Bash: Zero-pad a number with a variable for the digit count

Question

I need to zero-pad a sequence of numbers in a loop in Bash. I know how to do it with

seq -f "%03g" 5

or the comparable printf approach, also

for index in {003..006}

The problem I did not find an answer to is that I need the number of digits to be a variable:

read CNT seq -f "%0$CNTd" 3 6

Will return an error

seq: das Format »%0“ endet mit %

I have not found any way to insert a variable in a format string or any other way to produce a zero-padded sequence where the number of digits comes from a (user-provided) variable.

Walter A · Accepted Answer · 2019-05-01 09:58:43Z

2

I think you want seq, but did you know the * operator in printf?

printf "%0*d\n" ${CNT} 5

answered May 1, 2019 at 9:58

Walter A

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6 Comments

uli_1973 Over a year ago

No, I didn't. This is useful information too, but indeed I think the seq solution is more natural to the use case in question.

Benjamin W. Over a year ago

@uli_1973 ... but seq is an external tool, while printf is a Bash built-in.

uli_1973 Over a year ago

@BenjaminW. good point, but not relevant since I only need that for personal use.

Benjamin W. Over a year ago

@uli_1973 Sure, but answers here are not just for whoever asked them, but for everybody who finds them later when encountering a similar problem, so ideally they employ universal best practices.

Paul Over a year ago

So what is the * parameter to printf? An answer should include an explanation, not an unanswered question.

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oguz ismail · Accepted Answer · 2019-05-01 09:55:27Z

1

A variable name (CNT) should be enclosed in curly braces when it is followed by a character (d) which is not to be interpreted as part of its name,
seq doesn't support %d, you should use %g.

$ read -r CNT $ seq -f "%0${CNT}g" 3 6 00003 00004 00005 00006

edited May 1, 2019 at 9:55

answered May 1, 2019 at 9:00

oguz ismail

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2 Comments

uli_1973 Over a year ago

2. was a mis-copying, in the original example at the top of my question I had it correct. 1. I did try the curly braces - however, for some reason I only put them around the $CNT ...

uli_1973 Over a year ago

The indices are of course also variables, but they didn't cause any problems to me. seq -f "%0${CNT}g" $FROM $TO works.

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