adb shell logcat – Command to collect logs, debug in Android

Collecting logs on desktop from device

 $ adb shell logcat > logcat-for-debugging.txt 

You can also use “script” command to save all the logs, as well as commands you type on console while debugging using adb as,

 $ script debug_logs.txt 
 $ adb shell 
 $ logcat 

To stop log collection, exit from shell and then exit from script command

You can also save logs to local file in SDcard and then pull those on desktop using command,

 $ adb logcat -f /sdcard/debug_logs.txt 
 $ adb pull /sdcard/debug_logs.txt 
Usage: logcat [options] [filterspecs]
options include:
  -s              Set default filter to silent. Equivalent to filterspec '*:S'
  -f <file>, --file=<file>               Log to file. Default is stdout
  -r <kbytes>, --rotate-kbytes=<kbytes>
                  Rotate log every kbytes. Requires -f option
  -n <count>, --rotate-count=<count>
                  Sets max number of rotated logs to <count>, default 4
  --id=<id>       If the signature id for logging to file changes, then clear
                  the fileset and continue
  -v <format>, --format=<format>
                  Sets log print format verb and adverbs, where <format> is:
                    brief help long process raw tag thread threadtime time
                  and individually flagged modifying adverbs can be added:
                    color descriptive epoch monotonic printable uid
                    usec UTC year zone
                  Multiple -v parameters or comma separated list of format and
                  format modifiers are allowed.
  -D, --dividers  Print dividers between each log buffer
  -c, --clear     Clear (flush) the entire log and exit
                  if Log to File specified, clear fileset instead
  -d              Dump the log and then exit (don't block)
  -e <expr>, --regex=<expr>
                  Only print lines where the log message matches <expr>
                  where <expr> is a Perl-compatible regular expression
  -m <count>, --max-count=<count>
                  Quit after printing <count> lines. This is meant to be
                  paired with --regex, but will work on its own.
  --print         Paired with --regex and --max-count to let content bypass
                  regex filter but still stop at number of matches.
  -t <count>      Print only the most recent <count> lines (implies -d)
  -t '<time>'     Print most recent lines since specified time (implies -d)
  -T <count>      Print only the most recent <count> lines (does not imply -d)
  -T '<time>'     Print most recent lines since specified time (not imply -d)
                  count is pure numerical, time is 'MM-DD hh:mm:ss.mmm...'
                  'YYYY-MM-DD hh:mm:ss.mmm...' or 'sssss.mmm...' format
  -g, --buffer-size                      Get the size of the ring buffer.
  -G <size>, --buffer-size=<size>
                  Set size of log ring buffer, may suffix with K or M.
  -L, --last      Dump logs from prior to last reboot
  -b <buffer>, --buffer=<buffer>         Request alternate ring buffer, 'main',
                  'system', 'radio', 'events', 'crash', 'default' or 'all'.
                  Multiple -b parameters or comma separated list of buffers are
                  allowed. Buffers interleaved. Default -b main,system,crash.
  -B, --binary    Output the log in binary.
  -S, --statistics                       Output statistics.
  -p, --prune     Print prune white and ~black list. Service is specified as
                  UID, UID/PID or /PID. Weighed for quicker pruning if prefix
                  with ~, otherwise weighed for longevity if unadorned. All
                  other pruning activity is oldest first. Special case ~!
                  represents an automatic quicker pruning for the noisiest
                  UID as determined by the current statistics.
  -P '<list> ...', --prune='<list> ...'
                  Set prune white and ~black list, using same format as
                  listed above. Must be quoted.
  --pid=<pid>     Only prints logs from the given pid.
  --wrap          Sleep for 2 hours or when buffer about to wrap whichever
                  comes first. Improves efficiency of polling by providing
                  an about-to-wrap wakeup.
 
filterspecs are a series of 
  <tag>[:priority]
 
where <tag> is a log component tag (or * for all) and priority is:
  V    Verbose (default for <tag>)
  D    Debug (default for '*')
  I    Info
  W    Warn
  E    Error
  F    Fatal
  S    Silent (suppress all output)
 
'*' by itself means '*:D' and <tag> by itself means <tag>:V.
If no '*' filterspec or -s on command line, all filter defaults to '*:V'.
eg: '*:S <tag>' prints only <tag>, '<tag>:S' suppresses all <tag> log messages.
 
If not specified on the command line, filterspec is set from ANDROID_LOG_TAGS.
 
If not specified with -v on command line, format is set from ANDROID_PRINTF_LOG
or defaults to "threadtime"

You can find more information and its related command line arguments at https://developer.android.com/studio/command-line/logcat

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