Padding zeros in a string

Question

I'm writing a bash script to get some podcasts. The problem is that some of the podcast numbers are one digits while others are two/three digits, therefore I need to pad them to make them all 3 digits.

I tried the following:

n=1

n = printf %03d $n

wget http://aolradio.podcast.aol.com/sn/SN-$n.mp3

but the variable 'n' doesn't stay padded permanently. How can I make it permanent?

score 129 · Accepted Answer · edited Jun 02 '23 at 23:24

129

Use command substitution to assign the result of the printf command:

n=1
wget http://aolradio.podcast.aol.com/sn/SN-$(printf %03d $n).mp3

EDIT: Note that I removed one line which was not really necessary. If you want to assign the output of 'printf %...' to n, you could use

n=$(printf %03d $n)

and after that, use the $n variable substitution you used before.

edited Jun 02 '23 at 23:24

Gilles Quénot

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answered Jul 12 '09 at 22:55

ChristopheD

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Why not: `wget $(printf http://aolradio.podcast.aol.com/sn/SN-%03d $n.mp3, $n)`. Just put the whole URL in the `printf` statement. – David W. Jun 16 '14 at 01:43
What if you actually need to pad a string? for example, I have the string "Hello" but I need to always print 10 characters, I would need to print the string: "-----Hello" or if I have "yes" I still get 10 characters printed as: "-------yes" – m4l490n Jan 17 '19 at 18:22

score 49 · Answer 2 · edited Apr 17 '17 at 08:03

49

Seems you're assigning the return value of the printf command (which is its exit code), you want to assign the output of printf.

bash-3.2$ n=1
bash-3.2$ n=$(printf %03d $n)
bash-3.2$ echo $n
001

edited Apr 17 '17 at 08:03

vidstige

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answered Jul 12 '09 at 22:58

nos

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Why is $() preferable? – bli Nov 28 '16 at 15:40
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@bli It can be nested, whereas backticks cannot (at least not without some form of escaping) – Neil Dec 28 '16 at 18:59

UnSandpiper · Answer 3 · 2014-03-16T18:36:13.470

24

Attention though if your input string has a leading zero!
printf will still do the padding, but also convert your string to ~~hex~~ octal format.

# looks ok
$ echo `printf "%05d" 03`
00003

# but not for numbers over 8
$ echo `printf "%05d" 033`
00027

A solution to this seems to be printing a float instead of decimal.
The trick is omitting the decimal places with .0f.

# works with leading zero
$ echo `printf "%05.0f" 033`
00033

# as well as without
$ echo `printf "%05.0f" 33`
00033

edited Mar 16 '14 at 18:36

answered Oct 16 '13 at 14:35

UnSandpiper

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That's because printf takes "0n" as octal. octal 33 = decimal 27. Not hex. – cde Jan 25 '14 at 22:34

score 20 · Answer 4 · answered Jun 08 '17 at 13:00

20

to avoid context switching:

a="123"
b="00000${a}"
c="${b: -5}"

answered Jun 08 '17 at 13:00

cC Xx

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this! . . . . <3 – milahu Oct 15 '22 at 21:47
inverse: `while [ "${s:0:1}" = '0' ]; do s=${s:1}; done` – milahu Oct 15 '22 at 22:08
Be aware this will cut off numbers which have more than 5 digits. To avoid that, try `a="123456"; [[ ${#a} -lt 5 ]] && a="00000${a}" && a="${a: -5}"; echo "$a"` – Bryan Roach Mar 14 '23 at 16:57

score 4 · Answer 5 · answered Jul 12 '09 at 22:58

4

n=`printf '%03d' "2"`

Note spacing and backticks

answered Jul 12 '09 at 22:58

score 3 · Answer 6 · edited Jun 15 '14 at 23:15

3

As mentioned by noselad, please command substitution, i.e. $(...), is preferable as it supercedes backtics, i.e. `...`.

Much easier to work with when trying to nest several command substitutions instead of escaping, i.e. "backslashing", backtics.

edited Jun 15 '14 at 23:15

Camilo Martin

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answered Jul 12 '09 at 23:10

Rob Wells

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score 0 · Answer 7 · edited Oct 17 '20 at 18:52

0

This is in response to an answer given by cC Xx. It will work only until a's value less is than 5 digits.

Consider when a=12345678. It'll truncate the leading digits:

a="12345678" 
b="00000${a}" 
c="${b: -5}" 
echo "$a, $b, $c"

This gives the following output:

12345678, 0000012345678, 45678

Putting an if to check value of a is less than 5 digits and then doing it could be solution:

if [[ $a -lt 9999 ]] ; then b="00000${a}" ; c="${b: -5}" ;  else c=$a; fi

edited Oct 17 '20 at 18:52

fcdt

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answered Oct 17 '20 at 16:59

Siva

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Putting an if to check value of a is less than 5 digits and then doing it could be solution. – Siva Oct 17 '20 at 17:02

score 0 · Answer 8 · answered Nov 10 '22 at 22:55

0

Just typing this here for additional information.

If you know the number of zeroes you need, you can use the string concatenation:

let pad="0"
pad+=1
echo $pad # this will print 01

answered Nov 10 '22 at 22:55

Parakrama Dharmapala

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Padding zeros in a string

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