Bash scripting: parameter substitution problem

Question

I'm trying to write a simple bash script but something seems wrong, I'm testing the following on the command line:

DATE="2010-09-{10,11}"
result=`\ls *ext.$DATE.Z`

and results in ls: cannot access *ext.2010-09-{10,11}.Z: No such file or directory

but if I execute this:

result=`\ls *ext.2010-09-{10,11}.Z`

it works flawlessly...

I even tried to remove the quotation marks from DATE parameter but that isn't the problem, bash manual isn't helping, what am I doing wrong? Wasn't it supposed to execute parameter substitution and pass it to my command?

I thought I should have to escape the $ sign but that didn't work either.

EDIT - Explanation on purpose added

What I am trying to accomplish is to populate variable result with all filenames that match the given pattern (*ext.2010-09-{10,11}), I know I can solve this using a for cycle but I thought about using curly braces for shortness.

Why are you running it like \ls? Aliases do not carry over into the script. — Daenyth
– Daenyth, Commented Sep 17, 2010 at 15:49

Rudu · Accepted Answer · 2010-09-17 16:01:20Z

4

The issue is when you execute it directly on the command line \ls *ext.2010-09-{10,11}.Z is a short form that's expanded into two commands: ls *ext.2010-09-10.Z and ls *ext.2010-09-11.Z (by the command line - each subsequently called). ls itself doesn't directly support an expression like that, so when you build it into a script, it's getting the literal string which it doesn't understand.

Brace Expansion is not supported by all command lines, and isn't recommended for shell scripts:

Brace expansions should not be used in portable shell scripts, because the Bourne shell will not produce the same output.

Here's a solution-script:

#!/bin/sh
DAYS="10 11"
for i in $DAYS;
do
  ls *ext.2010-09-$i.Z
done

edited Sep 17, 2010 at 16:01

answered Sep 17, 2010 at 15:43

Rudu

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2 Comments

Giuseppe Guerrini Over a year ago

Correct, your post completes my short explanation. Regards

Dennis Williamson Over a year ago

But the question is tagged [bash] not [sh] or [bourne] and if you have a shebang that says #!/bin/bash you ought to be able to use Bash features.

Daenyth · Accepted Answer · 2010-09-17 15:46:45Z

3

This happens because brace expansion happens before variable expansion. First it expands the braces -- except there are none in your argument to ls. Then it expands the variables. Then it runs it. At this point it's too late to expand the braces.

If you elaborate on the specific problem you're trying to solve I might be able to help you find a better way.

answered Sep 17, 2010 at 15:46

Daenyth

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Comments

Giuseppe Guerrini · Accepted Answer · 2010-09-17 15:43:16Z

1

The {x,y} group is not expanded anymore if you assign it to a variabile. But you can compose a string like "ls ..." and submit it to the "eval" function. Bye!

answered Sep 17, 2010 at 15:43

Giuseppe Guerrini

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1 Comment

Daenyth Over a year ago

eval is unsafe and could potentially introduce security holes. There's probably a better way

Dennis Williamson · Accepted Answer · 2010-09-17 17:55:51Z

0

This will put the filenames you're looking for in the variable:

result=$(echo *ext.2010-09-{10,11}.Z)

answered Sep 17, 2010 at 17:55

Dennis Williamson

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