A shell assignment is a single word, with no space after the equal sign. So what you wrote assigns an empty value to
thefile; furthermore, since the assignment is grouped with a command, it makes
thefile an environment variable and the assignment is local to that particular command, i.e. only the call to
ls sees the assigned value.
You want to capture the output of a command, so you need to use command substitution:
thefile=$(ls -t -U | grep -m 1 "Screen Shot")
(Some literature shows an alternate syntax
thefile=`ls …` ; the backquote syntax is equivalent to the dollar-parentheses syntax except that quoting inside backquotes is weird sometimes, so just use
Other remarks about your script:
-t (sort by time) with
-U (don't sort with GNU
ls) doesn't make sense; just use
Rather than using
grep to match screenshots, it's clearer to pass a wildcard to
ls and use
head to capture the first file:
thefile=$(ls -td -- *"Screen Shot"* | head -n 1)
It's generally a bad idea to parse the output of
ls. This could fail quite badly if you have file names with nonprintable characters. However, sorting files by date is difficult without
ls, so it's an acceptable solution if you know you won't have unprintable characters or backslashes in file names.
Always use double quotes around variable substitutions, i.e. here write
echo "Most recent screenshot is: $thefile"
Without double quotes, the value of the variable is reexpanded, which will cause trouble if it contains whitespace or other special characters.
You don't need semicolons at the end of a line. They're redundant but harmless.
In a shell script, it's often a good idea to include
set -e. This tells the shell to exit if any command fails (by returning a nonzero status).
If you have GNU
sort (in particular if you're running non-embedded Linux or Cygwin), there's another approach to finding the most recent file: have
find list the files and their dates, and use
read (here assuming
-d '' to read a NUL-delimited record) to extract the youngest file.
IFS=/ read -rd '' ignored thefile < <(
find -maxdepth 1 -type f -name "*Screen Shot*" -printf "%T@/%p\0" |
If you're willing to write this script in zsh instead of bash, there's a much easier way to catch the newest file, because zsh has glob qualifiers that permit wildcard matches not only on names but also on file metadata. The
(om) part after the pattern is the glob qualifiers;
om sorts matches by increasing age (i.e. by modification time, newest first) and
 extracts the first match only. The whole match needs to be in parentheses because it's technically an array, since globbing returns a list of files, even if the
 means that in this particular case the list contains (at most) one file.
print -r "Most recent screenshot is: $thefile"