Bash command not working with ProcessBuilder

Question

The following command executes fine in bash:

Command:

bash -c "$(echo 'H4sIAArQ/mAAA1WMuw7CIBRAd77ihLJqtKuTg19hHIjetiQU0svl/1sn43weaeKJD4PnlI2R1w1bpOBA3kvF340ssX1Z1LmvUqyhsvWk8jl7nOQmP/2x9ZixSlXWqnLcYvlrw4VwJYxHOiW3AwCHgS2AAAAA' | base64 --decode | zcat)" - -a -b

Output:

Equal to or more than 2 arguments - -a -b

Wanted to know - how can I achieve this using Java's ProcessBuilder?

I tried the following:

ProcessBuilder processBuilder = new ProcessBuilder(args);

where args are:

bash
-c
"$(echo 'H4sIAArQ/mAAA1WMuw7CIBRAd77ihLJqtKuTg19hHIjetiQU0svl/1sn43weaeKJD4PnlI2R1w1bpOBA3kvF340ssX1Z1LmvUqyhsvWk8jl7nOQmP/2x9ZixSlXWqnLcYvlrw4VwJYxHOiW3AwCHgS2AAAAA' | base64 --decode | zcat)"
-
-a
-b

But I keep on getting the following error:

-: if: command not found

Process finished with exit code 127

Can someone please point out the issue here?

I suspect you're taking code that was built to be evaluated by an outer shell, and then using it in an environment where there's only one shell -- an inner one. — Charles Duffy
– Charles Duffy, Commented Jul 26, 2021 at 16:46
By the way: Your compressed base64-string alone is longer (156 bytes) than the uncompressed program itself (124 bytes). Why not execute the uncompressed program directly to safe some resources and avoid the problems? — Socowi
– Socowi, Commented Jul 26, 2021 at 17:12
Socowi, the main problem is on a long script (6000 chars). The compression ratio remains 60% for that case. The above problem is just for representation. — Rajarshi Sarkar
– Rajarshi Sarkar, Commented Jul 26, 2021 at 17:20
Makes sense. Thank you for the info and for posting a minimal example instead of the full script. — Socowi
– Socowi, Commented Jul 26, 2021 at 17:23

Charles Duffy · Accepted Answer · 2021-07-26 16:49:46Z

2

Command substitution results, in bash, don't go through all parsing steps. That means that compound commands like if aren't honored, command separators like ; have no syntactic meaning, etc.

If you want to override that and force an additional parsing pass, you need to use eval. Thus:

args = String[]{
  "bash",
  "-c",
  "eval \"$(echo 'H4sIAArQ/mAAA1WMuw7CIBRAd77ihLJqtKuTg19hHIjetiQU0svl/1sn43weaeKJD4PnlI2R1w1bpOBA3kvF340ssX1Z1LmvUqyhsvWk8jl7nOQmP/2x9ZixSlXWqnLcYvlrw4VwJYxHOiW3AwCHgS2AAAAA' | base64 --decode | zcat)\"",
  "-",
  "-a",
  "-b",
}

Why did this work when you ran it in a shell, instead of from a ProcessBuilder? Because that shell you ran it in would perform the command substitution in "$(...)", and put the results of that substitution in the text it passed to the child shell; so the substitution was already done at parsing time.

answered Jul 26, 2021 at 16:49

Charles Duffy

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3 Comments

Rajarshi Sarkar Over a year ago

Thanks for the wonderful explanation! Does it works the same way for the args as well? Let's say instead of "-b" I want to pass "eval \"$(echo 'H4sIALLp/mAAA9NN4gIAmuzKeAMAAAA=' | base64 --decode | zcat)\""?

Charles Duffy Over a year ago

Only the argument immediately after -c is parsed as code. All others keep their literal values. This is important from a security perspective -- otherwise, one could never write a bash script able to safely handle untrusted data.

Rajarshi Sarkar Over a year ago

Thanks, for the clarification.

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