667 lines
10 KiB
HTML
667 lines
10 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>Signals</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Bash Guide for Beginners"
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HREF="index.html"><LINK
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TITLE="Catching signals"
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HREF="chap_12.html"><LINK
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REL="PREVIOUS"
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TITLE="Catching signals"
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HREF="chap_12.html"><LINK
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REL="NEXT"
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TITLE="Traps"
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HREF="sect_12_02.html"></HEAD
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><BODY
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CELLSPACING="0"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Bash Guide for Beginners</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="chap_12.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 12. Catching signals</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="sect_12_02.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="sect1"
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><H1
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CLASS="sect1"
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><A
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NAME="sect_12_01"
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></A
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>12.1. Signals</H1
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="sect_12_01_01"
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></A
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>12.1.1. Introduction</H2
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><DIV
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CLASS="sect3"
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><H3
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CLASS="sect3"
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><A
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NAME="sect_12_01_01_01"
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></A
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>12.1.1.1. Finding the signal man page</H3
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><P
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>Your system contains a man page listing all the available signals, but depending on your operating system, it might be opened in a different way. On most Linux systems, this will be <B
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CLASS="command"
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>man <TT
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CLASS="option"
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>7</TT
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> signal</B
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>. When in doubt, locate the exact man page and section using commands like</P
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><P
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><B
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CLASS="command"
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>man <TT
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CLASS="option"
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>-k</TT
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> signal | grep <TT
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CLASS="option"
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>list</TT
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></B
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> </P
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><P
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>or</P
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><P
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><B
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CLASS="command"
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>apropos signal | grep <TT
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CLASS="option"
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>list</TT
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></B
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> </P
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><P
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>Signal names can be found using <B
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CLASS="command"
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>kill -l</B
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>.</P
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></DIV
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><DIV
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CLASS="sect3"
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><H3
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CLASS="sect3"
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><A
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NAME="sect_12_01_01_02"
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></A
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>12.1.1.2. Signals to your Bash shell</H3
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><P
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>In the absence of any traps, an interactive Bash shell ignores <EM
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>SIGTERM</EM
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> and <EM
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>SIGQUIT</EM
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>. <EM
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>SIGINT</EM
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> is caught and handled, and if job control is active, <EM
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>SIGTTIN</EM
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>, <EM
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>SIGTTOU</EM
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> and <EM
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>SIGTSTP</EM
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> are also ignored. Commands that are run as the result of a command substitution also ignore these signals, when keyboard generated.</P
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><P
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><EM
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>SIGHUP</EM
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> by default exits a shell. An interactive shell will send a <EM
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>SIGHUP</EM
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> to all jobs, running or stopped; see the documentation on the <B
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CLASS="command"
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>disown</B
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> built-in if you want to disable this default behavior for a particular process. Use the <TT
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CLASS="option"
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>huponexit</TT
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> option for killing all jobs upon receiving a <EM
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>SIGHUP</EM
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> signal, using the <B
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CLASS="command"
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>shopt</B
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> built-in.</P
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></DIV
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><DIV
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CLASS="sect3"
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><H3
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CLASS="sect3"
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><A
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NAME="sect_12_01_01_03"
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></A
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>12.1.1.3. Sending signals using the shell</H3
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><P
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>The following signals can be sent using the Bash shell:</P
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><DIV
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CLASS="table"
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><A
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NAME="tab_12_01"
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></A
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><P
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><B
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>Table 12-1. Control signals in Bash</B
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></P
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><TABLE
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BORDER="1"
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CLASS="CALSTABLE"
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><THEAD
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><TR
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><TH
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Standard key combination</TH
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><TH
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Meaning</TH
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></TR
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></THEAD
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><TBODY
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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><B
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CLASS="keycap"
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>Ctrl</B
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>+<B
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CLASS="keycap"
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>C</B
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>The interrupt signal, sends SIGINT to the job running in the foreground.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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><B
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CLASS="keycap"
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>Ctrl</B
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>+<B
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CLASS="keycap"
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>Y</B
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>The <EM
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>delayed suspend</EM
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> character. Causes a running process to be stopped when it attempts to read input from the terminal. Control is returned to the shell, the user can foreground, background or kill the process. Delayed suspend is only available on operating systems supporting this feature.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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><B
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CLASS="keycap"
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>Ctrl</B
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>+<B
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CLASS="keycap"
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>Z</B
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>The <EM
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>suspend</EM
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> signal, sends a <EM
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>SIGTSTP</EM
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> to a running program, thus stopping it and returning control to the shell.</TD
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></TR
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></TBODY
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></TABLE
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></DIV
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><DIV
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CLASS="note"
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><P
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></P
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><TABLE
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CLASS="note"
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WIDTH="100%"
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BORDER="0"
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><TR
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/note.gif"
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HSPACE="5"
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ALT="Note"></TD
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><TH
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ALIGN="LEFT"
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VALIGN="CENTER"
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><B
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>Terminal settings</B
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></TH
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></TR
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><TR
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><TD
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> </TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>Check your <B
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CLASS="command"
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>stty</B
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> settings. Suspend and resume of output is usually disabled if you are using <SPAN
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CLASS="QUOTE"
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>"modern"</SPAN
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> terminal emulations. The standard <B
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CLASS="command"
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>xterm</B
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> supports <B
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CLASS="keycap"
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>Ctrl</B
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>+<B
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CLASS="keycap"
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>S</B
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> and <B
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CLASS="keycap"
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>Ctrl</B
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>+<B
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CLASS="keycap"
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>Q</B
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> by default.</P
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></TD
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></TR
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></TABLE
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></DIV
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></DIV
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></DIV
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="sect_12_01_02"
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></A
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>12.1.2. Usage of signals with kill</H2
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><P
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>Most modern shells, Bash included, have a built-in <B
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CLASS="command"
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>kill</B
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> function. In Bash, both signal names and numbers are accepted as options, and arguments may be job or process IDs. An exit status can be reported using the <TT
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CLASS="option"
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>-l</TT
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> option: zero when at least one signal was successfully sent, non-zero if an error occurred.</P
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><P
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>Using the <B
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CLASS="command"
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>kill</B
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> command from <TT
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CLASS="filename"
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>/usr/bin</TT
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>, your system might enable extra options, such as the ability to kill processes from other than your own user ID and specifying processes by name, like with <B
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CLASS="command"
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>pgrep</B
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> and <B
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CLASS="command"
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>pkill</B
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>.</P
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><P
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>Both <B
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CLASS="command"
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>kill</B
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> commands send the <EM
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>TERM</EM
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> signal if none is given.</P
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><P
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>This is a list of the most common signals:</P
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><DIV
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CLASS="table"
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><A
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NAME="tab_12_02"
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></A
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><P
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><B
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>Table 12-2. Common kill signals</B
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></P
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><TABLE
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BORDER="1"
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CLASS="CALSTABLE"
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><THEAD
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><TR
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><TH
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Signal name</TH
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><TH
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Signal value</TH
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><TH
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Effect</TH
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></TR
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></THEAD
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><TBODY
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>SIGHUP</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>1</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Hangup</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>SIGINT</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>2</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Interrupt from keyboard</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>SIGKILL</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>9</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Kill signal</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>SIGTERM</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>15</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Termination signal</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>SIGSTOP</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>17,19,23</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Stop the process</TD
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></TR
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></TBODY
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></TABLE
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></DIV
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><DIV
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CLASS="note"
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><P
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></P
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><TABLE
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CLASS="note"
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WIDTH="100%"
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BORDER="0"
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><TR
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/note.gif"
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HSPACE="5"
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ALT="Note"></TD
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><TH
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ALIGN="LEFT"
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VALIGN="CENTER"
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><B
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>SIGKILL and SIGSTOP</B
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></TH
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></TR
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><TR
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><TD
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> </TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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><EM
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>SIGKILL</EM
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> and <EM
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>SIGSTOP</EM
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> can not be caught, blocked or ignored.</P
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></TD
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></TR
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></TABLE
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></DIV
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><P
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>When killing a process or series of processes, it is common sense to start trying with the least dangerous signal, <EM
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>SIGTERM</EM
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>. That way, programs that care about an orderly shutdown get the chance to follow the procedures that they have been designed to execute when getting the <EM
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>SIGTERM</EM
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> signal, such as cleaning up and closing open files. If you send a <EM
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>SIGKILL</EM
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> to a process, you remove any chance for the process to do a tidy cleanup and shutdown, which might have unfortunate consequences.</P
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><P
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>But if a clean termination does not work, the <EM
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>INT</EM
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> or<EM
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>KILL</EM
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> signals might be the only way. For instance, when a process does not die using <B
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CLASS="keycap"
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>Ctrl</B
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>+<B
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CLASS="keycap"
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>C</B
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>, it is best to use the <B
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CLASS="command"
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>kill <TT
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CLASS="option"
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>-9</TT
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></B
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> on that process ID:</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> <TT
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CLASS="prompt"
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>maud: ~></TT
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> <B
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CLASS="command"
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>ps <TT
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CLASS="option"
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>-ef</TT
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> | grep <TT
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CLASS="parameter"
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><I
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>stuck_process</I
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></TT
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></B
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>
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maud 5607 2214 0 20:05 pts/5 00:00:02 stuck_process
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<TT
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CLASS="prompt"
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>maud: ~></TT
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> <B
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CLASS="command"
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>kill <TT
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CLASS="option"
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>-9</TT
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> <TT
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CLASS="parameter"
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><I
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>5607</I
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></TT
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></B
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>
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<TT
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CLASS="prompt"
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>maud: ~></TT
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> <B
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CLASS="command"
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>ps <TT
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CLASS="option"
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>-ef</TT
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> | grep <TT
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CLASS="parameter"
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><I
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>stuck_process</I
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></TT
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></B
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>
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maud 5614 2214 0 20:15 pts/5 00:00:00 grep stuck_process
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[1]+ Killed stuck_process
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
|
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><P
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>When a process starts up several instances, <B
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CLASS="command"
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>killall</B
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> might be easier. It takes the same option as the <B
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CLASS="command"
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>kill</B
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> command, but applies on all instances of a given process. Test this command before using it in a production environment, since it might not work as expected on some of the commercial Unices.</P
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></DIV
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></DIV
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><DIV
|
|
CLASS="NAVFOOTER"
|
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="chap_12.html"
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>Prev</A
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HREF="index.html"
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>Home</A
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HREF="sect_12_02.html"
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VALIGN="top"
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>Catching signals</TD
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WIDTH="34%"
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>Traps</TD
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