Using variables to copy files with increasing numerical names.

Jovec · March 8, 2020, 10:04am

Hello, I have been learning Linux shell commands for about 4 months now. I am trying to write a simple shell script, but I have encountered an issue I do not understand the cause of.

#!/bin/bash

first=1
last=10

cp path/to/files/{$first..$last}.dat ./

When I run my script I get an error.
[user@x dir]$ ./myscript.sh
cp: cannot stat `path/to/file/{1..2}.dat': No such file or directory
[user@x dir]$

However when I run it without variables.

#!/bin/bash

cp path/to/files/{1..10}.dat ./

Running script without variables.
[user@x dir]$ ./myscript.sh

It copies the ten *.dat files from the path/to/files to the directory of myscript.sh. How can I get the script working with variables? Thanks in advance for any help given.

RudiC · March 8, 2020, 11:22am

This is a "brace expansion". It is expanded before any other expansions, and it cannot take any variables; read man bash very carefully:

Brace Expansion
Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to pathname expansion, but the filenames generated need not exist. Patterns to be brace expanded take the form of an optional preamble, followed by either a series of comma-separated strings or a sequence expression between a pair of braces, followed by an optional postscript.
.
.
.
A sequence expression takes the form {x..y[..incr]}, where x and y are either integers or single characters, and incr, an optional increment, is an integer.

.

.
.
Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is
strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces.

Those strings, integers, or characters are constants, not variables.

nezabudka · March 9, 2020, 4:38am

Hi
try this workaround

eval cp path/to/files/{$first..$last}.dat ./

MadeInGermany · March 9, 2020, 5:17am

... or a loop, that correctly handles special characters in file names, and can verify that the files exist:

for (( i=1; i<=10; i++ )); do
  f=/path/to/files/$i.dat
  [ -f "$f" ] && cp "$f" ./
done

nezabudka · March 9, 2020, 5:41am

Hi, @MadeInGermany
Then it will meet the condition of the task

for (( i=first; i<=last; i++ )); do
  f=/path/to/files/$i.dat
  [ -f $f -a ! -f $i.dat ] && cp $f ./
done

But closer to me is such a solution

cp path/to/files/[$first-$last].dat ./

Here I got excited. This is correct if the file numbers are in the range 0-9

drl · March 9, 2020, 8:26am

Hi.

Brace expansion with variables will work correctly with ksh and zsh ... cheers, drl

On a system like:

OS, ker|rel, machine: Linux, 3.16.0-7-amd64, x86_64
Distribution        : Debian 8.11 (jessie) 
ksh 93u+
zsh 5.0.7

MadeInGermany · March 9, 2020, 11:16am

Good to know. Bash was first with brace expansion, now ksh and zsh are ahead. Let's see when bash will catch up again.
BTW, Bash-4 has got a number + field alignment expansion:

echo {01..10}
01 02 03 04 05 06 07 08 09 10

that ksh-93 has NOT:

echo {01..10}
1 2 3 4 5 6 7 8 9 10

zsh has got it even with variables:

zsh$ first=1
zsh$ last=10
zsh$ echo {$first..$last}
1 2 3 4 5 6 7 8 9 10
zsh$ first=01
zsh$ echo {$first..$last}
01 02 03 04 05 06 07 08 09 10

RudiC · March 9, 2020, 5:57pm

Using "brace expansion",

(see above), as opposed to "file name globbing". So be careful and test for file existence (as done in other posts / methods above) to avoid running into errors.