dash reports 'Syntax error: "(" unexpected' when using process substitution

Question

I have following bash command

diff <(xzcat file1.xz) <(xzcat file2.xz)

and I need to execute it in dash. On my system (Debian Wheezy), dash is the default interpreter for cron (/bin/sh is a link to /bin/dash).

When I execute the command in dash, I get following error:

Syntax error: "(" unexpected

Debian also has a checkbashisms script. I forget the package name, though. — strugee
– strugee, Commented Oct 2, 2013 at 16:45
See also: '-sh: syntax error: unexpected "("' when attempting process substitution on an embedded Linux device with bash — Gabriel Staples
– Gabriel Staples, Commented Nov 9, 2021 at 2:13

slm · Accepted Answer · 2013-10-02 14:13:53Z

7

If you need a specific shell when running something from a cron job wrap it in a script and call the script from the cron.

#!/bin/bash

diff <(xzcat file1.xz) <(xzcat file2.xz)

*  *  *  *  * user-name  /path/to/above/script.bash

answered Oct 2, 2013 at 14:08

slm♦

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Stéphane Chazelas · Accepted Answer · 2013-10-02 15:25:44Z

6

Yes, process substitution is a non-standard feature originated in ksh and only available in ksh, bash and zsh.

On systems that support /dev/fd/n (like Debian), you can do:

xzcat < file1.xz | { xzcat < file2.xz | diff /dev/fd/3 -; } 3<&0

Or you can always do:

bash -c 'diff <(xzcat file1.xz) <(xzcat file2.xz)'

answered Oct 2, 2013 at 15:25

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Joseph R. · Accepted Answer · 2013-10-02 14:37:59Z

4

If you must use dash, this will work:

mkfifo file1
mkfifo file2
xzcat file1.xz >file1&
xzcat file2.xz >file2&
diff file1 file2
rm -f file1 file2 #remove the FIFOs

answered Oct 2, 2013 at 14:37

Joseph R.

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3 Answers 3