Run commands using the output of one command and using them as input to the next

Question

Based on the values in the second column, I need to run another command on these values.

[root@box ~]# for i in $(openstack server list --all -c ID -f value); do openstack server show $i -f value -c name -c project_id | xargs printf '%-60s %s\n'; done
cf1-0                                  ebf0f23f424c4417afd3e7bbf4e3900f
sf1-3                                  ebf0f23f424c4417afd3e7bbf4e3900f
sf1-2                                  ebf0f23f424c4417afd3e7bbf4e3900f
sf1-0                                  ebf0f23f424c4417afd3e7bbf4e3900f

Here's an example of what I'm trying to do.

[root@box ~]# for i in $(openstack server list --all -c ID -f value); do openstack server show $i -f value -c name -c project_id | xargs printf "%-60s eval(openstack project show %s -f value -c name)"; done
 cf1-0                                  eval(openstack project show ebf0f23f424c4417afd3e7bbf4e3900f -f value -c name)

The substitution from the second %s in printf works correctly, but I can't figure out how to execute the command.

The command needs to be executed in the same shell, and I cannot use environment variables.

Thanks.

** OUTPUT OF COMMANDS **

Here, -c var1 -c var2 can be placed in any order. -f just means print out the values, not the identifier.

[root@box~]# openstack server list --all -c Name -c ID -f value
2534ce5a-04da-4c7d-9ad5-b7bc466ae612 cf1-0

[root@box ~]# openstack server show 2534ce5a-04da-4c7d-9ad5-b7bc466ae612 -f value -c name -c project_id
cf1-0
ebf0f23f424c4417afd3e7bbf4e3900f

[root@box ~]# openstack project show ebf0f23f424c4417afd3e7bbf4e3900f -f value -c name
core

Desired output
===============
cf1-0 core

Re: for var in $(...anything...), see BashPitfalls #1, and BashFAQ #1 describing the approach used in Inian's answer as a preferable alternative. — Charles Duffy
– Charles Duffy, Commented Feb 19, 2020 at 16:46
btw, I'm curious about the "cannot use environment variables" -- why would environment variables be part of an answer to this otherwise? Why can't they be used? (Clearly, you have some environment variables set -- if you didn't have a PATH, you couldn't be using unqualified command names). — Charles Duffy
– Charles Duffy, Commented Feb 19, 2020 at 16:49
Thanks @CharlesDuffy. This server is a management node which is part of a cloud infrastructure running custom based Linux software. We have been asked not to treat it like an ordinary server. I believe that extends to setting variables within bash scripts, as those would be exposed to the shell(?). In the long-term we should obtain this information via REST API, but I am looking for a quick solution right now via bash. — user
– user, Commented Feb 19, 2020 at 16:58
No, variables in scripts are not exposed to the parent shell. A program can only change the environment of its child processes, not of its parents; that's true whether or not that program is implemented as a shell script. — Charles Duffy
– Charles Duffy, Commented Feb 19, 2020 at 17:01
I was thinking of storing the variables in a [x][y] awk array, to avoid storing them in the shell. Then iterating through the elements of the [y] awk array. — user
– user, Commented Feb 19, 2020 at 17:01

Inian · Accepted Answer · 2020-02-19 18:03:16Z

2

Don't make things further convoluted by adding further levels of xargs. Recommend using a while loop reading the output at each stage and processing them further. The below works in bash with <(..) process substitution

#!/usr/bin/env bash

while read -r id instname; do
    proj="$(openstack server show "$id" -f value -c project_id)"
    [ -z "$proj" ] && break
    name="$(openstack project show "$proj" -f value -c name)"
    printf '%s %s\n' "$instname" "$name"
done < <(openstack server list --all -c ID -c Name -f value)

edited Feb 19, 2020 at 18:03

answered Feb 19, 2020 at 16:41

Inian

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Maverick · Accepted Answer · 2020-02-19 17:12:08Z

0

Not very familiar with the openstack command, but a simple way of doing this in one line would be using awk with system function.

The command should look similar to this:

openstack server list --all -c ID -f value | awk -F'\t' '{ printf $1; system("eval openstack project show " $2 " -f value -c name") }'

answered Feb 19, 2020 at 17:12

Maverick

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4 Comments

user Over a year ago

Maverick, I believe I was not clear in the original question. We need two levels of recursion here. I've updated the question with the raw outputs and expected output. Thanks

Charles Duffy Over a year ago

Why would you put an eval in there? Calling system() already sends your data through a shell parsing pass (which is part of why it's insanely dangerous to use if you don't have complete control of all data substituted into the string).

Charles Duffy Over a year ago

(by comparison, even unquoted parameter expansions in shell skip the most dangerous parsing phases, performing only string splitting and glob expansion; but when system() in awk starts a shell, it has no way to indicate to that shell where the boundaries are between code and data).

Charles Duffy Over a year ago

As a starting point, see mywiki.wooledge.org/BashFAQ/048, keeping in mind that everything it says applies to system() even before you put an eval in it.

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