

score 3 · Answer 2 · answered Nov 15 '17 at 08:20

Hauke already answered why it's happening. For future reference, you can also troubleshoot what is happening with strace:

$ touch a b l
$ a='["apple","ball"]'                                                      
$ strace -e trace=execve echo $a
execve("/bin/echo", ["echo", "a", "b", "l"], [/* 82 vars */]) = 0
a b l
+++ exited with 0 +++

Or with set -x:

$ set -x; echo $a
+ set -x
+ echo a b l
a b l

It's might not immediately be clear, but at least you see that shell has converted the unquoted variable into a list of items; from there we can deduce that filename expansion occurred.

Echoing a string in bash with brackets surrounding it returns 'b'

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