Il client da riga di comando mysql

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Informazioni sul client da riga di comando mysql

mysql è una semplice shell SQL (che fa uso di GNU readline). Supporta sia l'utilizzo interattivo sia quello non interattivo. Quando è usato in modo interattivo, i risultati delle query sono presentati come tabelle con caratteri ASCII. Quando è usato in modo non interattivo (per esempio come filtro), i risultati sono presentati con un carattere tab come separatore. Il formato dell'output può essere cambiato tramite le opzioni.

Se con i grandi insiemi di risultati la memoria non è sufficiente, si usi l'opzione --quick. Essa fa sì che mysql estragga i risultati dal server una riga alla volta, piuttosto che riceverli tutti insieme e inserirli in un buffer in memoria in attesa di mostrarli. Questo si può ottenere anche utilizzando la funzione dell'API C mysql_use_result() nella libreria client/server, invece di mysql_store_result().

Utilizzare mysql è molto semplice. Per invocarlo dal prompt dei comandi:

shell> mysql nome_db

Oppure:

shell> mysql --user=nome_utente --password=pwd nome_db

Poi si possono scrivere le istruzioni SQL, che devono terminare con “;”, \g, o \G, e infine premere Invio.

A partire da MySQL 5.1.10, se si preme Control-C mysql tenta di terminare l'esecuzione dell'ultima istruzione. Se questo non è possibile, o se Control-C viene premuto di nuovo prima che il comando venga terminato, mysql termina l'esecuzione. Precedentemente, Control-C terminata mysql in ogni caso.

E' possibile eseguire le istruzioni SQL in uno script (file batch) in questo modo:

shell> mysql nome_db < script.sql > output.tab

mysql Options

Le opzioni possono essere specificate dalla riga di comando o nel file my.cnf, nei gruppi [mysql] e [client]. L'elenco completo delle opzioni supportate si può visualizzare così:

shell> mysql --verbose --help

In MariaDB 5.1.49 l'output di tale comando è il seguente:

shell> mysql --verbose --help mysql Ver 14.16 Distrib 5.1.49-MariaDB, for debian-linux-gnu (i486) using readline 5.1 This software comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to modify and redistribute it under the GPL v2 license Usage: mysql [OPTIONS] [database] -?, --help Display this help and exit. -I, --help Synonym for -? --abort-source-on-error Abort 'source filename' operations in case of errors --auto-rehash Enable automatic rehashing. One doesn't need to use 'rehash' to get table and field completion, but startup and reconnecting may take a longer time. Disable with --disable-auto-rehash. -A, --no-auto-rehash No automatic rehashing. One has to use 'rehash' to get table and field completion. This gives a quicker start of mysql and disables rehashing on reconnect. -B, --batch Don't use history file. Disable interactive behavior. (Enables --silent.) --character-sets-dir=name Directory for character set files. --column-type-info Display column type information. -c, --comments Preserve comments. Send comments to the server. The default is --skip-comments (discard comments), enable with --comments. -C, --compress Use compression in server/client protocol. -#, --debug[=#] This is a non-debug version. Catch this and exit. --debug-check Check memory and open file usage at exit. -T, --debug-info Print some debug info at exit. -D, --database=name Database to use. --default-character-set=name Set the default character set. --delimiter=name Delimiter to be used. -e, --execute=name Execute command and quit. (Disables --force and history file.) -E, --vertical Print the output of a query (rows) vertically. -f, --force Continue even if we get an SQL error. Sets abort-source-on-error to 0 -G, --named-commands Enable named commands. Named commands mean this program's internal commands; see mysql> help . When enabled, the named commands can be used from any line of the query, otherwise only from the first line, before an enter. Disable with --disable-named-commands. This option is disabled by default. -g, --no-named-commands Named commands are disabled. Use \* form only, or use named commands only in the beginning of a line ending with a semicolon (;). Since version 10.9, the client now starts with this option ENABLED by default. Disable with '-G'. Long format commands still work from the first line. WARNING: option deprecated; use --disable-named-commands instead. -i, --ignore-spaces Ignore space after function names. --local-infile Enable/disable LOAD DATA LOCAL INFILE. -b, --no-beep Turn off beep on error. -h, --host=name Connect to host. -H, --html Produce HTML output. -X, --xml Produce XML output. --line-numbers Write line numbers for errors. -L, --skip-line-numbers Don't write line number for errors. -n, --unbuffered Flush buffer after each query. --column-names Write column names in results. -N, --skip-column-names Don't write column names in results. -O, --set-variable=name Change the value of a variable. Please note that this option is deprecated; you can set variables directly with --variable-name=value. --sigint-ignore Ignore SIGINT (CTRL-C). -o, --one-database Only update the default database. This is useful for skipping updates to other database in the update log. --pager[=name] Pager to use to display results. If you don't supply an option, the default pager is taken from your ENV variable PAGER. Valid pagers are less, more, cat [> filename], etc. See interactive help (\h) also. This option does not work in batch mode. Disable with --disable-pager. This option is disabled by default. --no-pager Disable pager and print to stdout. See interactive help (\h) also. WARNING: option deprecated; use --disable-pager instead. -p, --password[=name] Password to use when connecting to server. If password is not given it's asked from the tty. -P, --port=# Port number to use for connection or 0 for default to, in order of preference, my.cnf, $MYSQL_TCP_PORT, /etc/services, built-in default (3306). --prompt=name Set the mysql prompt to this value. --protocol=name The protocol to use for connection (tcp, socket, pipe, memory). -q, --quick Don't cache result, print it row by row. This may slow down the server if the output is suspended. Doesn't use history file. -r, --raw Write fields without conversion. Used with --batch. --reconnect Reconnect if the connection is lost. Disable with --disable-reconnect. This option is enabled by default. -s, --silent Be more silent. Print results with a tab as separator, each row on new line. -S, --socket=name The socket file to use for connection. --ssl Enable SSL for connection (automatically enabled with other flags).Disable with --skip-ssl. --ssl-ca=name CA file in PEM format (check OpenSSL docs, implies --ssl). --ssl-capath=name CA directory (check OpenSSL docs, implies --ssl). --ssl-cert=name X509 cert in PEM format (implies --ssl). --ssl-cipher=name SSL cipher to use (implies --ssl). --ssl-key=name X509 key in PEM format (implies --ssl). --ssl-verify-server-cert Verify server's "Common Name" in its cert against hostname used when connecting. This option is disabled by default. -t, --table Output in table format. --tee=name Append everything into outfile. See interactive help (\h) also. Does not work in batch mode. Disable with --disable-tee. This option is disabled by default. --no-tee Disable outfile. See interactive help (\h) also. WARNING: Option deprecated; use --disable-tee instead. -u, --user=name User for login if not current user. -U, --safe-updates Only allow UPDATE and DELETE that uses keys. -U, --i-am-a-dummy Synonym for option --safe-updates, -U. -v, --verbose Write more. (-v -v -v gives the table output format). -V, --version Output version information and exit. -w, --wait Wait and retry if connection is down. --connect_timeout=# Number of seconds before connection timeout. --max_allowed_packet=# The maximum packet length to send to or receive from server. --net_buffer_length=# The buffer size for TCP/IP and socket communication. --select_limit=# Automatic limit for SELECT when using --safe-updates. --max_join_size=# Automatic limit for rows in a join when using --safe-updates. --secure-auth Refuse client connecting to server if it uses old (pre-4.1.1) protocol. --server-arg=name Send embedded server this as a parameter. --show-warnings Show warnings after every statement.

Default options are read from the following files in the given order: /etc/my.cnf /etc/mysql/my.cnf /usr/etc/my.cnf /.my.cnf The following groups are read: mysql client The following options may be given as the first argument: --print-defaults Print the program argument list and exit. --no-defaults Don't read default options from any option file. --defaults-file=# Only read default options from the given file #. --defaults-extra-file=# Read this file after the global files are read.

Variables (--variable-name=value) and boolean options {FALSE|TRUE} Value (after reading options) --------------------------------- ----------------------------- abort-source-on-error FALSE auto-rehash TRUE character-sets-dir (No default value) column-type-info FALSE comments FALSE compress FALSE debug-check FALSE debug-info FALSE database (No default value) default-character-set latin1 delimiter ; vertical FALSE force FALSE named-commands FALSE ignore-spaces FALSE local-infile FALSE no-beep FALSE host (No default value) html FALSE xml FALSE line-numbers TRUE unbuffered FALSE column-names TRUE sigint-ignore FALSE port 3306 prompt \N [\d]> quick FALSE raw FALSE reconnect TRUE socket /var/run/mysqld/mysqld.sock ssl FALSE ssl-ca (No default value) ssl-capath (No default value) ssl-cert (No default value) ssl-cipher (No default value) ssl-key (No default value) ssl-verify-server-cert FALSE table FALSE user (No default value) safe-updates FALSE i-am-a-dummy FALSE connect_timeout 0 max_allowed_packet 16777216 net_buffer_length 16384 select_limit 1000 max_join_size 1000000 secure-auth FALSE show-warnings FALSE

The output you get when running the above may differ depending on any customizations you may have in your local my.cnf file.

The mysql_history File

On Unix, the mysql client writes a record of executed statements to a history file. By default, this file is named .mysql_history and is created in your home directory. To specify a different file, set the value of the MYSQL_HISTFILE environment variable.

The .mysql_history should be protected with a restrictive access mode because sensitive information might be written to it, such as the text of SQL statements that contain passwords. See Section 5.3.2.2, “End-User Guidelines for Password Security”.

If you do not want to maintain a history file, first remove .mysql_history if it exists, and then use either of the following techniques:

  • Set the MYSQL_HISTFILE variable to /dev/null. To cause this setting to take effect each time you log in, put the setting in one of your shell's startup files.
  • Create .mysql_history as a symbolic link to /dev/null:
shell> ln -s /dev/null $HOME/.mysql_history

You need do this only once.

mysql Tips

This section describes some techniques that can help you use mysql more effectively.

Displaying Query Results Vertically

Some query results are much more readable when displayed vertically, instead of in the usual horizontal table format. Queries can be displayed vertically by terminating the query with \G instead of a semicolon. For example, longer text values that include newlines often are much easier to read with vertical output:

mysql> SELECT * FROM mails WHERE LENGTH(txt) < 300 LIMIT 300,1\G
*************************** 1. row ***************************
  msg_nro: 3068
    date: 2000-03-01 23:29:50
time_zone: +0200
mail_from: Monty
    reply: monty@no.spam.com
  mail_to: "Thimble Smith" <tim@no.spam.com>
      sbj: UTF-8
      txt: >>>>> "Thimble" == Thimble Smith writes:
Thimble> Hi.  I think this is a good idea.  Is anyone familiar
Thimble> with UTF-8 or Unicode? Otherwise, I´ll put this on my
Thimble> TODO list and see what happens.
Yes, please do that.
Regards,
Monty
    file: inbox-jani-1
    hash: 190402944
1 row in set (0.09 sec)

Using the --safe-updates Option

For beginners, a useful startup option is --safe-updates (or --i-am-a-dummy, which has the same effect). It is helpful for cases when you might have issued a 'DELETE FROM tbl_name' statement but forgotten the WHERE clause. Normally, such a statement deletes all rows from the table. With --safe-updates, you can delete rows only by specifying the key values that identify them. This helps prevent accidents.

When you use the --safe-updates option, mysql issues the following statement when it connects to the MySQL server:

SET sql_safe_updates=1, sql_select_limit=1000, sql_max_join_size=1000000;

The SET statement has the following effects:

  • You are not allowed to execute an UPDATE or DELETE statement unless you specify a key constraint in the WHERE clause or provide a LIMIT clause (or both). For example:
UPDATE tbl_name SET not_key_column=val WHERE key_column=val;
UPDATE tbl_name SET not_key_column=val LIMIT 1;
  • The server limits all large SELECT results to 1,000 rows unless the statement includes a LIMIT clause.
  • The server aborts multiple-table SELECT statements that probably need to examine more than 1,000,000 row combinations.

To specify limits different from 1,000 and 1,000,000, you can override the defaults by using the --select_limit and --max_join_size options:

shell> mysql --safe-updates --select_limit=500 --max_join_size=10000

Disabling mysql Auto-Reconnect

If the mysql client loses its connection to the server while sending a statement, it immediately and automatically tries to reconnect once to the server and send the statement again. However, even if mysql succeeds in reconnecting, your first connection has ended and all your previous session objects and settings are lost: temporary tables, the autocommit mode, and user-defined and session variables. Also, any current transaction rolls back. This behavior may be dangerous for you, as in the following example where the server was shut down and restarted between the first and second statements without you knowing it:

mysql> SET @a=1;
Query OK, 0 rows affected (0.05 sec)
mysql> INSERT INTO t VALUES(@a);
ERROR 2006: MySQL server has gone away
No connection. Trying to reconnect...
Connection id:    1
Current database: test
Query OK, 1 row affected (1.30 sec)
mysql> SELECT * FROM t;
+------+
| a    |
+------+
| NULL |
+------+
1 row in set (0.05 sec)

The @a user variable has been lost with the connection, and after the reconnection it is undefined. If it is important to have mysql terminate with an error if the connection has been lost, you can start the mysql client with the --skip-reconnect option.

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