Bash Shell: Extract Digits From a String

Posted on in Categories last updated February 16, 2011

How do I extract digits only from a given string under Bash shell?

You can use the sed, grep and other shell utilities as follows:

grep -o "[0-9]" <<<"input"
grep -o "[0-9]" <<<"$var"
grep -o "[0-9]" <<<"test123"
grep -o "[0-9]" <<<"1Th5is is a test. 5"
var="foo1bar2"
output=$(grep -o "[0-9]" <<<"$var")
echo "$output"
grep -oE "[[:digit:]]{1,}" input.file

See our grep command and grep regex tutorial for more information.

10 comment

  1. Try using ‘tr’, e.g.,
    $ echo ‘asd;lfj29834slkjajfds298124768ald;09290dsfasd098089adfs’ | tr -d [:alpha:] | tr -d [:punct:]
    2983429812476809290098089

  2. Can we use regex in grep? For example, I only want to extract the http links for every line of my file. Is it possible?

    1. Yes Cenk. RegEx are included.

      Read The Fantastic Manual, give it a try, and then share your results with us once you are done. Good reading.
      Looking forward to reading from you.

      GREP(1) User Commands GREP(1)

      NAME
      grep, egrep, fgrep, rgrep – print lines matching a pattern

      SYNOPSIS
      grep [OPTIONS] PATTERN [FILE…]
      grep [OPTIONS] [-e PATTERN | -f FILE] [FILE…]

      DESCRIPTION
      grep searches the named input FILEs (or standard input if no files are named, or if a single hyphen-minus (-) is given
      as file name) for lines containing a match to the given PATTERN. By default, grep prints the matching lines.

      In addition, three variant programs egrep, fgrep and rgrep are available. egrep is the same as grep -E. fgrep is the
      same as grep -F. rgrep is the same as grep -r. Direct invocation as either egrep or fgrep is deprecated, but is
      provided to allow historical applications that rely on them to run unmodified.

      OPTIONS
      Generic Program Information
      –help Print a usage message briefly summarizing these command-line options and the bug-reporting address, then exit.

      -V, –version
      Print the version number of grep to the standard output stream. This version number should be included in all
      bug reports (see below).

      Matcher Selection
      -E, –extended-regexp
      Interpret PATTERN as an extended regular expression (ERE, see below). (-E is specified by POSIX.)

      -F, –fixed-strings
      Interpret PATTERN as a list of fixed strings, separated by newlines, any of which is to be matched. (-F is
      specified by POSIX.)

      -G, –basic-regexp
      Interpret PATTERN as a basic regular expression (BRE, see below). This is the default.

      -P, –perl-regexp
      Interpret PATTERN as a Perl regular expression. This is highly experimental and grep -P may warn of
      unimplemented features.

      Matching Control
      -e PATTERN, –regexp=PATTERN
      Use PATTERN as the pattern. This is useful to protect patterns beginning with hyphen-minus (-). (-e is
      specified by POSIX.)

    1. Real cool Chris.
      I knew Brace Expansion tricks with search/replace pattern, but I never read [bash] accepted RegEx as a pattern. (It does not seem to be written into [bash] [man] page.
      Where did you read it first? That would be great to know. Bash-Hackers site ?

    1. Bash does not use regular expressions (execpt in [[ xxx =~ regex ]]); it uses filename expansion rules, which also accepts character classes.

      I could just as well have used [!0-9] instead of the character class:

      printf "%s\n" "${var//[!0-9]/}"
      
      1. For digits usually okay, but for everything else it’s not recommended to use an explicit set like [A-Z] instead of [[:upper:]]

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