Call Programs with Python Output

Question

I want to call the c program display_output with python generated arguments, however I'm not sure how to formulate the syntax. I tried this

./display_output (python -c "print 'A' * 20")

but I get

bash: syntax error near unexpected token `python'

I suppose this goes along with my original question and could help me out with that. The only way I could find to try running python cmd line output as a bash command was by appending | bash to the command. However, is there a better way to do this?

(python -c "print 'ls'") | bash

I clearly don't know my way around Bash, but I was certain there was a more appropraite way to do this.

Figured it out. I added quotes like so: ./display_output '(python -c "print 'A' * 20")' -_- — WhiteMask
– WhiteMask, Commented Apr 5, 2015 at 0:00
Nevermind, now have weird output. This doesn't work python -c "print 'A' * 20" | ./display_output — WhiteMask
– WhiteMask, Commented Apr 5, 2015 at 0:01
still think you would find this easier to do using python itself. — Padraic Cunningham
– Padraic Cunningham, Commented Apr 5, 2015 at 0:06

glenn jackman · Accepted Answer · 2015-04-05 00:03:23Z

When bash sees an open parenthesis at a place where a command can be, it will launch a subshell to run the enclosed commands. Where you currently have them is not a place where a command can go. What you want instead is command substitution

./display_output $(python -c "print 'A' * 20") # ...............^

You will get into trouble if any of the arguments generated contain whitespace (obviously not the case for this toy example.

To generate a string of 20 "A"s in bash, you would write:

a20=$(printf "%20s" "") # generate a string of 20 spaces # or, the less readable but more efficient: printf -v a20 "%20s" "" a20=${a20// /A} # replace all spaces with A's

The last line is pattern replacement in shell parameter expansion

What shell are you using? What do you get with echo $(python -c "print 'A' * 20")
Sorry, the illegal instruction error went away when I changed the amount of generated A's and flipped the double parentheses with singles, and vice versa.

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