Argthtjtr

Using common bash tools as part of a shell script, I want to repeatedly insert a newline char ('n') into a long string at intervals of every N chars.

For example, given this string, how would I insert a newline char every 20 chars?

head -n 50 /dev/urandom | tr -dc A-Za-z0-9

Example of the results I am trying to achieve:

ZL1WEV72TTO0S83LP2I2

MTQ8DEIU3GSSYJOI9CFE

6GEPWUPCBBHLWNA4M28D

P2DHDI1L2JQIZJL0ACFV

UDYEK7HN7HQY4E2U6VFC

RH68ZZJGMSSC5YLHO0KZ

94LMELDIN1BAXQKTNSMH

0DXLM7B5966UEFGZENLZ

4917Y741L2WRTG5ACFGQ

GRVDVT3CYOLYKNT2ZYUJ

EAVN1EY4O161VTW1P3OY

Q17T24S7S9BDG1RMKGBX

WOZSI4D35U81P68NF5SB

HH7AOYHV2TWQP27A40QC

QW5N4JDK5001EAQXF41N

FKH3Q5GOQZ54HZG2FFZS

Q89KGMQZ46YBW3GVROYH

AIBOU8NFM39RYP1XBLQM

YLG8SSIW6J6XG6UJEKXO

A use-case is to quickly make a set of random passwords or ID's of a fixed length. The way I did the above example is:

for i in {1..30}; do head /dev/random | tr -dc A-Z0-9 | head -c 20 ; echo ''; done

However, for learning purposes, I want to do it a different way. I want to start with an arbitrarily long string and insert newlines, thus breaking one string into multiple small strings of fixed char length.

The venerable fold command ("written by Bill Joy on June 28, 1977") can wrap lines:

$ printf "foobarzotn" | fold -w 3

foo

bar

zot

However, there are some edge cases

BUGS
Traditional roff(7) output semantics, implemented both by GNU nroff and
by mandoc(1), only uses a single backspace for backing up the previous
character, even for double-width characters. The fold backspace
semantics required by POSIX mishandles such backspace-encoded sequences,
breaking lines early. The fmt(1) utility provides similar functionality
and does not suffer from that problem, but isn't standardized by POSIX.

so if your input has backspace characters you may need to filter or remove those

$ printf "abcbdben" | col -b | fold -w 1

e

$ printf "abcbdben" | tr -d "b" | fold -w 1

a

c

d

e

I like the fold answer but just in case you want with sed, here it is:

sed 's/.{20}/&

/g' filename

You can use -i for in-place insertion.

If you have the contents in a variable, such as:

var=$(head -n 50 /dev/urandom | tr -dc A-Za-z0-9)

Then you could use a bash loop over the length of the variable in chunks of 20, printing out each chunk:

for((start=1;start < ${#var}; start += 20)); do printf '%sn' "${var:start:20}"; done

If you want them as individual variables, consider assigning the output to an array:

readarray -t passwords < <(for((start=1;start < ${#var}; start += 20)); do printf '%sn' "${var:start:20}"; done)

Instead of doing that tr -dc thing to get rid of non-printable chars, I would just use base64 from coreutils:

$ base64 -w20 /dev/urandom | head -8

ckXkWvb0zJknz2zi4fRS

3Jv0dDbKiX8fef7SOfbH

QJySlGUzzhi32wvrGliK

YEiuz6v+EFaRYRMjvnJq

HCXIPiP9wmgONLRqm9uK

iHYwo5xIs8gGjQQEQBeX

8NkL4EkmOAHdmWhGvZYl

AcxD2DaTq2TZRsDL+UMx

If the + and / are problem you can replace them:

$ base64 -w20 /dev/urandom | tr +/ pq | head -8

zr7MgiEr7xBd7h9ihK30

IRNvDuT2H9HsHVq9yFqh

S1cihgfAInjfFspMNXVC

qUUwGErD7nZqtzQtLOo7

DNDp4TVWvHmbEh7HLDGX

GtqqDdEoceY8m5U7FGu0

TvGtTukm6Whr7VHN1mZG

DW5TUH525IA52zLKYACV