AppleScript doesn't make it easy:
log only logs while running in AppleScript Editor or when running via osascript (to stderr in that case) - the output will be lost in other cases, such as when applications run a script with the NSAppleScript Cocoa class.
log only accepts one argument; while it does accept any object type, it doesn't make it easy to get a meaningful representation of non-built-in types: try log me to get information about the script itself, for instance; frequently, log (get properties of <someObj>) must be used to get meaningful information; note the cumbersome syntax, which is required, because just using log properties of <someObj> typically merely prints the name of the reference form instead of the properties it points to (e.g, log properties of me uselessly outputs just (*properties*)).
In general, AppleScript makes it very hard to get meaningful text representations of objects of non-built-in types: <someObj> as text (same as: <someObj> as string) annoyingly breaks - throws a runtime error - for such objects; try me as text.
Below are helper subroutines that address these issues:
dlog() is a subroutine that combines deriving meaningful text representations of any objects with the ability to write to multiple log targets (including syslog and files) based on a global config variable.toString() (effectively embedded in dlog()) is a subroutine that takes a single object of any type and derives a meaningful text representation from it.
Tip of the hat to @1.61803; his answer provided pointers for implementing the various logging targets.
Examples:
# Setup: Log to syslog and a file in the home dir.
# Other targets supported: "log", "alert"
# Set to {} to suppress logging.
set DLOG_TARGETS to { "syslog", "~/as.log" }
# Log properties of the front window of frontmost application.
dlog(front window of application (path to frontmost application as text))
# Log properties of own front window; note the *list* syntax for multiple args.
dlog({"my front window: ", front window})
# Get properties of the running script as string.
toString(me) # ->, e.g.: [script name="sandbox"] {selection:insertion point after character 2475 of text of document "sandbox2.scpt", frontmost:true, class:application, name:"AppleScript Editor", version:"2.6"}
See the source-code comments above each subroutine for details.
dlog() source code
# Logs a text representation of the specified object or objects, which may be of any type, typically for debugging.
# Works hard to find a meaningful text representation of each object.
# SYNOPSIS
# dlog(anyObjOrListOfObjects)
# USE EXAMPLES
# dlog("before") # single object
# dlog({ "front window: ", front window }) # list of objects
# SETUP
# At the top of your script, define global variable DLOG_TARGETS and set it to a *list* of targets (even if you only have 1 target).
# set DLOG_TARGETS to {} # must be a list with any combination of: "log", "syslog", "alert", <posixFilePath>
# An *empty* list means that logging should be *disabled*.
# If you specify a POSIX file path, the file will be *appended* to; variable references in the path
# are allowed, and as a courtesy the path may start with "~" to refer to your home dir.
# Caveat: while you can *remove* the variable definition to disable logging, you'll take an additional performance hit.
# SETUP EXAMPLES
# For instance, to use both AppleScript's log command *and* display a GUI alert, use:
# set DLOG_TARGETS to { "log", "alert" }
# Note:
# - Since the subroutine is still called even when DLOG_TARGETS is an empty list,
# you pay a performancy penalty for leaving dlog() calls in your code.
# - Unlike with the built-in log() method, you MUST use parentheses around the parameter.
# - To specify more than one object, pass a *list*. Note that while you could try to synthesize a single
# output string by concatenation yourself, you'd lose the benefit of this subroutine's ability to derive
# readable text representations even of objects that can't simply be converted with `as text`.
on dlog(anyObjOrListOfObjects)
global DLOG_TARGETS
try
if length of DLOG_TARGETS is 0 then return
on error
return
end try
# The following tries hard to derive a readable representation from the input object(s).
if class of anyObjOrListOfObjects is not list then set anyObjOrListOfObjects to {anyObjOrListOfObjects}
local lst, i, txt, errMsg, orgTids, oName, oId, prefix, logTarget, txtCombined, prefixTime, prefixDateTime
set lst to {}
repeat with anyObj in anyObjOrListOfObjects
set txt to ""
repeat with i from 1 to 2
try
if i is 1 then
if class of anyObj is list then
set {orgTids, AppleScript's text item delimiters} to {AppleScript's text item delimiters, {", "}} # '
set txt to ("{" & anyObj as string) & "}"
set AppleScript's text item delimiters to orgTids # '
else
set txt to anyObj as string
end if
else
set txt to properties of anyObj as string
end if
on error errMsg
# Trick for records and record-*like* objects:
# We exploit the fact that the error message contains the desired string representation of the record, so we extract it from there. This (still) works as of AS 2.3 (OS X 10.9).
try
set txt to do shell script "egrep -o '\\{.*\\}' <<< " & quoted form of errMsg
end try
end try
if txt is not "" then exit repeat
end repeat
set prefix to ""
if class of anyObj is not in {text, integer, real, boolean, date, list, record} and anyObj is not missing value then
set prefix to "[" & class of anyObj
set oName to ""
set oId to ""
try
set oName to name of anyObj
if oName is not missing value then set prefix to prefix & " name=\"" & oName & "\""
end try
try
set oId to id of anyObj
if oId is not missing value then set prefix to prefix & " id=" & oId
end try
set prefix to prefix & "] "
set txt to prefix & txt
end if
set lst to lst & txt
end repeat
set {orgTids, AppleScript's text item delimiters} to {AppleScript's text item delimiters, {" "}} # '
set txtCombined to lst as string
set prefixTime to "[" & time string of (current date) & "] "
set prefixDateTime to "[" & short date string of (current date) & " " & text 2 thru -1 of prefixTime
set AppleScript's text item delimiters to orgTids # '
# Log the result to every target specified.
repeat with logTarget in DLOG_TARGETS
if contents of logTarget is "log" then
log prefixTime & txtCombined
else if contents of logTarget is "alert" then
display alert prefixTime & txtCombined
else if contents of logTarget is "syslog" then
do shell script "logger -t " & quoted form of ("AS: " & (name of me)) & " " & quoted form of txtCombined
else # assumed to be a POSIX file path to *append* to.
set fpath to contents of logTarget
if fpath starts with "~/" then set fpath to "$HOME/" & text 3 thru -1 of fpath
do shell script "printf '%s\\n' " & quoted form of (prefixDateTime & txtCombined) & " >> \"" & fpath & "\""
end if
end repeat
end dlog
toString() source code
# Converts the specified object - which may be of any type - into a string representation for logging/debugging.
# Tries hard to find a readable representation - sadly, simple conversion with `as text` mostly doesn't work with non-primitive types.
# An attempt is made to list the properties of non-primitive types (does not always work), and the result is prefixed with the type (class) name
# and, if present, the object's name and ID.
# EXAMPLE
# toString(path to desktop) # -> "[alias] Macintosh HD:Users:mklement:Desktop:"
# To test this subroutine and see the various representations, use the following:
# repeat with elem in {42, 3.14, "two", true, (current date), {"one", "two", "three"}, {one:1, two:"deux", three:false}, missing value, me, path to desktop, front window of application (path to frontmost application as text)}
# log my toString(contents of elem)
# end repeat
on toString(anyObj)
local i, txt, errMsg, orgTids, oName, oId, prefix
set txt to ""
repeat with i from 1 to 2
try
if i is 1 then
if class of anyObj is list then
set {orgTids, AppleScript's text item delimiters} to {AppleScript's text item delimiters, {", "}}
set txt to ("{" & anyObj as string) & "}"
set AppleScript's text item delimiters to orgTids # '
else
set txt to anyObj as string
end if
else
set txt to properties of anyObj as string
end if
on error errMsg
# Trick for records and record-*like* objects:
# We exploit the fact that the error message contains the desired string representation of the record, so we extract it from there. This (still) works as of AS 2.3 (OS X 10.9).
try
set txt to do shell script "egrep -o '\\{.*\\}' <<< " & quoted form of errMsg
end try
end try
if txt is not "" then exit repeat
end repeat
set prefix to ""
if class of anyObj is not in {text, integer, real, boolean, date, list, record} and anyObj is not missing value then
set prefix to "[" & class of anyObj
set oName to ""
set oId to ""
try
set oName to name of anyObj
if oName is not missing value then set prefix to prefix & " name=\"" & oName & "\""
end try
try
set oId to id of anyObj
if oId is not missing value then set prefix to prefix & " id=" & oId
end try
set prefix to prefix & "] "
end if
return prefix & txt
end toString
