grep lines that start with a specific string

Question

I want to find all the lines in a file that start with a specific string. The problem is, I don't know what's in the string beforehand. The value is stored in a variable.

The naïve solution would be the following:

grep "^${my_string}" file.txt;

Because if the Bash variable my_string contains ANY regular expression special characters, grep will cry, and everyone will have a bad day.

You don't want to make grep cry, do you?

Solution

You should use awk instead of grep for non-regex search using index function:

awk -v s="$my_string" 'index($0, s) == 1' file

index($0, s) == 1 ensures search string is found only at start.

Comments

[anubhava]

Problem is that using -F we cannot force grep to find match only at start of line.

[James Brown]

@anubhava Oh yeah, that is true. Good morning.

[Silvia Justi]

what if I want to find the string at the beginning of the line, but I need the whole line to be printed?

[anubhava]

That's what this command does. Test it out

[Silvia Justi]

Found my mistake!