The phrase "a fair command of" is correct and usable in written English.
It can be used to describe someone's reasonable or adequate level of skill or knowledge in a particular subject or language.
Example: "She has a fair command of Spanish, which allows her to communicate effectively during her travels."
Alternatives: "a good grasp of" or "a solid understanding of".
He has a fair command of the language, unlike some deportees who arrived with no knowledge of Khmer.
He had a fair command of English, provided the other party to the conversation didn't talk too fast.
Li Shanlin, a dapper 80-year-old professor of mathematics and physics in English tweeds and with a fair command of English, will tell the story of his restaurant with minimal prompting.
This week her colleagues in the four-party cabinet, which rarely deals with education, cautiously backed her idea that all Swiss children should have a fair command of three languages, including two of the country's own.But the language war has barely begun.
Despite having low keyboarding skills in English, they also have a fair command of knowledge and practice of general Web 2.0 tools for daily life, but not for educational purposes.
I have noticed that he has a fair command of most of what a 9-year-old needs to use a computer.
A reasonable command of the issues.
That, too, is a view that throughout Western Europe seems to command a fair degree of sympathy.
Their work is almost invisible here, though it commands a fair amount of attention in the flourishing and contentious cinephile wing of the blogosphere.
Placing someone as famous as Styles in the fray, and giving him a fair share of close-ups and commanding dialogue, bonds him with the audience.
Third, Ukip high command puts on a good front about being thick-skinned, but behind the scenes Farage has spent a fair amount of political capital expelling people and dissolving branches where necessary.
