Filtering a result from a command

Question

I am having trouble writing a little program; when I run the following command from my program:

gsettings get org.gnome.desktop.background picture-uri

I get:

'file:///home/thamara/.config/variety/Favorites/wallpaper-2448037.jpg'

But that needs to become:

/home/thamara/.config/variety/Favorites/wallpaper-2448037.jpg

Or the program will not work. Can someone please help me?

The program:

#!/bin/bash
## Blogry for GDM - Blurs your current wallpaper and puts it on your loginscreen

## Some Settings
effect='0x8' # Change this to anything you like http://www.imagemagick.org/Usage/blur/
save='~/Pictures/blur.jpg'

## Step one -  getting the current wallpaper
dir=$(gsettings get org.gnome.desktop.background picture-uri)

## Step two - Blurring the wallpaper
convert $dir -blur $effect -blur $effect -blur $effect $save

## Step three - exit (Since GDM automatically loads the new wallpaper there is no need for seting it.)
exit 0

## Links:
# http://www.imagemagick.org/Usage/blur/

Answer 1

When I run this command on Ubuntu 12.10 I get the following:

$ dir=$(gsettings get org.gnome.desktop.background picture-uri)
$ echo $dir
'file:///usr/share/backgrounds/warty-final-ubuntu.png'

I would just cleanup the value getting stored in $dir like so:

$ dir="${dir/file:\/\//}"
$ echo $dir
'/usr/share/backgrounds/warty-final-ubuntu.png'

This will truncate the file:// from the beginning of the string. You can change this if you're getting something different. In your case:

$ dir="${dir/\/\//}"

Details

The above is using a pattern substitution, ${var/pattern/} which will remove pattern from the variable $var.

Alternatives

@jthill also had a good suggestion of using Bash's "remove matching pattern prefix` notation instead. It's a little trickier to understand, IMO, but works equally as well.

Example

$ dir="\'${dir#\'file://}"

The above is removing the prefix, \'file:// from $dir. It's replacing it with a tick, ', followed by the remainder of $dir without the 'file://.

Bash man page

If you want to read up more on these features of Bash, I'd encourage you to do so. These are features that we're using above.

excerpts from Bash man page

${parameter#word}
${parameter##word}
       Remove matching prefix pattern.  The word is expanded to produce a 
       pattern just as in pathname expansion.  If the pattern matches the 
       beginning of  the  value  of  parameter, then  the  result  of  the  
       expansion is the expanded value of parameter with the shortest 
       matching pattern (the ``#'' case) or the longest matching pattern 
       (the ``##'' case) deleted.  If parameter is @ or *, the pattern 
       removal operation is applied to each positional parameter in turn, 
       and the expansion is the resultant list.  If parameter is  an array 
       variable subscripted with @ or *, the pattern removal operation is 
       applied to each member of the array in turn, and the expansion is the 
       resultant list.

${parameter/pattern/string}
       Pattern substitution.  The pattern is expanded to produce a pattern 
       just as in pathname expansion.  Parameter is expanded and the longest 
       match of pattern against  its  value is replaced with string.  If 
       pattern begins with /, all matches of pattern are replaced with 
       string.  Normally only the first match is replaced.  If pattern 
       begins with #, it must match at the beginning of the expanded value 
       of parameter.  If pattern begins with %, it must match at the end of 
       the expanded value of parameter.  If  string  is  null, matches  of  
       pattern  are  deleted  and the / following pattern may be omitted.  
       If parameter is @ or *, the substitution operation is applied to each 
       positional parameter in turn, and the expansion is the resultant 
       list.  If parameter is an array variable subscripted with @ or *, the 
       substitution operation is applied to each member of  the  array in 
       turn, and the expansion is the resultant list.

Follow-up Question #1

The OP asked the following in comments below.

now i am having the following problem.. unable to open image '/home/thamara/.config/variety/Downloaded/wallbase_type_all_order_random_nsfw_1‌​00_board_1/wallpaper-2249773.jpg''

The issue, if you'll notice, is that there are 2 tick marks at the end of the string. I have no idea why those are there, but if you'd like to get rid of the trailing tick marks you can use this sed command right after the previous substitution I gave you. I couldn't figure out a way to deal with the 2 single tick marks at the end just using Bash's substitution features.

dir=$(echo "$dir" | sed "s/''/'/")

Example

$ echo "$dir"
'/home/thamara/.config/variety/Downloaded/wallbase_type_all_order_random_nsfw_1‌​00_board_1/wallpaper-2249773.jpg''

$ dir=$(echo "$dir" | sed "s/''/'/")
$ echo "$dir"
'/home/thamara/.config/variety/Downloaded/wallbase_type_all_order_random_nsfw_1‌​00_board_1/wallpaper-2249773.jpg'

Answer 2

The output from the gsettings command complies with the GVariant text syntax. This parameter is a string, printed out in single quotes.

You need to remove the quotes.

dir_uri=$(gsettings get org.gnome.desktop.background picture-uri |
          sed "s/^'//; s/'\$//; s/\\\\\\(.\\)/\\1/")

You then get a URI. If the file name doesn't contain any special characters, it's enough to remove file:// at the beginning (or even file:).

dir=$(gsettings get org.gnome.desktop.background picture-uri |
      sed "s~^'file://~~; s~'\$~~")

Answer 3

If all you want to do is eliminate the extra two // at the start of the file name, and the '' that is oddly showing up at the end of the file name you can use sed:

dir=$(gsettings get org.gnome.desktop.background picture-uri|sed -e 's/\/\/\//\//' -e "s/'//g; s/^/'/; s/$/'/")

This will replace /// with / and delete any erroneously formatted instances or duplicates of '. Finally, it will add a ' at the beginning and end of the line as to properly encapsulate the file name if it has any spaces. This solution is a bit long but it should properly do the job.

I will also echo what slm mentioned and tell you that when I ran:

gsettings get org.gnome.desktop.background picture-uri

I got a result in the format of:

'file:///usr/share/backgrounds/warty-final-ubuntu.png'