jshn: a JSON parsing and generation library in for shell scripts

jshn (JSON SHell Notation), a small utility and shell library for parsing and generating JSON data.

Shell scripts (ash, bash, zsh) doesn't have built-in functions to work with JSON or other hierarchical structures so OpenWrt provides a shell library /usr/share/libubox/jshn.sh from libubox package. You need to include it into your scripts and then you can call it's functions:

#!/bin/sh
 
# source jshn shell library
. /usr/share/libubox/jshn.sh
 
# generating json data
json_init
json_add_string "msg" "Hello, world!"
json_add_object "test"
json_add_int "testdata" "1"
json_close_object
MSG=`json_dump`
# MSG now contains: { "msg": "Hello, world!", "test": { "testdata": 1 } }
 
 
# parsing json data
json_load "$MSG"
json_select test
json_get_var var1 testdata
json_select ..
json_get_var var2 msg
echo "msg: $var2 - testdata: $var1"

Another example:

#!/bin/sh
 
# source jshn shell library
. /usr/share/libubox/jshn.sh
 
# initialize JSON output structure
json_init
 
# add a boolean field
json_add_boolean foo 0
 
# add an integer field
json_add_int code 123
 
# add a string, take care of escaping
json_add_string result "Some complex string\n with newlines\n and even command output: $(date)"
 
# add an array with three integers
json_add_array alist
json_add_int "" 1
json_add_int "" 2
json_add_int "" 3
json_close_array
 
# add an object (dictionary)
json_add_object adict
json_add_string foo bar
json_add_string bar baz
json_close_object
 
# build JSON object and print to stdout
json_dump
 
# will output something like this:
# { "foo": false, "code": 123, "result": "Some complex string\\n with newlines\\n and even command output: Fri Jul 13 07:11:39 CEST 2018", "alist": [ 1, 2, 3 ], "adict": { "foo": "bar", "bar": "baz" } }

Given the file:

/data1.json

{
  "status": 200,
  "msg": "ok",
  "result": {
    "number": "99",
    "mac": "d85d4c9c2f1a",
    "last_seen": 1363777473407
  }
}

Script to parse it:

#!/bin/sh
. /usr/share/libubox/jshn.sh
 
json_init
json_load_file /data2.txt  ## Load JSON from file
## json_get_var <local_var> <json_var>
json_get_var status status    ## Get Value of status inside JSON string (i.e. MSG) into local "status" variable
json_get_var msg msg
json_select result            ## Select "result" object of JSON string (i.e. MSG)
json_get_var number number
json_get_var mac mac
json_get_var last_seen last_seen

Given the file:

/data2.json

{
  "ip_addrs": {
    "lan": ["192.168.0.100","192.168.0.101","192.168.0.200"],
    "wan": ["10.11.12.22","10.11.12.23", "10.11.12.25"]
  }
}

Script to parse it:

#!/bin/sh
. /usr/share/libubox/jshn.sh
 
json_init
json_load_file /data2.json
json_select "ip_addrs"
 
if json_get_type Type lan && [ "$Type" == array ]
then
	json_select lan
	local idx="1"
	while json_get_type Type "$idx" && [ "$Type" == string ]	## iterate over data inside "lan" object
	do
		json_get_var ip_addr $idx 
		echo "$ip_addr"
		$((idx++)) 2> /dev/null
	done
fi

Internally the /usr/share/libubox/jshn.sh is just a wrapper for /usr/bin/jshn utility. You can use it directly:

root@OpenWrt:/# jshn
Usage: jshn [-n] [-i] -r <message>|-R <file>|-o <file>|-p <prefix>|-w
root@OpenWrt:/# jshn -i -R /etc/board.json 
json_init;
json_add_object 'model';
json_add_string 'id' 'innotek-gmbh-virtualbox';
json_add_string 'name' 'innotek GmbH VirtualBox';
json_close_object;
json_add_object 'network';
json_add_object 'lan';
json_add_string 'ifname' 'eth0';
json_add_string 'protocol' 'static';
json_close_object;
json_add_object 'wan';
json_add_string 'ifname' 'eth1';
json_add_string 'protocol' 'dhcp';
json_close_object;
json_close_object;

Options:

• -r <message> parse from string <message>
• -R <file> parse from file <file>
• -w write to stdout
• -o <file> write to file <file>
• -p <prefix> set prefix
• -n no newline
• -i indent

OpenWrt have a command devstatus to check network device status:

root@OpenWrt:~# devstatus br-lan
{
        "external": false,
        "present": true,
        "type": "bridge",
        "up": true,
        "carrier": true,
        "bridge-members": [
                "eth0.1",
                "wlan0"
        ],
        "mtu": 1500,
        "mtu6": 1500,
        "macaddr": "84:16:f9:9b:e0:7a",
        "txqueuelen": 1000,
        "ipv6": true,
        "promisc": false,
        "rpfilter": 0,
        "acceptlocal": false,
        "igmpversion": 0,
        "mldversion": 0,
        "neigh4reachabletime": 30000,
        "neigh6reachabletime": 30000,
        "neigh4gcstaletime": 60,
        "neigh6gcstaletime": 60,
        "neigh4locktime": 100,
        "dadtransmits": 1,
        "multicast": true,
        "sendredirects": true,
        "statistics": {
                "collisions": 0,
                "rx_frame_errors": 0,
                "tx_compressed": 0,
                "multicast": 0,
                "rx_length_errors": 0,
                "tx_dropped": 0,
                "rx_bytes": 10609108786,
                "rx_missed_errors": 0,
                "tx_errors": 0,
                "rx_compressed": 0,
                "rx_over_errors": 0,
                "tx_fifo_errors": 0,
                "rx_crc_errors": 0,
                "rx_packets": 39594607,
                "tx_heartbeat_errors": 0,
                "rx_dropped": 0,
                "tx_aborted_errors": 0,
                "tx_packets": 91154927,
                "rx_errors": 0,
                "tx_bytes": 121051584071,
                "tx_window_errors": 0,
                "rx_fifo_errors": 0,
                "tx_carrier_errors": 0
        }
}

And you can check it's sources which internally uses the jshn:

cat /sbin/devstatus
#!/bin/sh
. /usr/share/libubox/jshn.sh
DEVICE="$1"
 
[ -n "$DEVICE" ] || {
	echo "Usage: $0 <device>"
	exit 1
}
 
json_init
json_add_string name "$DEVICE"
ubus call network.device status "$(json_dump)"

#!/bin/sh
 
. /usr/share/libubox/jshn.sh
 
WANDEV="$(uci get network.wan.ifname)"
 
json_load "$(devstatus $WANDEV)"
 
json_get_var var1 speed
json_get_var var2 link
 
echo "Speed: $var1"
echo "Link: $var2"