Why isn't echo printing the output of $(grep ... /proc/net/bonding/bond0)?-CodePudding

When I run a grep pipeline directly I get two lines of output:

$ grep Interface /proc/net/bonding/bond0 | cut -d : -f 2 | sed 's/ //g'
eno1np0
eno2np1
$

Strangely, the output is empty when I capture it and call echo:

$ echo $(grep Interface /proc/net/bonding/bond0 | cut -d : -f 2 | sed 's/ //g')
              
$

A for loop through grep's output shows many empty lines:

$ for a in $(grep Interface /proc/net/bonding/bond0 | cut -d : -f 2 | sed 's/ //g'); do echo $a; done
 
 
 
 
 
 
 
 
 
 
 
 
 
 
$

For what it's worth, I have tried redirecting stderr to stdout with no luck:

echo $(grep Interface /proc/net/bonding/bond0 2>&1 | cut -d : -f 2 | sed 's/ //g')

I even tried using cat instead of grep.

How can I get echo to actually print what's being output by the grep command? Normally, echo works. I can echo output just I like normally would, for other things. I can echo text into a file. I just can't get it to work with this grep pipeline.

CodePudding user response：

My hunch is that the results from /proc/net actually contain null bytes, which throw off the shell.

What you are trying is quite similar to the infamous useless use of echo though I suppose if the goal is to get all the output on a single line, it's not entirely useless (but still broken, because the shell will expand any wildcards in the results).

Perhaps try this:

grep Interface /proc/net/bonding/bond0 |
tr '\000' '\012' |
sed 's/^[^:]*://;s/ //g;N;s/^[^:]*://;s/ //g;s/\n/ /'

though the s/\n/ / thing is probably not portable to all sed variants.

CodePudding user response：

Figured this out, based on information from @CharlesDuffy and @tripleee, and a colleague of mine.

I am not sure it's a common occurrence, but posting here in case someone else runs into this in the future.

The cause was that the environment variable IFS was set to a non-standard character. While cat and grep were working correctly - presumably because neither depends on IFS to work - echo was not.

I am not entirely sure how this happened, but for anyone that faces this issue, check your environment for a variable named IFS (env | grep IFS), and set it back to the default (' \t\n').

PS: bad start to a mid-week morning /facepalm