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PROGRAM:

NAME


jshon — JSON parser for the shell

SYNOPSIS


jshon -[P|S|Q|V|C|I|0] [-F path] -[t|l|k|u|p|a] -[s|n] value -[e|i|d] index

DESCRIPTION


jshon parses, reads and creates JSON. It is designed to be as usable as possible from
within the shell and replaces fragile adhoc parsers made from grep/sed/awk as well as
heavyweight one-line parsers made from perl/python.

jshon loads json text from stdin, performs actions, then displays the last action on stdout.
Some of the options output json, others output plain text summaries. Because Bash has very
poor nested datastructures, jshon does not return the JSON as a native object as a typical
library would. Instead jshon retains a history of edits in a stack, and you manipulate the
topmost JSON element.

ACTIONS


Each action takes the form of a short option. Some require arguments. While many instances
of jshon can be piped through each other, actions should be chained sequentially to reduce
calls. All examples use this json sample:

{"a":1,"b":[true,false,null,"str"],"c":{"d":4,"e":5}}
jshon [actions] < sample.json

Most common read-only uses will only need several -e actions and one -a in the middle of
them.

-t (type) returns string, object, array, number, bool, null

jshon -t -> object

-l (length) returns an integer. Only works on string, object, array.

jshon -l -> 3

-k (keys) returns a newline separated list of keys. Only works on object.

jshon -k -> a b c

-e index
(extract) returns json value at "index". Only works on object, array. The index of an
array is an integer.

jshon -e c -> {"d":4,"e":5}

-a (across) maps the remaining actions across the selected element. Only works on objects
and arrays. Multiple -a calls can be nested, though the need is rare in practice.

jshon -e b -a -t -> bool bool null string

-s value
(string) returns a json encoded string. Can later be (-i)nserted to an existing
structure.

jshon -s "back\slash" -> "back\\slash"

-n value
(nonstring/number) returns a json element. Can later be (-i)nserted to an existing
structure. Valid values are 'true', 'false', 'null', 'array', 'object', integers and
floats. Abbreviations t, f, n, [] and {} respectively also work.

jshon -n object -> {}

-u (unstring) returns a decoded string. Only works on simple types: string, int, real,
boolean, null.

jshon -e b -e 3 -u -> str

-p (pop) pops the last manipulation from the stack, rewinding the history. Useful for
extracting multiple values from one object.

jshon -e c -e d -u -p -e e -u -> 4 5

-d index
(delete) removes an item in an array or object. Negative array indexes will wrap
around.

jshon -d b -> {"a":1,"c":{"d":4,"e":5}}

-i index
(insert) is complicated. It is the reverse of extract. Extract puts a json sub-element
on the stack. Insert removes a sub-element from the stack, and inserts that bit of json
into the larger array/object underneath. Use extract to dive into the json tree,
delete/string/nonstring to change things, and insert to push the changes back into the
tree.

jshon -e a -i a -> the orginal json
jshon -s one -i a -> {"a":"one", ...}

Arrays are handled in a special manner. Passing integers will insert a value without
overwriting. Negative integers are acceptable, as is the string 'append'. To overwrite
a value in an array: delete the index, -n/s the new value, and then insert at the index.

jshon -e b -d 0 -s q -i 0 -> {"b":"q",false,null,"str"}

NON-MANIPULATION


There are several meta-options that do not directly edit json. Call these at most once per
invocation.

-F <path>
(file) reads from a file instead of stdin. The only non-manipulation option to take an
argument.

-P (jsonp) strips a jsonp callback before continuing normally.

-S (sort) returns json sorted by key, instead of the original ordering.

-Q (quiet) disables error reporting on stderr, so you don't have to sprinkle "2> /dev/null"
throughout your script.

-V (by-value) enables pass-by-value on the edit history stack. In extreme cases with
thousands of deeply nested values this may result in jshon running several times slower
while using several times more memory. However by-value is safer than by-reference and
generally causes less surprise. By-reference is enabled by default because there is no
risk during read-only operations and generally makes editing json more convenient.

jshon -e c -n 7 -i d -p -> c["d"] == 7
jshon -V -e c -n 7 -i d -p -> c["d"] == 5
jshon -V -e c -n 7 -i d -i c -> c["d"] == 7

With -V , changes must be manually inserted back through the stack instead of simply
popping off the intermediate values.

-C (continue) on potentially recoverable errors. For example, extracting values that don't
exist will add 'null' to the edit stack instead of aborting. Behavior may change in the
future.

-I (in-place) file editing. Requires a file to modify and so only works with -F. This is
meant for making slight changes to a json file. When used, normal output is suppressed
and the bottom of the edit stack is written out.

-0 (null delimiters) Changes the delimiter of -u from a newline to a null. This option
only affects -u because that is the only time a newline may legitimately appear in the
output.

--version
Returns a YYYYMMDD timestamp and exits.

OTHER TOOLS


jshon always outputs one field per line. Many unix tools expect multiple tab separated
fields per line. Pipe the output through 'paste' to fix this. However, paste can not
handle empty lines so pad those with a placeholder. Here is an example:

jshon ... | sed 's/^$/-/' | paste -s -d '\t\t\n'

This replaces blanks with '-' and merges every three lines into one.

There are more and more tools that produce json output. Often these use a line-oriented
json/plaintext hybrid where each line is an independent json structure. Sadly this means
the output as a whole is not legitimate json. Either loop though the data line by line
(calling jshon once for each line) or convert it to a legitimate json array. For example:

while read line; do jshon <<< "$line"; done < <(journalctl -o json)

journalctl -o json | sed -e '1i[' -e '$!s/$/,/' -e '$a]' | jshon

GOLF


If you care about extremely short one liners, arguments can be condensed when it does not
cause ambiguity. The example from -p(op) can be golfed as follows:

jshon -e c -e d -u -p -e e -u == jshon -ec -ed -upee -u

I do not recommend doing this (it makes things much harder to understand) but some people
golf despite the consequences.

CREATING JSON


jshon can create json by passing an empty object as input:

jshon -s one -i a <<< "{}"

AUTHORS


jshon was written by Kyle Keen <keenerd@gmail.com> with patches from Dave Reisner
<d@falconindy.com>, AndrewF (BSD, OSX, jsonp, sorting), and Jean-Marc A (solaris).

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