# Increment a number (sed, a stream editor)

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### 7.3 Increment a Number

This script is one of a few that demonstrate how to do arithmetic in `sed`

. This is indeed possible,9 but must be done manually.

To increment one number you just add 1 to last digit, replacing it by the following digit. There is one exception: when the digit is a nine the previous digits must be also incremented until you don’t have a nine.

This solution by Bruno Haible is very clever and smart because it uses a single buffer; if you don’t have this limitation, the algorithm used in Numbering lines, is faster. It works by replacing trailing nines with an underscore, then using multiple `s`

commands to increment the last digit, and then again substituting underscores with zeros.

#!/usr/bin/sed -f /[^0-9]/ d

# replace all trailing 9s by _ (any other character except digits, could # be used) :d s/9\(_*\)$/_\1/ td

# incr last digit only. The first line adds a most-significant # digit of 1 if we have to add a digit.

s/^\(_*\)$/1\1/; tn s/8\(_*\)$/9\1/; tn s/7\(_*\)$/8\1/; tn s/6\(_*\)$/7\1/; tn s/5\(_*\)$/6\1/; tn s/4\(_*\)$/5\1/; tn s/3\(_*\)$/4\1/; tn s/2\(_*\)$/3\1/; tn s/1\(_*\)$/2\1/; tn s/0\(_*\)$/1\1/; tn

:n y/_/0/

#### Footnotes

### (9)

`sed`

guru Greg Ubben wrote an implementation of the `dc`

RPN calculator! It is distributed together with sed.