How to understand the output of echo $-
? It looks like some kind of flag characters. I can't get a clue by googling.
bash – How to Understand the Output of `echo $-`
bashvariable
Related Solutions
A shell assignment is a single word, with no space after the equal sign. So what you wrote assigns an empty value to thefile
; furthermore, since the assignment is grouped with a command, it makes thefile
an environment variable and the assignment is local to that particular command, i.e. only the call to ls
sees the assigned value.
You want to capture the output of a command, so you need to use command substitution:
thefile=$(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 $(…)
.)
Other remarks about your script:
Combining
-t
(sort by time) with-U
(don't sort with GNUls
) doesn't make sense; just use-t
.Rather than using
grep
to match screenshots, it's clearer to pass a wildcard tols
and usehead
to capture the first file:thefile=$(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 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
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 find
and 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 sort
and read
(here assuming bash
or zsh
for -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" |
sort -rnz)
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.
#!/bin/zsh
set -e
cd ~/Desktop
thefile=(*"Screen Shot"*(om[1]))
print -r "Most recent screenshot is: $thefile"
Quote your braces:
bash-3.2$ echo "${X:-"{}"}"
{}
bash-3.2$ X=y
bash-3.2$ echo "${X:-"{}"}"
y
bash-3.2$ unset X
bash-3.2$ echo "${X:-"{}"}"
{}
Inner double quotes are required here, which looks funny but is syntactically fine.
Single quotes won't work, and I'm not entirely sure why not. This is real nested quoting, not end-and-resume, which you can verify by putting spaces in. Double will work fine though.
Best Answer
They represent the values of the shell's flags; this is defined by POSIX:
The Zsh manual mentions it briefly:
as does the Bash manual in the description of
set
:To understand the output of
echo $-
you need to look up the options in your shell's manual. For example, in Bash,echo $-
outputshimBHs
for me, which means that the-h
,-m
,-B
and-H
options are enabled (seehelp set
for details), that the shell is interactive (-i
) and reading from standard input (-s
).