Linux bash: How can I use the eval command correctly?

Question

I can run the following command on the Linux command line and it correctly lists all the files that have the ABC- and XYZ- prefixes:

    ls {ABC-,XYZ-}*.xml

    Result: ABC-01.xml  ABC-02.xml  ABC-03.xml  XYZ-01.xml  XYZ-02.xml

Now, I figured that I could use a variable as follows:

    file_pfx={ABC-,XYZ-}

and then use the variable in the "ls" command combined with "eval":

    eval 'ls ${file_pfx}*.xml'

    Result: ls: {ABC-,XYZ-}*.xml: No such file or directory

What am I doing wrong?

Thanks

You can't store the braced expressioni in a variable because brace expansion takes place *before* parameter substitution. See the [order of expansions in the manual](https://www.gnu.org/software/bash/manual/bashref.html#Shell-Expansions). — glenn jackman, Aug 19 '17 at 20:42
Before actually using `eval`, see ["Why should eval be avoided in Bash, and what should I use instead?"](https://stackoverflow.com/questions/17529220/why-should-eval-be-avoided-in-bash-and-what-should-i-use-instead) — John1024, Aug 19 '17 at 20:57

score 4 · Answer 1 · answered Aug 19 '17 at 20:35

4

Don't use eval. You can use extended globbing instead:

#! /bin/bash
shopt -s extglob
prefix='ABC-|XYZ-'
ls @($prefix)*.xml

answered Aug 19 '17 at 20:35

choroba

231,213
25
204
289

score 3 · Answer 2 · answered Aug 19 '17 at 20:40

3

I would advise you to use an array

$ touch ABC-01.xml  ABC-02.xml  ABC-03.xml  XYZ-01.xml  XYZ-02.xml

$ files=( {ABC-,XYZ-}*.xml )

$ printf "%s\n" "${files[@]}"
ABC-01.xml
ABC-02.xml
ABC-03.xml
XYZ-01.xml
XYZ-02.xml

answered Aug 19 '17 at 20:40

glenn jackman

238,783
38
220
352

whoan · Accepted Answer · 2017-08-19T20:48:03.643

1

You should use double quotes to allow variable expansion ($file_pfx):

eval "ls ${file_pfx}*.xml"

Notice I'm assuming your filenames don't contain spaces.

Anyway, be careful with the use of eval.

edited Aug 19 '17 at 20:48

answered Aug 19 '17 at 20:29

whoan

8,143
4
39
48

2

It will not work if any of the filenames contains a space – glenn jackman Aug 19 '17 at 20:40

Linux bash: How can I use the eval command correctly?

3 Answers3