Using regex in bash scripting

Question

I'm trying to use regex in an if statement in a bash script, but I am getting different values.

The script:

#!/bin/bash

a="input2.txt"

paramCheck(){
  while read -r line; do
    d=( $line )
    e=${d[@]:1}
    for i in "$e"; do
      if [ "$i" == $[st][0-9] ]; then
         echo "$i"
      fi
    done
  done < "$a"
}

echo `paramCheck`

The text file:

add $s1 $s2 $s3
sub $t0 
sub $t1 $t0 
addi $t1 $t0 $s5

The predicted results:

$s1 $s2 $s3 $t0 $t1 $t0 $t1 $t0 $s5

The actual result was: nothing printed out.

samthegolden · Accepted Answer · 2019-03-28 14:05:06Z

1

You have to use double brackets for regex matching and escape the dollar, as it is a special bash character. Substitute

if [ "$i" == $[st][0-9] ]; then

for

if [[ "$i" = \$[st][0-9] ]]; then

answered Mar 28, 2019 at 14:05

samthegolden

1,5181 gold badge12 silver badges30 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Gordon Davisson Over a year ago

[[ = ]] uses wildcard ("glob") pattern syntax, not regular expressions. For regular expression matching, use [[ =~ ]]' instead.

samthegolden Over a year ago

@GordonDavisson maybe you're right, but why this works then? I'm not using wildcards.

Gordon Davisson Over a year ago

Both glob patterns and RE use [] character classes, but otherwise they're quite a bit different. For instance, in an RE, "?" means "the last thing is optional", but in a glob it means "any single character" (in an RE, you'd use "." for that). In an RE, "*" means "zero or more of the last thing", but in a glob it means "any string of zero or more characters" (in an RE, you'd use ".*" for that). (And of course, there are multiple variations of both glob and RE syntax...) Another difference here is that [[ = ]] must match the whole string, but [[ =~ ]] allows substring matches.

Benjamin W. · Accepted Answer · 2019-03-28 14:57:04Z

Here's one way you could do this using various standard utilities:

$ cut -d' ' -f2- infile | grep -o '\$[st][[:digit:]]' | paste -sd ' '
$s1 $s2 $s3 $t0 $t1 $t0 $t1 $t0 $s

cut removes the first space separated column
grep finds all matches of the pattern and prints them one per line
paste gets the output on a single line

In pure Bash:

#!/usr/bin/env bash

while read -ra line; do
    for word in "${line[@]:1}"; do
        [[ $word == \$[st][[:digit:]] ]] && printf '%s ' "$word"
    done
done < 'input2.txt'

reads directly into an array with read -a
no intermediate assignment, loop directly over elements of "${line[@]:1}"
use [[ ]] for pattern matching, escape $, use locale-safe [[:digit:]] instead of [0-9]
use printf instead of echo to suppress linebreaks

Notice that this'll add a trailing blank.

A few pointers for your code:

d=( $line ) relies on word splitting and is subject to filename expansion; if you have a word * in $line, it'll expand to all files in the directory.
e=${d[@]:1} assigns the second and later elements of the array to a single string – now we don't have an array any longer. To keep the array, use e=("${d[@]:1}") instead.
for i in "$e" now has $e containing all the elements in a single string, and the quoting suppresses word splitting, so for the first line, this'll put all of $s1 $s2 $s3 into i instead of just $s1. The intent is probably for i in $e, but that's again subject to word splitting and glob expansion; use an array instead.
[ ] doesn't support pattern matching, use [[ ]] instead. $ has to be escaped.
Glob patterns (used here) are not regular expressions. Check the "Patterns" article in the references for a good overview of the differences.
Bash does understand both == and = within [ ], but == isn't portable (as in "POSIX conformant") – it's a good habit to use = instead. Within [[ ]], it's debatable what to use, as [[ ]] isn't portable itself.
echo `cmd` is the same as just cmd.

References:

Collectives™ on Stack Overflow

Using regex in bash scripting

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related