I want to assign the result of an expression
(i.e., the output from a command)
to a variable and then manipulate it – for example,
concatenate it with a string, then echo it.
Here's what I've got:
#!/bin/bash
cd ~/Desktop;
thefile= ls -t -U | grep -m 1 "Screen Shot";
echo "Most recent screenshot is: "$thefile;
But that outputs:
Screen Shot 2011-07-03 at 1.55.43 PM.png
Most recent screenshot is:
So, it looks like that isn't getting assigned to $thefile
, and is being printed as it's executed.
What am I missing?
Best Answer
A shell assignment is a single word, with no space after the equal sign. So what you wrote assigns an empty value to
thefile
; furthermore, since the assignment is grouped with a command, it makesthefile
an environment variable and the assignment is local to that particular command, i.e. only the call tols
sees the assigned value.You want to capture the output of a command, so you need to use command substitution:
(Some literature shows an alternate syntax
thefile=`ls …`
; the backquote syntax is equivalent to the dollar-parentheses syntax except that quoting inside backquotes is weird sometimes, so just use$(…)
.)Other remarks about your script:
Combining
-t
(sort by time) with-U
(don't sort with GNUls
) doesn't make sense; just use-t
.Rather than using
grep
to match screenshots, it's clearer to pass a wildcard tols
and usehead
to capture the first file: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 withoutls
, 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
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
find
andsort
(in particular if you're running non-embedded Linux or Cygwin), there's another approach to finding the most recent file: havefind
list the files and their dates, and usesort
andread
(here assumingbash
orzsh
for-d ''
to read a NUL-delimited record) to extract the youngest file.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[1])
part after the pattern is the glob qualifiers;om
sorts matches by increasing age (i.e. by modification time, newest first) and[1]
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[1]
means that in this particular case the list contains (at most) one file.