CODEWITHSUNDEEP
bashshell
Kagemand Andersen
Kagemand Andersen

Reputation: 1693

Conditionally prepend variable with a slash

I thought I could figure this out. I've written more complex shell scripts before, so I thought it'd be easy to do. I've wasted an hour now and tearing my hair out because nothing I do works and I cannot find any usable help online.

All I want to do is the equivalent to this simple little C# code snippet here:

var splat = "some/path";
if (!splat.StartsWith("/"))
{
    splat = $"/{splat}";
}

Take a string argument. If it starts with a slash, do nothing. If it doesn't start with a slash, add it to the front of the string variable.

How do I accomplish this apparent herculean task in a bash script?

My attempt:

#!/bin/bash

dir="/root/home"
sub_dir=$1

echo "argument given: '"$sub_dir"'"

if [ ! -z $sub_dir ]
then
        echo "before: "$dir$1
        if [[ "${sub_dir}" != *'/' ]]; then
                echo "starts with slash!"
        else
                echo "doesn't start with slash!"
                $sub_dir="\/"${sub_dir}
                echo "slash added: "$sub_dir
        fi
else
        echo "argument not given"
        echo $dir
fi

echo "$dir$sub_dir"

I have also tried using regex with no success for the conditional:

if [[ "$HOST" =~ ^user.* ]]; then

I also have not found a way to pass an argument that does start with a slash or create a string that starts with a slash without it for some reason treating it as a file path instead of a string (and subsequently resulting in an error).

EDIT: bash --version says it's GNU bash verison 5.0.17, if that makes any difference.

Upvotes: 1

Views: 556

Answers (2)

oguz ismail
oguz ismail

Reputation: 50750

Use a parameter expansion that removes the leading character if it is a slash or leaves the value intact otherwise, and prepend the result with a slash.

$ s1=some/path s2=/some/path
$ echo /${s1#/}
/some/path
$ echo /${s2#/}
/some/path

Simply put, you can replace that if command with this line:

sub_dir=/${sub_dir#/}

Upvotes: 4

ajinzrathod
ajinzrathod

Reputation: 940

#!/bin/bash

dir="/root/home"
sub_dir=$1

echo "argument given: '"$sub_dir"'"

if [ ! -z $sub_dir ]
then
    echo "before: "$dir$1
    if [[ $sub_dir = '/'* ]]; then
        echo "Start with slash"
    else
        echo "doesn't start with slash!"
        slash="/"
        sub_dir="${slash}${sub_dir}"
        echo "slash added: "$sub_dir
    fi
else
    echo "argument not given"
    # echo $dir
fi

echo "$dir$sub_dir"

Upvotes: 2

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