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:
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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 |
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.
Posted by: Vivek Gite
The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.
10 comment
Try using ‘tr’, e.g.,
$ echo ‘asd;lfj29834slkjajfds298124768ald;09290dsfasd098089adfs’ | tr -d [:alpha:] | tr -d [:punct:]
2983429812476809290098089
Or simply doing this:
echo “ljhdfkldkfs23094823sdklnklsd23984nks8d8d8s” | tr -cd [:digit:]
tr -cd [:digit:] #both more effective and easier to read +1
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?
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.)
In bash, you don’t need any external command:
var=1qa2ws3ed4rf printf "%s\n" "${var//[![:digit:]]/}"Real cool Chris.
I knew Brace Expansion tricks with search/replace pattern, but I never read [bash] accepted RegEx as a pattern. (It does not seem to be written into [bash] [man] page.
Where did you read it first? That would be great to know. Bash-Hackers site ?
So I read here
http://wiki.bash-hackers.org/syntax/pattern
that bash patterns are not exactly equivalent to RegEx, but accepts POSIX character class, as you good remembered us.
Thanks a lot, Chris, and thanks Vivek for his Fantastic Site!
Bash does not use regular expressions (execpt in [[ xxx =~ regex ]]); it uses filename expansion rules, which also accepts character classes.
I could just as well have used [!0-9] instead of the character class:
printf "%s\n" "${var//[!0-9]/}"For digits usually okay, but for everything else it’s not recommended to use an explicit set like [A-Z] instead of [[:upper:]]