Why does redirection in crontab result in a zero byte file?

Question

System: Xubuntu 13.10

When I have this crontab entry

*/5 * * * * cat /home/dbk/.bash_aliases &> /home/dbk/Desktop/junk

junk has a byte size of 0.

Running

$ cat /home/dbk/.bash_aliases &> /home/dbk/Desktop/junk

gives a file with a proper size and content.

score 5 · Accepted Answer · edited Oct 05 '15 at 15:08

5

The probem because cron run task with sh. &> is a shortcut to redirect both stderr and stdout to the same file in bash, not in sh.

In sh, your command:

cat /home/dbk/.bash_aliases &> /home/dbk/Desktop/junk

meaning run two commands separately:

Run cat /home/dbk/.bash_aliases in background

cat /home/dbk/.bash_aliases &
Truncate the junk file.

> /home/dbk/Desktop/junk

So you should use bash to run your command in crontab:

*/5 * * * * bash -c "cat /home/dbk/.bash_aliases &> /home/dbk/Desktop/junk"

or using more portable way:

*/5 * * * * cat /home/dbk/.bash_aliases > /home/dbk/Desktop/junk 2>&1

edited Oct 05 '15 at 15:08

Community

answered Mar 23 '14 at 18:11

cuonglm

Its true that >&1 2>&1 is the analog to &>, but i think that in the portable cat case it is useless - if cat errors it should quit and write nothing anyway so >file is enough. – mikeserv Mar 23 '14 at 20:44
@mikeserv: Yes, you can see man bash, REDIRECTION section for more details. No, if cat produce any errors, these errors will be saved to file. – cuonglm Mar 24 '14 at 01:27
What kind of errors will be saved to the file? – mikeserv Mar 24 '14 at 01:28
@mikeserv: any errors produce by cat, i.e you cat a file which doesn't exist. – cuonglm Mar 24 '14 at 01:31
Much better than that test -e $file && cat $_ >$tgt – mikeserv Mar 24 '14 at 01:49
@mikeserv: not if you actually want to write the error message into the file. – Adaephon Mar 25 '14 at 07:22

1 Answers1