CODEWITHSUNDEEP
regexbash
Zack
Zack

Reputation: 139

bash: parsing a number out of a text string

I'm writing a little bash script that scan a list of text lines, each of which has the format:

num1 num2 num3 filename

For each line, I only want to parse out the first numerical token. This is my code:

printf "input line: %s\n" "${line}"
let number="${line//^[0-9]+/}"
printf "regexp parsed %s\n" "${number}"

Well, it does parse out the first number in the line, but also outputs an error message:

input line: 11531          1008      16   12555    310b /usr/bin/gresource
./statistics.sh: line 21: let: number=11531           1008      16   12555    310b /usr/bin/gresource: syntax error in expression (error token is "1008          16   12555    310b /usr/bin/gresource")
regexp parsed 11531

Why do I get this error message? How can I apply the regexp $[0-9]+ on $line without getting the error?

Upvotes: 0

Views: 832

Answers (2)

chepner
chepner

Reputation: 530920

Parameter expansions expect patterns, not regular expressions. Further, your attempt would remove the number rather than capturing it. What's really happening is that let is converting the entire line to a number by commenting on, but ignoring, the non-numeric part of the line. (That is, it only "works" because the line actually starts with a number.)

Consider the following, using the extended pattern equivalent to the regular expression [0-9]+. Note that your regular expression, treated as a pattern, doesn't match anything.

$ echo "$line"
11531          1008      16   12555    310b /usr/bin/gresource
$ echo "${line//^[0-9]+/}"
11531          1008      16   12555    310b /usr/bin/gresource
$ shopt -s extglob
$ echo "${line/+([0-9])}"
          1008      16   12555    310b /usr/bin/gresource

Use a regular expression match.

[[ $line =~ [0-9]+ ]] && number=${BASH_REMATCH[0]}

Upvotes: 1

agc
agc

Reputation: 8406

If the lines are all that format, use cut, since there'd be no need to parse for numbers:

cut -d ' ' -f 1 <<< 'num1 num2 num3 filename'

Output:

num1

For an input file do:

cut -d ' ' -f 1  inputfile.txt

Upvotes: 1

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