📄 exit-status.html
字号:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><HTML><HEAD><TITLE>Exit and Exit Status</TITLE><METANAME="GENERATOR"CONTENT="Modular DocBook HTML Stylesheet Version 1.57"><LINKREL="HOME"TITLE="Advanced Bash-Scripting Guide"HREF="index.html"><LINKREL="UP"TITLE="Basics"HREF="part2.html"><LINKREL="PREVIOUS"TITLE="Escaping"HREF="escapingsection.html"><LINKREL="NEXT"TITLE="Tests"HREF="tests.html"><METAHTTP-EQUIV="Content-Style-Type"CONTENT="text/css"><LINKREL="stylesheet"HREF="common/kde-common.css"TYPE="text/css"><METAHTTP-EQUIV="Content-Type"CONTENT="text/html; charset=iso-8859-1"><METAHTTP-EQUIV="Content-Language"CONTENT="en"><LINKREL="stylesheet"HREF="common/kde-localised.css"TYPE="text/css"TITLE="KDE-English"><LINKREL="stylesheet"HREF="common/kde-default.css"TYPE="text/css"TITLE="KDE-Default"></HEAD><BODYCLASS="CHAPTER"BGCOLOR="#FFFFFF"TEXT="#000000"LINK="#AA0000"VLINK="#AA0055"ALINK="#AA0000"STYLE="font-family: sans-serif;"><DIVCLASS="NAVHEADER"><TABLEWIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><THCOLSPAN="3"ALIGN="center">Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH></TR><TR><TDWIDTH="10%"ALIGN="left"VALIGN="bottom"><AHREF="escapingsection.html">Prev</A></TD><TDWIDTH="80%"ALIGN="center"VALIGN="bottom"></TD><TDWIDTH="10%"ALIGN="right"VALIGN="bottom"><AHREF="tests.html">Next</A></TD></TR></TABLE><HRALIGN="LEFT"WIDTH="100%"></DIV><DIVCLASS="CHAPTER"><H1><ANAME="EXIT-STATUS">Chapter 6. Exit and Exit Status</A></H1><TABLEBORDER="0"WIDTH="100%"CELLSPACING="0"CELLPADDING="0"CLASS="EPIGRAPH"><TR><TDWIDTH="45%"> </TD><TDWIDTH="45%"ALIGN="LEFT"VALIGN="TOP"><I><P><I>...there are dark corners in the Bourne shell, and people use all of them.</I></P></I></TD></TR><TR><TDWIDTH="45%"> </TD><TDWIDTH="45%"ALIGN="RIGHT"VALIGN="TOP"><I><SPANCLASS="ATTRIBUTION">Chet Ramey</SPAN></I></TD></TR></TABLE><P><ANAME="EXITCOMMANDREF"></A>The <BCLASS="COMMAND"> exit </B> command may be used to terminate a script, just as in a <ICLASS="EMPHASIS">C</I> program. It can also return a value, which is available to the script's parent process.</P><P><ANAME="EXITSTATUSREF"></A>Every command returns an <ICLASS="FIRSTTERM"> exit status </I> (sometimes referred to as a <ICLASS="FIRSTTERM"> return status </I>). A successful command returns a <SPANCLASS="RETURNVALUE">0</SPAN>, while an unsuccessful one returns a <SPANCLASS="RETURNVALUE">non-zero</SPAN> value that usually may be interpreted as an error code. Well-behaved UNIX commands, programs, and utilities return a <SPANCLASS="RETURNVALUE">0</SPAN> exit code upon successful completion, though there are some exceptions.</P><P>Likewise, functions within a script and the script itself return an exit status. The last command executed in the function or script determines the exit status. Within a script, an <TTCLASS="USERINPUT"><B>exit <TTCLASS="REPLACEABLE"><I>nnn</I></TT></B></TT> command may be used to deliver an <SPANCLASS="RETURNVALUE"><TTCLASS="REPLACEABLE"><I>nnn</I></TT></SPAN> exit status to the shell (<SPANCLASS="RETURNVALUE"><TTCLASS="REPLACEABLE"><I>nnn</I></TT></SPAN> must be a decimal number in the <SPANCLASS="RETURNVALUE">0</SPAN> - <SPANCLASS="RETURNVALUE">255</SPAN> range).</P><DIVCLASS="NOTE"><TABLECLASS="NOTE"WIDTH="100%"BORDER="0"><TR><TDWIDTH="25"ALIGN="CENTER"VALIGN="TOP"><IMGSRC="common/note.png"HSPACE="5"ALT="Note"></TD><TDALIGN="LEFT"VALIGN="TOP"><P>When a script ends with an <BCLASS="COMMAND">exit</B> that has no parameter, the exit status of the script is the exit status of the last command executed in the script (previous to the <BCLASS="COMMAND">exit</B>).</P><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #!/bin/bash 2 3 COMMAND_1 4 5 . . . 6 7 # Will exit with status of last command. 8 COMMAND_LAST 9 10 exit</PRE></TD></TR></TABLE></P><P>The equivalent of a bare <BCLASS="COMMAND">exit</B> is <BCLASS="COMMAND">exit $?</B> or even just omitting the <BCLASS="COMMAND">exit</B>.</P><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #!/bin/bash 2 3 COMMAND_1 4 5 . . . 6 7 # Will exit with status of last command. 8 COMMAND_LAST 9 10 exit $?</PRE></TD></TR></TABLE></P><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #!/bin/bash 2 3 COMMAND1 4 5 . . . 6 7 # Will exit with status of last command. 8 COMMAND_LAST</PRE></TD></TR></TABLE></P></TD></TR></TABLE></DIV><P><ANAME="EXSREF"></A></P><P> <TTCLASS="VARNAME"> $?</TT> reads the exit status of the last command executed. After a function returns, <TTCLASS="VARNAME">$?</TT> gives the exit status of the last command executed in the function. This is Bash's way of giving functions a <SPANCLASS="QUOTE">"return value."</SPAN> After a script terminates, a <TTCLASS="VARNAME">$?</TT> from the command line gives the exit status of the script, that is, the last command executed in the script, which is, by convention, <TTCLASS="USERINPUT"><B>0</B></TT> on success or an integer in the range <SPANCLASS="RETURNVALUE">1 - 255</SPAN> on error.</P><DIVCLASS="EXAMPLE"><HR><ANAME="EX5"></A><P><B>Example 6-1. exit / exit status</B></P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #!/bin/bash 2 3 echo hello 4 echo $? # Exit status 0 returned because command executed successfully. 5 6 lskdf # Unrecognized command. 7 echo $? # Non-zero exit status returned because command failed to execute. 8 9 echo 10 11 exit 113 # Will return 113 to shell. 12 # To verify this, type "echo $?" after script terminates. 13 14 # By convention, an 'exit 0' indicates success, 15 #+ while a non-zero exit value means an error or anomalous condition.</PRE></TD></TR></TABLE><HR></DIV><P><AHREF="variables2.html#XSTATVARREF">$?</A> is especially useful for testing the result of a command in a script (see <AHREF="filearchiv.html#FILECOMP">Example 12-32</A> and <AHREF="textproc.html#LOOKUP">Example 12-17</A>).</P><DIVCLASS="NOTE"><TABLECLASS="NOTE"WIDTH="100%"BORDER="0"><TR><TDWIDTH="25"ALIGN="CENTER"VALIGN="TOP"><IMGSRC="common/note.png"HSPACE="5"ALT="Note"></TD><TDALIGN="LEFT"VALIGN="TOP"><P>The <AHREF="special-chars.html#NOTREF">!</A>, the logical <SPANCLASS="QUOTE">"not"</SPAN> qualifier, reverses the outcome of a test or command, and this affects its <AHREF="exit-status.html#EXITSTATUSREF">exit status</A>. <DIVCLASS="EXAMPLE"><HR><ANAME="NEGCOND"></A><P><B>Example 6-2. Negating a condition using <SPANCLASS="TOKEN">!</SPAN></B></P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 true # the "true" builtin. 2 echo "exit status of \"true\" = $?" # 0 3 4 ! true 5 echo "exit status of \"! true\" = $?" # 1 6 # Note that the "!" needs a space. 7 # !true leads to a "command not found" error 8 # 9 # The '!' operator prefixing a command invokes the Bash history mechanism. 10 11 true 12 !true 13 # No error this time, but no negation either. 14 # It just repeats the previous command (true). 15 16 # Thanks, St閜hane Chazelas and Kristopher Newsome.</PRE></TD></TR></TABLE><HR></DIV> </P></TD></TR></TABLE></DIV><DIVCLASS="CAUTION"><TABLECLASS="CAUTION"WIDTH="100%"BORDER="0"><TR><TDWIDTH="25"ALIGN="CENTER"VALIGN="TOP"><IMGSRC="common/caution.png"HSPACE="5"ALT="Caution"></TD><TDALIGN="LEFT"VALIGN="TOP"><P>Certain exit status codes have <AHREF="exitcodes.html#EXITCODESREF">reserved meanings</A> and should not be user-specified in a script. </P></TD></TR></TABLE></DIV></DIV><DIVCLASS="NAVFOOTER"><HRALIGN="LEFT"WIDTH="100%"><TABLEWIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top"><AHREF="escapingsection.html">Prev</A></TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><AHREF="index.html">Home</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top"><AHREF="tests.html">Next</A></TD></TR><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top">Escaping</TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><AHREF="part2.html">Up</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top">Tests</TD></TR></TABLE></DIV></BODY></HTML>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -