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I have a string dk of length N, and a numeric value NDK.

If N is less than NDK I want to append the difference from the start of another string called SYMBOL.

if (( n < ndk )); then
  df=$(( ndk - n ))
fi

Thus if the difference df is 2, I want to append the first two characters from the string SYMBOL to the string DK.

symbol="!@#$%^&"

df=2
dk="FIRST"

With the resulting value of dk being

"FIRST!@"

How can I append the additional df characters from the beginning of string SYMBOL ?

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  • What would you want happen if $df is larger than ${#symbol} (the length of the $symbol string)? Commented Mar 22, 2023 at 15:25
  • I have ensured that condition is never true. Thus DF is always less than the length of SYMBOL. Commented Mar 22, 2023 at 15:36

2 Answers 2

Reset to default
0 
df=3
dk=hello
symbol=123456

dk+=${symbol:0:df}

This appends the $df first characters from $symbol to the end of the string in $dk.

The value in $dk at the end of the above code would be hello123.

The += operator appends the value to the right of the operator to the end of the value of the variable to the left of the operator.

The ${variable:offset:length} expansion in bash returns the length long substring starting at offset (a zero-based index) from the variable variable. Both offset and length are evaluated in an arithmetic context.

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  • Can the length of a string be included in arithmetic operation df=$(( ${#str} - n )). Because usually one does not use $ in front of the variable in the arithmetic construct ((...)). Commented Mar 22, 2023 at 17:22
  • @Randy, yes (try it), and that's a case where you have to use the expansion form with the $. You could also do the arithmetic as part of the substring expansion, like ${symbol:0: ${#str} - n }. Except of course that if the count goes negative, the meaning changes, with s=12345678, ${s:0: -3} gives 12345, i.e. ending at three chars before the end of the string Commented Mar 22, 2023 at 17:45
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If what you want is to right-pad $dk to the length of $ndk using characters from $symbols, in zsh:

$ dk=FIRST ndk=7 symbol='!@#$%^&'
$ printf '%s\n' ${(pr[ndk][$symbol])dk}
FIRST!@
$ ndk=20
$ printf '%s\n' ${(pr[ndk][$symbol])dk}
FIRST!@#$%^&!@#$%^&!

That uses the r[length][string] right-padding parameter expansion flag. The p flags changes how the $symbol is interpreted. Without p, the padding string would be $symbol literally.

Beware that if $dk is longer than $ndk characters, it will truncate it.

To pad to a width of $ndk columns instead of to a length of $ndk characters, that is to take into account the display width of each character, add the m parameter expansion flag.

To pad to a number of byte instead of characters (which would make a difference if there were multibyte characters in either $dk or $symbols), you'd disable the multibyte option of set the locale to C (and not use the m flag).

See info zsh flags for details.

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