10-15-2012
Shell quoting rules for other languages programmers
I've seen so many times that programmers were confused about shell quoting and white spaces interpretation, that I decided to investigate that problem deeper. And my conclusion is that quoting in shells is
very different from other programming languages. Programmers who have bigger experience in C/Python/Java/PHP than in Shell should remember about two
unintuitive rules:
- everything in shell is a string, and one need to quote everything properly. Also when reading variables: "$X", not only storing X="hi there"
- shell will parse lines of code just one time, leaving unresolved nested variables and commands. In this case programmer need to explicitly use "eval" function.
I've written short article about this if anyone will be more curious you can find it under "white shell cofoh"
This User Gave Thanks to wyderkat For This Post:
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LEARN ABOUT POSIX
upscli_list_start
UPSCLI_LIST_START(3) NUT Manual UPSCLI_LIST_START(3)
NAME
upscli_list_start - begin multi-item retrieval from a UPS
SYNOPSIS
#include <upsclient.h>
int upscli_list_start(UPSCONN_t *ups, int numq, const char **query)
DESCRIPTION
The upscli_list_start() function takes the pointer ups to a UPSCONN_t state structure, and the pointer query to an array of numq query
elements. It builds a properly-formatted request from those elements and transmits it to upsd(8).
Upon success, the caller must call upscli_list_next(3) to retrieve the elements of the list. Failure to retrieve the list will most likely
result in the client getting out of sync with the server due to buffered data.
USES
This function implements the "LIST" command in the protocol. As a result, you can use it to request many different things from the server.
Some examples are:
o LIST UPS
o LIST VAR <ups>
o LIST RW <ups>
o LIST CMD <ups>
o LIST ENUM <ups> <var>
o LIST RANGE <ups> <var>
QUERY FORMATTING
To see the list of variables on a UPS called su700, the protocol command would be LIST VAR su700. To start that list with this function,
you would populate query and numq as follows:
int numq;
const char *query[2];
query[0] = "VAR";
query[1] = "su700";
numq = 2;
All escaping of special characters and quoting of elements with spaces are handled for you inside this function.
ERROR CHECKING
This function checks the response from upsd(8) against your query. If it is not starting a list, or is starting the wrong type of list, it
will return an error code.
When this happens, upscli_upserror(3) will return UPSCLI_ERR_PROTOCOL.
RETURN VALUE
The upscli_list_start() function returns 0 on success, or -1 if an error occurs.
SEE ALSO
upscli_fd(3), upscli_get(3), upscli_readline(3), upscli_sendline(3), upscli_ssl(3), upscli_strerror(3), upscli_upserror(3)
Network UPS Tools 05/31/2012 UPSCLI_LIST_START(3)