Linux Security Cookbook

9.32. Writing Log Entries via Shell Scripts

Problem

You want to add information to the system log using a shell script.

Solution

Use logger and this handy API, which emulates that of Perl and C:

               syslog-api.sh:
#!/bin/sh
ident="$USER"
facility="user"
openlog( ) {
        if [ $# -ne 3 ]
        then
                echo "usage: openlog ident option[,option,...] facility" 1>&2
                return 1
        fi
        ident="$1"
        local option="$2"
        facility="$3"
        case ",$option," in
                *,pid,*)            ident="$ident[$$]";;
        esac
}

syslog( ) {
        if [ $# -lt 2 ]
        then
                echo "usage: syslog [facility.]priority format ..." 1>&2
                return 1
        fi
        local priority="$1"
        local format="$2"
        shift 2
        case "$priority" in
                *.*)    ;;
                *)      priority="$facility.$priority";;
        esac
        printf "$format" "$@" | logger -t "$ident" -p "$priority"
}

closelog( ) {
        ident="$USER"
        facility="user"
}

To use the functions in a shell script:

#!/bin/sh
source syslog-api.sh
openlog `basename "$0"` pid local3
syslog warning "%d connections from %s" $count $host
syslog authpriv.err "intruder alert!"
closelog

The syslog API

The standard API for the system logger provides the following three functions for Perl scripts and C programs, and we provide an implementation for Bash shell scripts as well. [Recipe 9.32]

openlog

Specify the identifier prepended to each message, conventionally the basename of the program or script. An option is provided to add the process ID as well; other options are less commonly used. Finally, a default facility is established for subsequent messages: local0 through local6 are good choices. ...