If your command line tool prints something, make it friendly to shell scripts. This example was sent to me last week:
virsh # pool-list --details --all Name State Autostart Persistent Capacity Allocation Available --------------------------------------------------------------------------- default running yes yes 1.79 TB 1.49 TB 304.77 GB image_dir running yes yes 1.79 TB 1.49 TB 304.77 GB tmp inactive no yes - - -
And this was my reply:
One good thing, and several bad things about that. The good thing is
that empty columns are presented with ‘-‘ which means you can use awk
and sort -k to parse the output columnwise.
The bad things:
- Space within fields “1.79 TB” (awk / sort -k in fact won’t work).
- Numeric fields aren’t numbers: You can’t
sort -non “1.79 TB”, and you can’t read that number into a script and do math on it. Most tools have a “-h” or “–human” option in order to generate human-readable numbers (without spaces), but default to just printing the raw numbers. - Unnecessary “——-” line.
- Title line should be optional. Have a –no-title option or something like that to suppress it.
- Does virsh still print an unnecessary blank line after the output? If so, stop doing that.
In tools like virt-df we also provide CSV format output. CSV is a fine format for two-dimensional output, but beware the caveats.
Richard WM Jones 
Totally disagree. Make your output friendly to humans. Add a –parseable option for computers (and insist on also specifying –output-fields or whatever). Don’t try to do both; it will most likely be a disaster.
You’re assuming that (can be parsed) XOR (friendly to humans). I don’t think any of the things I suggested would have made the output less friendly to humans.
The problem with machine-readable output, and also with CSV output, is that you have a separate and less-used code path, one which is more likely to bit rot. To see what I mean, try the “-m” option in parted some time.
Good points. Note `sort` recently got the -h option to sort 1.79TB etc. (without spaces).
Note also that one can align output for human consumption external to each tool, with `column`. Consider: mount | column -t
Hi Richard, and it went even worse – on EL7 (libvirt-client-1.2.17) there is space added before listed items. so if you were not trimming the lines, it’s required now ;o)