how to check if a substring exist

Question

I have the following string

msg="bbb. aaa.ccc. bbb.dddd. aaa.eee."

the separator between the sub strings is the space.

I want to check for example if "aaa." exist. in the above msg it does not exist.

I want to check for example if "bbb." exist. in the above msg it exists.

I tried with grep, but grep works with newlines as the separators between substrings

How to do that?

glenn jackman · Accepted Answer · 2013-09-30 17:20:03Z

2

This can be done in bash using pattern matching. You want to check if

the substring is at the start of the string
the substring is in the middle, or
the substring is at the end

# pass the string, the substring, and the word separator
matches() {
    [[ $1 == $2$3* ]] || [[ $1 == *$3$2$3* ]] || [[ $1 == *$3$2 ]]
}
msg="bbb. aaa.ccc. bbb.dddd. aaa.eee."
matches "$msg" "aaa." " " && echo y || echo n
matches "$msg" "bbb." " " && echo y || echo n

n
y

This works with dash, so it should work with ash too:

str_contains_word() {
    sep=${3:-" "}
    case "$1" in
        "$2$sep"* | *"$sep$2$sep"* | *"$sep$2") return 0;;
        *) return 1;;
    esac
}

msg="bbb. aaa.ccc. bbb.dddd. aaa.eee."
for substr in aaa. bbb.; do
    printf "%s: " "$substr"
    if str_contains_word "$msg" "$substr"; then echo yes; else echo no; fi
done

edited Sep 30, 2013 at 17:20

answered Sep 30, 2013 at 16:34

glenn jackman

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1 Comment

glenn jackman Over a year ago

I don't know if ash does pattern matching the same way within double brackets.

chepner · Accepted Answer · 2013-09-30 16:35:10Z

0

The simplest way is to use the -w option with grep, which would prevent aaa. from matching aaa.ccc.

if fgrep -qw 'aaa.' <<< "$msg"; then
    # Found aaa.
else:
    # Did not find aaa.
fi

answered Sep 30, 2013 at 16:35

chepner

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4 Comments

Sven Marnach Over a year ago

You need to escape the dot.

chepner Over a year ago

Oops, I think I intended to use fgrep and forgot.

MOHAMED Over a year ago

my ash does not support fgrep

chepner Over a year ago

How about grep -F? (fgrep is typically just a synonym for that option, so you may not have it either). Note that grep is a separate program, so the options it supports are independent of which shell you are using (but very dependent on what version of grep is installed on your system).

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