Someone asked me why I am looking for a new job while I am currently working. This got me thinking if there is a idiom that says something about it being wise to not leave your current job until you have another sure thing lined up etc.
It's better to have a lesser but certain advantage than the possibility of a greater one that may come to nothing.
This 16th century proverb is one of the oldest and best-known in English. It warns against taking unnecessary risks - it is better to keep what you have (a bird) than to risk getting more and ending with nothing (two birds out of your reach).
The more classic idiom don't count your chickens (before they're hatched) may apply here. Don't count on your new job and leave the old one till everything is settled:
something that you say in order to warn someone to wait until a good thing they are expecting has really happened before they make any plans about it.
- You might be able to get a loan from the bank, but don't count your chickens.
(Cambridge Idioms Dictionary)
Which basically means "Don't do things in the wrong order". Leaving a job before applying to another one is the wrong order.
Another possibility is "don't burn your bridges".
If you burn your bridges, you do something that makes it impossible to go back from the position you have taken.
Don't put all your eggs in one basket
Which means "don't concentrate all your prospects or resources in one thing or place, or you could lose everything."
It came from Don Quixote (Part I, Book III, Chapter 9) by Miguel de Cervantes [1547-1616].
“It is the part of a wise man to keep himself today for tomorrow, and not venture all his eggs in one basket.”—Sancho Panza
Don't throw away your old shoes before you have new ones.
This is an idiom in my language, and I always assumed it was one too in English, but I cannot find any evidence of that. In any case, I believe anyone would understand what you mean, if you'd say this.
Edit 1: The original Dutch phrase is: "Je moet geen oude schoenen weggooien voor je nieuwe hebt."
Edit 2: Sven Yargs mentions even better sources to the use of this idiom in English here on Meta: https://english.meta.stackexchange.com/a/10101/224220
Leaving your current job before you have a sure thing lined up is a leap into the dark!
leap in the dark
an action of which the consequences are unknown:
The experiment was a leap in the dark.
"Don't take a leap into the dark": the final EU referendum newspaper front pages
"Don't quit the day job" is often used of career paths that have a high degree of risk, particularly those in the arts. It would be applicable by analogy to the situation you describe.
(It sometimes suggests that the person shouldn't quit their job because they aren't any good at the artistic endeavour at all, and hence are certain to fail if they pursue it, and as such is an insult; it depends on context).
I've heard the phrase "Don't jump without somewhere to land" more than once, in conversations about leaving jobs - no citations available, I'm afraid.
Googling this (exact) phrase brings up a single result, using the phrase in a similar sense but regarding leaving a marriage.
I believe this might also be applicable:
- to hedge your bets
hedge-your-bets: to protect yourself against making the wrong choice