Extracting last ten characters of a string (e.g., file name) inside a parameter expansion

Question

I have a file name (including an extension, but not a full path name) in variable file, and I want to only get the last 10 characters of the base name from a parameter expansion.

Example: file contains the filename A_text_document_1234567890.txt.

Desired output: 1234567890.

I know that I can use ${file%.*} to remove the extension: echo ${file%.*} outputs A_text_document_1234567890.

If I do base=${file%.*}, then I can use ${base: -10} to get 1234567890. Can I do this inside the parameter expansion all in one step without using a second variable?

I guess my real question is, how do I combine these two parameter expansions into one?

You'd better show us the before and after: what is the filename, and what do you want to "pass it"? — glenn jackman, Feb 26 '20 at 23:37
Does this answer your question? Renaming a file in Bash using regular expressions — Jetchisel, Feb 26 '20 at 23:49
That is Parameter Expansion the question is misleading... which has answered on some post here. — Jetchisel, Feb 26 '20 at 23:49
You can't combine them. Take the result of one and apply the other expansion to that — muru, Feb 27 '20 at 01:03
You can do that in standard shell, without using the non-POSIX ${var:off:len} form, with "${file#"${file%??????????}"}" — , Feb 27 '20 at 11:22
"I guess my real question is, how do I combine these two parameter expansions into one?" -- You don't, not in Bash or ksh, but you can do it in Zsh. See the linked questions. — ilkkachu, Feb 27 '20 at 17:49
@ilkkachu my comment should've probably went below the answer, since it was meant as a portable alternative to ${file: -10}. — , Feb 27 '20 at 17:51
If the extension of the file is .txt, the solution in bash/ksh/zsh is obviously "${file: -14:10}" ;-) — , Feb 27 '20 at 18:02

glenn jackman · Accepted Answer · 2020-02-26T23:58:39.607

3

file=abcdefghijklm
echo ${file: -10}

defghijklm

The space after the colon is required to differentiate that parameter expansion from the ${var:-default} variety.

Using a 2nd variable:

$ tmp=${file%.*}; echo "${tmp:(-10)}"
1234567890

Using a regular expression:

$ [[ $file =~ (.{10})(\.[^.]*)?$ ]] && echo "${BASH_REMATCH[1]}"
1234567890

edited Feb 26 '20 at 23:58

answered Feb 26 '20 at 23:36

glenn jackman

85,964

1

${file:(-10)} will work down to bash2+. – Jetchisel Feb 26 '20 at 23:46

Extracting last ten characters of a string (e.g., file name) inside a parameter expansion

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