How to programmatically create output redirection in bash

Question

I'm writing a test script and it has a lot of repetition in it, and I'm looking for ways to reduce these code duplication. So I thought it would be nice to programmatically create the redirect part of the commands.

Here is a minimal example: (In the real script I would generate different output file names in x())

#!/bin/bash

set -x

x() {
  echo '> out.txt 2> err.txt'
}

./someProgram $(x)

My hope would be that stdout would end up in out.txt and stderr in err.txt. But bash quotes the string. So the resulting command is ./someProgram '>' out.txt '2>' err.txt instead of ./someProgram > out.txt 2> err.txt.

Here is the output of the example:

++ x
++ echo '> out.txt   2> err.txt'
+ ./someProgram '>' out.txt '2>' err.txt
.....

So is there a possibility that '>' and '2>' won't get quoted?

Have you considered to invert the flow? Instead of someProgram $(x) you can do it like this: x someProgram, i.e. the x function accepts an executable along with optional arguments and redirects its stdout and stderr as you wish — neshkeev
– neshkeev, Commented Mar 21, 2020 at 23:55

William Pursell · Accepted Answer · 2020-03-21 23:56:28Z

2

It's hard to way without more context, but probably you just want to use exec judiciously. I won't vouch for the portability of the following, but in bash 5.0.16 you can do:

x() { shift; "$@"; } > $1.out 2> $1.err
x filename ./someProgram

To run ./someProgram with the output directed to filename.out and the error stream written to filenmae.err

answered Mar 21, 2020 at 23:56

William Pursell

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2 Comments

oguz ismail Over a year ago

Wrt I won't vouch for the portability of the following standard is a bit unclear on this, it says The operands to the command temporarily shall become the positional parameters during the execution of the compound-command; but I think they should support this usage, it's really cool.

Scolytus Over a year ago

exec was not the right approach but re-thinking the flow. I use a similar approach to yours with shift. Thanks for your insight/inspiration!

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