I have a watch on
jq -r '.' $(ls -t | head -1)"
The goal is to apply "jq -r '.'" to the most recently modified file in directory. Unfortunately, the command used to know the last modified file is evaluated when watch is launched, but not at refresh. How should i write it, so that all of the command is reevaluated at each refresh ?
Thanks
If you pass the command in single quotes it will be expanded on each cycle of watch rather than when you run the watch command, for example:
watch echo $(date)
watch 'echo $(date)'
The first command will print the original date over and over, the second command will run the date on each cycle and print a different date.
This is because you are passing the literal string echo $(date) to the watch command, which runs it each cycle and echos the correct date. If you don't use quotes, the shell will expand the $(date) and replace it with the output of the date command and pass that to watch, so it will print the same output over and over.
The main issue with your command is that when you execute
watch jq -r '.' "$(ls -t | head -1)"
... then the command substitution is executed once, as part of evaluating the command line arguments for the watch command.
You must stop the current shell from evaluating the command substitution when launching watch to have watch run a command that evaluates the substitution each time the command is run.
You can do that with
watch 'jq -r . "$(ls -t | head -1)"'
The most recently modified regular file (possibly symbolically linked) in a directory is given by the filename globbing pattern *(.om[1]) in the zsh shell. The parenthesis qualifies the way that the preceding * matches names; The dot selects exclusively regular files (possibly symbolically linked), om orders (o) by last-modified timestamp (m), and [1] picks out the first name only from the generated list.
Another way to write your command would therefore be
watch -x zsh -c 'jq . *(.om[1])'
Here, the -x causes watch to exec the given command instead of running it with sh -c.
