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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><HTML><HEAD><TITLE>Assorted Tips</TITLE><METANAME="GENERATOR"CONTENT="Modular DocBook HTML Stylesheet Version 1.57"><LINKREL="HOME"TITLE="Advanced Bash-Scripting Guide"HREF="index.html"><LINKREL="UP"TITLE="Miscellany"HREF="miscellany.html"><LINKREL="PREVIOUS"TITLE="Optimizations"HREF="optimizations.html"><LINKREL="NEXT"TITLE="Security Issues"HREF="securityissues.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="SECT1"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="optimizations.html">Prev</A></TD><TDWIDTH="80%"ALIGN="center"VALIGN="bottom">Chapter 33. Miscellany</TD><TDWIDTH="10%"ALIGN="right"VALIGN="bottom"><AHREF="securityissues.html">Next</A></TD></TR></TABLE><HRALIGN="LEFT"WIDTH="100%"></DIV><DIVCLASS="SECT1"><H1CLASS="SECT1"><ANAME="ASSORTEDTIPS">33.7. Assorted Tips</A></H1><UL><LI><P>To keep a record of which user scripts have run during a particular session or over a number of sessions, add the following lines to each script you want to keep track of. This will keep a continuing file record of the script names and invocation times. </P><P> <TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 # Append (>>) following to end of each script tracked. 2 3 whoami>> $SAVE_FILE # User invoking the script. 4 echo $0>> $SAVE_FILE # Script name. 5 date>> $SAVE_FILE # Date and time. 6 echo>> $SAVE_FILE # Blank line as separator. 7 8 # Of course, SAVE_FILE defined and exported as environmental variable in ~/.bashrc 9 #+ (something like ~/.scripts-run)</PRE></TD></TR></TABLE> </P></LI><LI><P><ANAME="PREPENDREF"></A></P><P>The <SPANCLASS="TOKEN">>></SPAN> operator appends lines to a file. What if you wish to <ICLASS="EMPHASIS">prepend</I> a line to an existing file, that is, to paste it in at the beginning?</P><P> <TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 file=data.txt 2 title="***This is the title line of data text file***" 3 4 echo $title | cat - $file >$file.new 5 # "cat -" concatenates stdout to $file. 6 # End result is 7 #+ to write a new file with $title appended at *beginning*.</PRE></TD></TR></TABLE> </P><P>This is a simplified variant of the <AHREF="here-docs.html#PREPENDEX">Example 17-13</A> script given earlier. And, of course, <AHREF="sedawk.html#SEDREF">sed</A> can also do this.</P></LI><LI><P>A shell script may act as an embedded command inside another shell script, a <ICLASS="EMPHASIS">Tcl</I> or <ICLASS="EMPHASIS">wish</I> script, or even a <AHREF="filearchiv.html#MAKEFILEREF">Makefile</A>. It can be invoked as an external shell command in a C program using the <TTCLASS="REPLACEABLE"><I>system()</I></TT> call, i.e., <TTCLASS="REPLACEABLE"><I>system("script_name");</I></TT>.</P></LI><LI><P>Setting a variable to the contents of an embedded <ICLASS="FIRSTTERM">sed</I> or <ICLASS="FIRSTTERM">awk</I> script increases the readability of the surrounding <AHREF="wrapper.html#SHWRAPPER">shell wrapper</A>. See <AHREF="contributed-scripts.html#MAILFORMAT">Example A-1</A> and <AHREF="internal.html#COLTOTALER3">Example 11-18</A>.</P></LI><LI><P>Put together files containing your favorite and most useful definitions and functions. As necessary, <SPANCLASS="QUOTE">"include"</SPAN> one or more of these <SPANCLASS="QUOTE">"library files"</SPAN> in scripts with either the <AHREF="special-chars.html#DOTREF">dot</A> (<BCLASS="COMMAND">.</B>) or <AHREF="internal.html#SOURCEREF">source</A> command.</P><P> <TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 # SCRIPT LIBRARY 2 # ------ ------- 3 4 # Note: 5 # No "#!" here. 6 # No "live code" either. 7 8 9 # Useful variable definitions 10 11 ROOT_UID=0 # Root has $UID 0. 12 E_NOTROOT=101 # Not root user error. 13 MAXRETVAL=255 # Maximum (positive) return value of a function. 14 SUCCESS=0 15 FAILURE=-1 16 17 18 19 # Functions 20 21 Usage () # "Usage:" message. 22 { 23 if [ -z "$1" ] # No arg passed. 24 then 25 msg=filename 26 else 27 msg=$@ 28 fi 29 30 echo "Usage: `basename $0` "$msg"" 31 } 32 33 34 Check_if_root () # Check if root running script. 35 { # From "ex39.sh" example. 36 if [ "$UID" -ne "$ROOT_UID" ] 37 then 38 echo "Must be root to run this script." 39 exit $E_NOTROOT 40 fi 41 } 42 43 44 CreateTempfileName () # Creates a "unique" temp filename. 45 { # From "ex51.sh" example. 46 prefix=temp 47 suffix=`eval date +%s` 48 Tempfilename=$prefix.$suffix 49 } 50 51 52 isalpha2 () # Tests whether *entire string* is alphabetic. 53 { # From "isalpha.sh" example. 54 [ $# -eq 1 ] || return $FAILURE 55 56 case $1 in 57 *[!a-zA-Z]*|"") return $FAILURE;; 58 *) return $SUCCESS;; 59 esac # Thanks, S.C. 60 } 61 62 63 abs () # Absolute value. 64 { # Caution: Max return value = 255. 65 E_ARGERR=-999999 66 67 if [ -z "$1" ] # Need arg passed. 68 then 69 return $E_ARGERR # Obvious error value returned. 70 fi 71 72 if [ "$1" -ge 0 ] # If non-negative, 73 then # 74 absval=$1 # stays as-is. 75 else # Otherwise, 76 let "absval = (( 0 - $1 ))" # change sign. 77 fi 78 79 return $absval 80 } 81 82 83 tolower () # Converts string(s) passed as argument(s) 84 { #+ to lowercase. 85 86 if [ -z "$1" ] # If no argument(s) passed, 87 then #+ send error message 88 echo "(null)" #+ (C-style void-pointer error message) 89 return #+ and return from function. 90 fi 91 92 echo "$@" | tr A-Z a-z 93 # Translate all passed arguments ($@). 94 95 return 96 97 # Use command substitution to set a variable to function output. 98 # For example: 99 # oldvar="A seT of miXed-caSe LEtTerS" 100 # newvar=`tolower "$oldvar"` 101 # echo "$newvar" # a set of mixed-case letters 102 # 103 # Exercise: Rewrite this function to change lowercase passed argument(s) 104 # to uppercase ... toupper() [easy]. 105 }</PRE></TD></TR></TABLE> </P></LI><LI><P>Use special-purpose comment headers to increase clarity and legibility in scripts.</P><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 ## Caution. 2 rm -rf *.zzy ## The "-rf" options to "rm" are very dangerous, 3 ##+ especially with wildcards. 4 5 #+ Line continuation. 6 # This is line 1 7 #+ of a multi-line comment, 8 #+ and this is the final line. 9 10 #* Note. 11 12 #o List item. 13 14 #> Another point of view. 15 while [ "$var1" != "end" ] #> while test "$var1" != "end"</PRE></TD></TR></TABLE></P></LI><LI><P>A particularly clever use of <AHREF="tests.html#TESTCONSTRUCTS1">if-test</A> constructs is commenting out blocks of code.</P><P> <TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #!/bin/bash 2 3 COMMENT_BLOCK= 4 # Try setting the above variable to some value 5 #+ for an unpleasant surprise. 6 7 if [ $COMMENT_BLOCK ]; then 8 9 Comment block -- 10 ================================= 11 This is a comment line. 12 This is another comment line. 13 This is yet another comment line. 14 ================================= 15 16 echo "This will not echo." 17 18 Comment blocks are error-free! Whee! 19 20 fi 21 22 echo "No more comments, please." 23 24 exit 0</PRE></TD></TR></TABLE> </P><P>Compare this with <AHREF="here-docs.html#CBLOCK1">using here documents to comment out code blocks</A>.</P></LI><LI><P>Using the <AHREF="variables2.html#XSTATVARREF">$? exit status variable</A>, a script may test if a parameter contains only digits, so it can be treated as an integer.</P><P> <TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"⌨️ 快捷键说明
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