Bash script creating an unexpected file

Question

I am creating a simple script that will help my Ubuntu-Server manage my backups. I compress my documents from my local machine every x-hours and scp it over to my backup machine. I want to have a maximum amount of backups that can stay in my backup directory. I am writing a script that will delete older backups if the maximum amount of backups have been reached. This is what I have so far, and it's generating a file called MAX_backups when I run the script. Any ideas why this file is being created? I am very far from experienced when it comes to bash programming, but any help would be greatly appreciated. Thanks.

#!/bin/bash

backup_count=$(ls ~/backup | wc -w)

MAX_backups='750'

extra_count=$((backup_count - MAX_backups))

if [ backup_count > MAX_backups ]
then
        for ((i=0; i <= extra_count; i++))
        do
                file=$(ls ~/backup -t -r -1 | head --lines 1)
                rm ~/backup/"$file"
        done
fi

John Kugelman · Accepted Answer · 2012-09-01 06:24:04Z

7

if [ backup_count > MAX_backups ]

The > is being interpreted as a file redirection. Try one of these:

# ((...)) is best for arithmetic comparisons. It is parsed specially by the shell and so a
# raw `>` is fine, unlike within `[ ... ]` which does not get special parsing.
if (( backup_count > MAX_backups ))

# [[ ... ]] is a smarter, fancier improvement over single brackets. The arithmetic operator
# is `-gt`, not `>`.
if [[ $backup_count -gt $MAX_backups ]]

# [ ... ] is the oldest, though most portable, syntax. Avoid it in new bash scripts as you
# have to worry about properly quoting variables, among other annoyances.
if [ "$backup_count" -gt "$MAX_backups" ]

edited Sep 1, 2012 at 6:24

answered Sep 1, 2012 at 6:16

John Kugelman

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Comments

Mark · Accepted Answer · 2012-09-01 06:25:42Z

0

Not sure why the file is created, but I would suppose your version of 'test' (the bracket operator in the if statement) creates this file. I think, the comparison should be changed to

if [ $backup_count -gt $MAX_backups ]

Edit: Of course! I missed the file redirection, this is why the file is created.

answered Sep 1, 2012 at 6:25

Mark

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