I have some configuration in file config and would like to cat that file. However, sometimes config doesn't exist. In this case, I would like to have my command output a default value.
Perhaps something that worked like this:
$ ls
$ cat config || echo 42
42
$ echo 73 > config
$ cat config || echo 42
73
Your construct is fine. You could even do someting like
cat config || cat defaultconfig
If you use some random command (like the ./get_config_from_web in comments), you'll have to make sure the command does give a sensible return status. That can be tricky, shell scripts just return the result of the last command executed, you'd have to do a exit if you want something else as result.
defaultconfig is guaranteed to exist.
cat config gives an error cat: config: No such file or directory if config doesn't exist. So I end up getting the output of both commands instead of just the second one.
/dev/null and it works: cat config 2> /dev/null || echo 42.
The following will echo 42 for any noncaught error condition:
trap "echo 42" ERR
You can make this a configurable variable:
trap 'echo "${CONFIG:=42}"' ERR # if $CONFIG is not set, it will be defaulted to 42
trap logic would be combined with the cat config command from the example. Can you elaborate?
$cat config -> "cat: config: No such file or directory\n" (from cat's error) "42" (from the trap's echo)
You could use the exit code in an if statement:
#!/bin/bash
cat config
if [[ "$?" -ne "0" ]]; then
echo "VALUE"
fi
If the previous command succeeds it will exit with a "0" value. So you are telling it that on any other exit (failures) it will output whatever is in the "VALUE".
if [ -a config ]; then cat config; else echo ${DEFAULT};fi
assuming the variable DEFAULT is set to something beforehand.
cat config example, but I was hoping for a solution that works regardless of whether the failure is due to a missing file. For example ./get_config_from_web.sh || echo 42.
bash syntax that supported this kind of a thing and I just hadn't learned about it yet. If no such bash syntax exists I guess the next best thing is a script, and the best script would be an elegant succinct one.
|| and && constructs to determine the success or failure of an action in the same command space. But again with the enhancements in shell and me being an old timer, it might entirely be possible in a nook and cranny that eludes me and probably you as well