The Linux logger command provides an easy way to add log files to /var/log/syslog — from the command line, from scripts, or from other files. In today's post, we'll take a look at how it works.
How easy is easy?
This easy. Just type logger <message> on the command line and your message will be added to the end of the /var/log/syslog file.
$ logger comment to be added to log $ tail -1 /vvar/log/syslog May 21 18:02:16 butterfly shs: comment to be added to log
Command output
You can also add the output from commands by enclosing the commands in backticks.
$ logger `who` $ tail -1 /var/log/syslog May 21 18:02:43 butterfly shs: shs pts/0 2018-05-21 15:57 (192.168.0.15)
Content from a file
The contents of text files can be added by using the -f option. Put the name of the file to be added to the log following the -f option as shown below.
$ cat msg Backups to off-site facility will run this coming weekend. System availability will not be affected. $ logger -f msg $ tail -2 /var/log/syslog May 21 18:06:01 butterfly shs: Backups to off-site facility will run this coming weekend. May 21 18:06:01 butterfly shs: System availability will not be affected.
Using logger in scripts
You can add logger commands to scripts to make it easier to track the completion of important tasks.
$ grep logger /bin/runme logger "$0 completed at `date`" $ sudo runme $ tail -1 /var/log/syslog May 21 17:57:36 butterfly shs: ./runme completed at Mon May 21 17:57:36 EDT 2018
Limiting the size of logger entries
If you're concerned about how much data will be added to your log file, especially if you're dumping content from a file, you can use the --size option to limit it. In this example, the size is artificially small to make a point.
$ logger --size 10 12345678901234567890123456789012345678901234567890 $ tail -1 /var/log/syslog May 21 18:18:02 butterfly shs: 1234567890
This option works differently than you might expect in that, given input that includes blanks, it will constrain the content on a per-line basis rather than an overall length basis.
$ logger --size 5 `date` $ tail -5 /var/log/syslog May 22 08:35:51 butterfly shs: May May 22 08:35:51 butterfly shs: 22 May 22 08:35:51 butterfly shs: 08:35 May 22 08:35:51 butterfly shs: EDT May 22 08:35:51 butterfly shs: 2018
Don't be misled by these simple examples. The --size option is generally used to limit large amounts of text. The default maximum is 1KiB (1024 bytes).
Ignoring blank lines
The -e option allows you to avoid dumping empty lines into your log file. They will simply be ignored. Note, however, that a line that contains blanks will not be considered blank.
$ cat appts
Appts
<=== file includes blank line
8 AM -- get to office
8:30 AM -- meet with boss
11:00 AM -- staff meeting
$ logger -e -f appts
May 22 08:17:31 butterfly shs: Appts <=== log does not
May 22 08:17:31 butterfly shs: 8 AM -- get to office
May 22 08:17:31 butterfly shs: 8:30 AM -- meet with boss
May 22 08:17:31 butterfly shs: 11:00 AM -- staff meeting
May 22 08:17:33 butterfly kernel: [58833.758599] [UFW BLOCK] IN=enp0s25 OUT= MAC=01:00:5e:00:00:fb:ac:63:be:ca:10:cf:08:00 SRC=192.168.0.9 DST=224.0.0.251 LEN=32 TOS=0x00 PREC=0xC0 TTL=1 ID=0 DF PROTO=2
Other options
The logger tool offers others as well — such as writing to a log on another server using -n or --no-act for testing. Check your man page for more details.