Command Line Interface

FRR features a flexible modal command line interface. Often when adding new features or modifying existing code it is necessary to create or modify CLI commands. FRR has a powerful internal CLI system that does most of the heavy lifting for you.

Modes

FRR’s CLI is organized by modes. Each mode is associated with some set of functionality, e.g. EVPN, or some underlying object such as an interface. Each mode contains a set of commands that control the associated functionality or object. Users move between the modes by entering a command, which is usually different for each source and destination mode.

A summary of the modes is given in the following figure.

/*
 * FRR CLI modes and their relationships.
 *
 * Each edge is labeled with the command that causes a transition along that
 * edge. Exit commands and their back edges are implicit.
 */
digraph climodes {
	ratio = "auto"
	mincross = 2.0
	graph [fontsize = 9]
	rankdir = LR

	AUTH_NODE -> VIEW_NODE -> ENABLE_NODE;
	VIEW_NODE -> AUTH_ENABLE_NODE -> ENABLE_NODE;
	ENABLE_NODE -> CONFIG_NODE [ label="configure terminal" ];
	CONFIG_NODE -> RIP_NODE [ label="router rip" ];
	CONFIG_NODE -> RIPNG_NODE [ label="router ripng" ];
	CONFIG_NODE -> BABEL_NODE [ label="router babel" ];
	CONFIG_NODE -> EIGRP_NODE [ label="router eigrp (1-65535)" ];
	CONFIG_NODE -> BGP_NODE [ label="router bgp ASN" ];
	subgraph cluster0 {
		BGP_NODE -> BGP_VPNV4_NODE [ label="address-family vpnv4 [unicast]" ];
		BGP_NODE -> BGP_VPNV6_NODE [ label="address-family vpnv6 [unicast]" ];
		BGP_NODE -> BGP_IPV4_NODE [ label="address-family ipv4 [unicast]" ];
		BGP_NODE -> BGP_IPV4L_NODE [ label="address-family ipv4 labeled-unicast" ];
		BGP_NODE -> BGP_IPV4M_NODE [ label="address-family ipv4 multicast" ];
		BGP_NODE -> BGP_IPV6_NODE [ label="address-family ipv6 [unicast]" ];
		BGP_NODE -> BGP_IPV6L_NODE [ label="address-family ipv6 labeled-unicast" ];
		BGP_NODE -> BGP_IPV6M_NODE [ label="address-family ipv6 multicast" ];
		BGP_NODE -> BGP_FLOWSPECV4_NODE [ label="address-family ipv4 flowspec" ];
		BGP_NODE -> BGP_FLOWSPECV6_NODE [ label="address-family ipv6 flowspec" ];
		BGP_NODE -> BGP_EVPN_NODE [ label="address-family l2vpn evpn" ];
		BGP_EVPN_NODE -> BGP_EVPN_VNI_NODE [ label="vni (1-16777215)" ];
		BGP_NODE -> BGP_VRF_POLICY_NODE [ label="vrf-policy NAME" ];
		BGP_NODE -> BGP_VNC_DEFAULTS_NODE [ label="vnc defaults" ];
		BGP_NODE -> BGP_VNC_NVE_GROUP_NODE [ label="vnc nve-group NAME" ];
		BGP_NODE -> BGP_VNC_L2_GROUP_NODE [ label="vnc l2-group NAME" ];
	}
	subgraph cluster1 {
		LDP_NODE -> LDP_IPV4_NODE [ label="address-family ipv4" ];
		LDP_NODE -> LDP_IPV6_NODE [ label="address-family ipv6" ];
		LDP_IPV4_NODE -> LDP_IPV4_IFACE_NODE [ label="interface IFNAME" ];
		LDP_IPV6_NODE -> LDP_IPV6_IFACE_NODE [ label="interface IFNAME" ];
		LDP_NODE -> LDP_L2VPN_NODE [ label="address-family l2vpn WORD type vpls" ];
		LDP_NODE -> LDP_PSEUDOWIRE_NODE [ label="member pseudowire IFNAME" ];
	}
	CONFIG_NODE -> OSPF_NODE [ label="router ospf [(1-65535)] [vrf NAME]" ];
	CONFIG_NODE -> OSPF6_NODE [ label="router ospf6" ];
	CONFIG_NODE -> LDP_NODE [ label="mpls ldp" ];
	CONFIG_NODE -> ISIS_NODE [ label="router isis WORD" ];
	CONFIG_NODE -> RMAP_NODE [ label="route-map WORD <deny|permit> (1-65535)" ];
	CONFIG_NODE -> PW_NODE [ label="pseudowire IFNAME" ];
	CONFIG_NODE -> VTY_NODE [ label="line vty" ];
	CONFIG_NODE -> KEYCHAIN_NODE [ label="key chain WORD" ];
	CONFIG_NODE -> KEYCHAIN_KEY_NODE [ label="key (0-2147483647)" ];
	KEYCHAIN_NODE -> KEYCHAIN_KEY_NODE [ label="key (0-2147483647)" ];
	KEYCHAIN_KEY_NODE -> KEYCHAIN_NODE [ label="no key (0-2147483647)" ];
	CONFIG_NODE -> VRF_NODE [ label="vrf NAME" ];
	CONFIG_NODE -> INTERFACE_NODE [ label="interface IFNAME vrf NAME" ];
	INTERFACE_NODE -> LINK_PARAMS_NODE [ label="link-params" ];
	CONFIG_NODE -> NH_GROUP_NODE [ label="nexthop-group NAME" ];
	CONFIG_NODE -> RPKI_NODE [ label="rpki" ];
}

See also

Data Structures

Walkup

FRR exhibits, for historical reasons, a peculiar behavior called ‘walkup’. Suppose a user is in OSPF_NODE, which contains only OSPF-specific commands, and enters the following command:

ip route 192.168.100.0/24 10.0.2.2

This command is not defined in OSPF_NODE, so the matcher will fail to match the command in that node. The matcher will then check “parent” nodes of OSPF_NODE. In this case the direct parent of OSPF_NODE is CONFIG_NODE, so the current node switches to CONFIG_NODE and the command is tried in that node. Since static route commands are defined in CONFIG_NODE the command succeeds. The procedure of attempting to execute unmatched commands by sequentially “walking up” to parent nodes only happens in children (direct and indirect) below CONFIG_NODE and stops at CONFIG_NODE.

Unfortunately, the internal representation of the various modes is not actually a graph. Instead, there is an array. The parent-child relationships are not explicitly defined in any datastructure but instead are hard-coded into the specific commands that switch nodes. For walkup, there is a function that takes a node and returns the parent of the node. This interface causes all manner of insidious problems, even for experienced developers, and needs to be fixed at some point in the future.

Defining Commands

All definitions for the CLI system are exposed in lib/command.h. In this header there are a set of macros used to define commands. These macros are collectively referred to as “DEFUNs”, because of their syntax:

DEFUN(command_name,
      command_name_cmd,
      "example command FOO...",
      "Examples\n"
      "CLI command\n"
      "Argument\n")
{
    // ...command handler...
}

DEFUNs generally take four arguments which are expanded into the appropriate constructs for hooking into the CLI. In order these are:

  • Function name - the name of the handler function for the command
  • Command name - the identifier of the struct cmd_element for the command. By convention this should be the function name with _cmd appended.
  • Command definition - an expression in FRR’s CLI grammar that defines the form of the command and its arguments, if any
  • Doc string - a newline-delimited string that documents each element in the command definition

In the above example, command_name is the function name, command_name_cmd is the command name, "example..." is the definition and the last argument is the doc string. The block following the macro is the body of the handler function, details on which are presented later in this section.

In order to make the command show up to the user it must be installed into the CLI graph. To do this, call:

install_element(NODE, &command_name_cmd);

This will install the command into the specified CLI node. Usually these calls are grouped together in a CLI initialization function for a set of commands, and the DEFUNs themselves are grouped into the same source file to avoid cluttering the codebase. The names of these files follow the form *_vty.[ch] by convention. Please do not scatter individual CLI commands in the middle of source files; instead expose the necessary functions in a header and place the command definition in a *_vty.[ch] file.

Definition Grammar

FRR uses its own grammar for defining CLI commands. The grammar draws from syntax commonly seen in *nix manpages and should be fairly intuitive. The parser is implemented in Bison and the lexer in Flex. These may be found in lib/command_lex.l and lib/command_parse.y, respectively.

ProTip: if you define a new command and find that the parser is throwing syntax or other errors, the parser is the last place you want to look. Bison is very stable and if it detects a syntax error, 99% of the time it will be a syntax error in your definition.

The formal grammar in BNF is given below. This is the grammar implemented in the Bison parser. At runtime, the Bison parser reads all of the CLI strings and builds a combined directed graph that is used to match and interpret user input.

Human-friendly explanations of how to use this grammar are given a bit later in this section alongside information on the Data Structures constructed by the parser.

command                ::=  cmd_token_seq
                            cmd_token_seq placeholder_token "..."
cmd_token_seq          ::=  *empty*
                            cmd_token_seq cmd_token
cmd_token              ::=  simple_token
                            selector
simple_token           ::=  literal_token
                            placeholder_token
literal_token          ::=  WORD varname_token
varname_token          ::=  "$" WORD
placeholder_token      ::=  placeholder_token_real varname_token
placeholder_token_real ::=  IPV4
                            IPV4_PREFIX
                            IPV6
                            IPV6_PREFIX
                            VARIABLE
                            RANGE
                            MAC
                            MAC_PREFIX
selector               ::=  "<" selector_seq_seq ">" varname_token
                            "{" selector_seq_seq "}" varname_token
                            "[" selector_seq_seq "]" varname_token
selector_seq_seq       ::=  selector_seq_seq "|" selector_token_seq
                            selector_token_seq
selector_token_seq     ::=  selector_token_seq selector_token
                            selector_token
selector_token         ::=  selector
                            simple_token

Tokens

The various capitalized tokens in the BNF above are in fact themselves placeholders, but not defined as such in the formal grammar; the grammar provides the structure, and the tokens are actually more like a type system for the strings you write in your CLI definitions. A CLI definition string is broken apart and each piece is assigned a type by the lexer based on a set of regular expressions. The parser uses the type information to verify the string and determine the structure of the CLI graph; additional metadata (such as the raw text of each token) is encoded into the graph as it is constructed by the parser, but this is merely a dumb copy job.

Here is a brief summary of the various token types along with examples.

Token type Syntax Description
WORD show ip bgp Matches itself. In the given example every token is a WORD.
IPV4 A.B.C.D Matches an IPv4 address.
IPV6 X:X::X:X Matches an IPv6 address.
IPV4_PREFIX A.B.C.D/M Matches an IPv4 prefix in CIDR notation.
IPV6_PREFIX X:X::X:X/M Matches an IPv6 prefix in CIDR notation.
MAC X:X:X:X:X:X Matches a 48-bit mac address.
MAC_PREFIX X:X:X:X:X:X/M Matches a 48-bit mac address with a mask.
VARIABLE FOOBAR Matches anything.
RANGE (X-Y) Matches numbers in the range X..Y inclusive.

When presented with user input, the parser will search over all defined commands in the current context to find a match. It is aware of the various types of user input and has a ranking system to help disambiguate commands. For instance, suppose the following commands are defined in the user’s current context:

example command FOO
example command (22-49)
example command A.B.C.D/X

The following table demonstrates the matcher’s choice for a selection of possible user input.

Input Matched command Reason
example command eLi7eH4xx0r example command FOO eLi7eH4xx0r is not an integer or IPv4 prefix, but FOO is a variable and matches all input.
example command 42 example command (22-49) 42 is not an IPv4 prefix. It does match both (22-49) and FOO, but RANGE tokens are more specific and have a higher priority than VARIABLE tokens.
example command 10.3.3.0/24 example command A.B.C.D/X The user entered an IPv4 prefix, which is best matched by the last command.

Rules

There are also constructs which allow optional tokens, mutual exclusion, one-or-more selection and repetition.

  • <angle|brackets> – Contain sequences of tokens separated by pipes and provide mutual exclusion. User input matches at most one option.
  • [square brackets] – Contains sequences of tokens that can be omitted. [<a|b>] can be shortened to [a|b].
  • {curly|braces} – similar to angle brackets, but instead of mutual exclusion, curly braces indicate that one or more of the pipe-separated sequences may be provided in any order.
  • VARIADICS... – Any token which accepts input (anything except WORD) which occurs as the last token of a line may be followed by an ellipsis, which indicates that input matching the token may be repeated an unlimited number of times.
  • $name – Specify a variable name for the preceding token. See “Variable Names” below.

Some general notes:

  • Options are allowed at the beginning of the command. The developer is entreated to use these extremely sparingly. They are most useful for implementing the ‘no’ form of configuration commands. Please think carefully before using them for anything else. There is usually a better solution, even if it is just separating out the command definition into separate ones.
  • The developer should judiciously apply separation of concerns when defining commands. CLI definitions for two unrelated or vaguely related commands or configuration items should be defined in separate commands. Clarity is preferred over LOC (within reason).
  • The maximum number of space-separated tokens that can be entered is presently limited to 256. Please keep this limit in mind when implementing new CLI.

Variable Names

The parser tries to fill the “varname” field on each token. This can happen either manually or automatically. Manual specifications work by appending $name after the input specifier:

foo bar$cmd WORD$name A.B.C.D$ip

Note that you can also assign variable names to fixed input tokens, this can be useful if multiple commands share code. You can also use “$name” after a multiple-choice option:

foo bar <A.B.C.D|X:X::X:X>$addr [optionA|optionB]$mode

The variable name is in this case assigned to the last token in each of the branches.

Automatic assignment of variable names works by applying the following rules:

  • manual names always have priority
  • a [no] at the beginning receives no as varname on the no token
  • VARIABLE tokens whose text is not WORD or NAME receive a cleaned lowercase version of the token text as varname, e.g. ROUTE-MAP becomes route_map.
  • other variable tokens (i.e. everything except “fixed”) receive the text of the preceding fixed token as varname, if one can be found. E.g. ip route A.B.C.D/M INTERFACE assigns “route” to the A.B.C.D/M token.

These rules should make it possible to avoid manual varname assignment in 90% of the cases.

Doc Strings

Each token in a command definition should be documented with a brief doc string that informs a user of the meaning and/or purpose of the subsequent command tree. These strings are provided as the last parameter to DEFUN macros, concatenated together and separated by an escaped newline (\n). These are best explained by example.

DEFUN (config_terminal,
       config_terminal_cmd,
       "configure terminal",
       "Configuration from vty interface\n"
       "Configuration terminal\n")

The last parameter is split into two lines for readability. Two newline delimited doc strings are present, one for each token in the command. The second string documents the functionality of the terminal command in the configure subtree.

Note that the first string, for configure does not contain documentation for ‘terminal’. This is because the CLI is best envisioned as a tree, with tokens defining branches. An imaginary start token is the root of every command in a CLI node. Each subsequent written token descends into a subtree, so the documentation for that token ideally summarizes all the functionality contained in the subtree.

A consequence of this structure is that the developer must be careful to use the same doc strings when defining multiple commands that are part of the same tree. Commands which share prefixes must share the same doc strings for those prefixes. On startup the parser will generate warnings if it notices inconsistent doc strings. Behavior is undefined; the same token may show up twice in completions, with different doc strings, or it may show up once with a random doc string. Parser warnings should be heeded and fixed to avoid confusing users.

The number of doc strings provided must be equal to the amount of tokens present in the command definition, read left to right, ignoring any special constructs.

In the examples below, each arrowed token needs a doc string.

"show ip bgp"
 ^    ^  ^

"command <foo|bar> [example]"
 ^        ^   ^     ^

DEFPY

DEFPY(...) is an enhanced version of DEFUN() which is preprocessed by python/clidef.py. The python script parses the command definition string, extracts variable names and types, and generates a C wrapper function that parses the variables and passes them on. This means that in the CLI function body, you will receive additional parameters with appropriate types.

This is best explained by an example. Invoking DEFPY like this:

DEFPY(func, func_cmd, "[no] foo bar A.B.C.D (0-99)$num", "...help...")

defines the handler function like this:

func(self, vty, argc, argv,  /* standard CLI arguments */
     const char *no,         /* unparsed "no" */
     struct in_addr bar,     /* parsed IP address */
     const char *bar_str,    /* unparsed IP address */
     long num,               /* parsed num */
     const char *num_str)    /* unparsed num */

Note that as documented in the previous section, bar is automatically applied as variable name for A.B.C.D. The Python script then detects this as an IP address argument and generates code to parse it into a struct in_addr, passing it in bar. The raw value is passed in bar_str. The range/number argument works in the same way with the explicitly given variable name.

Type rules

Token(s) Type Value if omitted by user
A.B.C.D struct in_addr 0.0.0.0
X:X::X:X struct in6_addr ::
A.B.C.D + X:X::X:X const union sockunion * NULL
A.B.C.D/M const struct prefix_ipv4 * NULL
X:X::X:X/M const struct prefix_ipv6 * NULL
A.B.C.D/M + X:X::X:X/M const struct prefix * NULL
(0-9) long 0
VARIABLE const char * NULL
word const char * NULL
all other const char * NULL

Note the following details:

  • Not all parameters are pointers, some are passed as values.
  • When the type is not const char *, there will be an extra _str argument with type const char *.
  • You can give a variable name not only to VARIABLE tokens but also to word tokens (e.g. constant words). This is useful if some parts of a command are optional. The type will be const char *.
  • [no] will be passed as const char *no.
  • Pointers will be NULL when the argument is optional and the user did not use it.
  • If a parameter is not a pointer, but is optional and the user didn’t use it, the default value will be passed. Check the _str argument if you need to determine whether the parameter was omitted.
  • If the definition contains multiple parameters with the same variable name, they will be collapsed into a single function parameter. The python code will detect if the types are compatible (i.e. IPv4 + IPv6 variants) and choose a corresponding C type.
  • The standard DEFUN parameters (self, vty, argc, argv) are still present and can be used. A DEFUN can simply be edited into a DEFPY without further changes and it will still work; this allows easy forward migration.
  • A file may contain both DEFUN and DEFPY statements.

Getting a parameter dump

The clidef.py script can be called to get a list of DEFUNs/DEFPYs with the parameter name/type list:

lib/clippy python/clidef.py --all-defun --show lib/plist.c > /dev/null

The generated code is printed to stdout, the info dump to stderr. The --all-defun argument will make it process DEFUN blocks as well as DEFPYs, which is useful prior to converting some DEFUNs. The dump does not list the ``_str`` arguments to keep the output shorter.

Note that the clidef.py script cannot be run with python directly, it needs to be run with clippy since the latter makes the CLI parser available.

Include & Makefile requirements

A source file that uses DEFPY needs to include the *_clippy.c file before all DEFPY statements:

/* GPL header */
#include ...
...
#ifndef VTYSH_EXTRACT_PL
#include "daemon/filename_clippy.c"
#endif

DEFPY(...)
DEFPY(...)

install_element(...)

This dependency needs to be marked in Makefile.am or subdir.am: (there is no ordering requirement)

# ...

# if linked into a LTLIBRARY (.la/.so):
filename.lo: filename_clippy.c

# if linked into an executable or static library (.a):
filename.o: filename_clippy.c

Handlers

The block that follows a CLI definition is executed when a user enters input that matches the definition. Its function signature looks like this:

int (*func) (const struct cmd_element *, struct vty *, int, struct cmd_token *[]);

The first argument is the command definition struct. The last argument is an ordered array of tokens that correspond to the path taken through the graph, and the argument just prior to that is the length of the array.

The arrangement of the token array has changed from Quagga’s CLI implementation. In the old system, missing arguments were padded with NULL so that the same parts of a command would show up at the same indices regardless of what was entered. The new system does not perform such padding and therefore it is generally incorrect to assume consistent indices in this array. As a simple example:

Command definition:

command [foo] <bar|baz>

User enters:

command foo bar

Array:

[0] -> command
[1] -> foo
[2] -> bar

User enters:

command baz

Array:

[0] -> command
[1] -> baz

Data Structures

On startup, the CLI parser sequentially parses each command string definition and constructs a directed graph with each token forming a node. This graph is the basis of the entire CLI system. It is used to match user input in order to generate command completions and match commands to functions.

There is one graph per CLI node (not the same as a graph node in the CLI graph). The CLI node struct keeps a reference to its graph (see lib/command.h).

While most of the graph maintains the form of a tree, special constructs outlined in the Rules section introduce some quirks. <>, [] and {} form self-contained ‘subgraphs’. Each subgraph is a tree except that all of the ‘leaves’ actually share a child node. This helps with minimizing graph size and debugging.

As a working example, here is the graph of the following command:

show [ip] bgp neighbors [<A.B.C.D|X:X::X:X|WORD>] [json]
_images/cligraph.png

Graph of example CLI command

FORK and JOIN nodes are plumbing nodes that don’t correspond to user input. They’re necessary in order to deduplicate these constructs where applicable.

Options follow the same form, except that there is an edge from the FORK node to the JOIN node. Since all of the subgraphs in the example command are optional, all of them have this edge.

Keywords follow the same form, except that there is an edge from JOIN to FORK. Because of this the CLI graph cannot be called acyclic. There is special logic in the input matching code that keeps a stack of paths already taken through the node in order to disallow following the same path more than once.

Variadics are a bit special; they have an edge back to themselves, which allows repeating the same input indefinitely.

The leaves of the graph are nodes that have no out edges. These nodes are special; their data section does not contain a token, as most nodes do, or NULL, as in FORK/JOIN nodes, but instead has a pointer to a cmd_element. All paths through the graph that terminate on a leaf are guaranteed to be defined by that command. When a user enters a complete command, the command matcher tokenizes the input and executes a DFS on the CLI graph. If it is simultaneously able to exhaust all input (one input token per graph node), and then find exactly one leaf connected to the last node it reaches, then the input has matched the corresponding command and the command is executed. If it finds more than one node, then the command is ambiguous (more on this in deduplication). If it cannot exhaust all input, the command is unknown. If it exhausts all input but does not find an edge node, the command is incomplete.

The parser uses an incremental strategy to build the CLI graph for a node. Each command is parsed into its own graph, and then this graph is merged into the overall graph. During this merge step, the parser makes a best-effort attempt to remove duplicate nodes. If it finds a node in the overall graph that is equal to a node in the corresponding position in the command graph, it will intelligently merge the properties from the node in the command graph into the already-existing node. Subgraphs are also checked for isomorphism and merged where possible. The definition of whether two nodes are ‘equal’ is based on the equality of some set of token properties; read the parser source for the most up-to-date definition of equality.

When the parser is unable to deduplicate some complicated constructs, this can result in two identical paths through separate parts of the graph. If this occurs and the user enters input that matches these paths, they will receive an ‘ambiguous command’ error and will be unable to execute the command. Most of the time the parser can detect and warn about duplicate commands, but it will not always be able to do this. Hence care should be taken before defining a new command to ensure it is not defined elsewhere.

struct cmd_token

/* Command token struct. */
struct cmd_token
{
        enum cmd_token_type type; // token type
        uint8_t attr;             // token attributes
        bool allowrepeat;         // matcher can match token repetitively?

        char *text;               // token text
        char *desc;               // token description
        long long min, max;       // for ranges
        char *arg;                // user input that matches this token
        char *varname;            // variable name
};

This struct is used in the CLI graph to match input against. It is also used to pass user input to command handler functions, as it is frequently useful for handlers to have access to that information. When a command is matched, the sequence of cmd_tokens that form the matching path are duplicated and placed in order into *argv[]. Before this happens the ->arg field is set to point at the snippet of user input that matched it.

For most nontrivial commands the handler function will need to determine which of the possible matching inputs was entered. Previously this was done by looking at the first few characters of input. This is now considered an anti-pattern and should be avoided. Instead, the ->type or ->text fields for this logic. The ->type field can be used when the possible inputs differ in type. When the possible types are the same, use the ->text field. This field has the full text of the corresponding token in the definition string and using it makes for much more readable code. An example is helpful.

Command definition:

command <(1-10)|foo|BAR>

In this example, the user may enter any one of: - an integer between 1 and 10 - “foo” - anything at all

If the user enters “command f”, then:

argv[1]->type == WORD_TKN
argv[1]->arg  == "f"
argv[1]->text == "foo"

Range tokens have some special treatment; a token with ->type == RANGE_TKN will have the ->min and ->max fields set to the bounding values of the range.

struct cmd_element

struct cmd_node {
        /* Node index. */
        enum node_type node;

        /* Prompt character at vty interface. */
        const char *prompt;

        /* Is this node's configuration goes to vtysh ? */
        int vtysh;

        /* Node's configuration write function */
        int (*func)(struct vty *);

        /* Node's command graph */
        struct graph *cmdgraph;

        /* Vector of this node's command list. */
        vector cmd_vector;

        /* Hashed index of command node list, for de-dupping primarily */
        struct hash *cmd_hash;
};

This struct corresponds to a CLI mode. The last three fields are most relevant here.

cmdgraph
This is a pointer to the command graph that was described in the first part of this section. It is the datastructure used for matching user input to commands.
cmd_vector
This is a list of all the struct cmd_element defined in the mode.
cmd_hash
This is a hash table of all the struct cmd_element defined in the mode. When install_element is called, it checks that the element it is given is not already present in the hash table as a safeguard against duplicate calls resulting in a command being defined twice, which renders the command ambiguous.

All struct cmd_node are themselves held in a static vector defined in lib/command.c that defines the global CLI space.

Command Abbreviation & Matching Priority

It is possible for users to elide parts of tokens when the CLI matcher does not need them to make an unambiguous match. This is best explained by example.

Command definitions:

command dog cow
command dog crow

User input:

c d c         -> ambiguous command
c d co        -> match "command dog cow"

The parser will look ahead and attempt to disambiguate the input based on tokens later on in the input string.

Command definitions:

show ip bgp A.B.C.D
show ipv6 bgp X:X::X:X

User enters:

s i b 4.3.2.1         -> match "show ip bgp A.B.C.D"
s i b ::e0            -> match "show ipv6 bgp X:X::X:X"

Reading left to right, both of these commands would be ambiguous since ‘i’ does not explicitly select either ‘ip’ or ‘ipv6’. However, since the user later provides a token that matches only one of the commands (an IPv4 or IPv6 address) the parser is able to look ahead and select the appropriate command. This has some implications for parsing the *argv[] that is passed to the command handler.

Now consider a command definition such as:

command <foo|VAR>

‘foo’ only matches the string ‘foo’, but ‘VAR’ matches any input, including ‘foo’. Who wins? In situations like this the matcher will always choose the ‘better’ match, so ‘foo’ will win.

Consider also:

show <ip|ipv6> foo

User input:

show ip foo

ip partially matches ipv6 but exactly matches ip, so ip will win.

Inspection & Debugging

Permutations

It is sometimes useful to check all the possible combinations of input that would match an arbitrary definition string. There is a tool in tools/permutations that reads CLI definition strings on stdin and prints out all matching input permutations. It also dumps a text representation of the graph, which is more useful for debugging than anything else. It looks like this:

$ ./permutations "show [ip] bgp [<view|vrf> WORD]"

show ip bgp view WORD
show ip bgp vrf WORD
show ip bgp
show bgp view WORD
show bgp vrf WORD
show bgp

This functionality is also built into VTY/VTYSH; list permutations will list all possible matching input permutations in the current CLI node.

Graph Inspection

When in the Telnet or VTYSH console, show cli graph will dump the entire command space of the current mode in the DOT graph language. This can be fed into one of the various GraphViz layout engines, such as dot, neato, etc.

For example, to generate an image of the entire command space for the top-level mode (ENABLE_NODE):

sudo vtysh -c 'show cli graph' | dot -Tjpg -Grankdir=LR > graph.jpg

To do the same for the BGP mode:

sudo vtysh -c 'conf t' -c 'router bgp' -c 'show cli graph' | dot -Tjpg -Grankdir=LR > bgpgraph.jpg

This information is very helpful when debugging command resolution, tracking down duplicate / ambiguous commands, and debugging patches to the CLI graph builder.