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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 7] 7.13 Pre-Prompt Commands in bash </TITLE><METANAME="DC.title"CONTENT="UNIX Power Tools"><METANAME="DC.creator"CONTENT="Jerry Peek, Tim O'Reilly & Mike Loukides"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1998-08-04T21:34:02Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-260-3"SCHEME="ISBN"><METANAME="DC.language"CONTENT="en-US"><METANAME="generator"CONTENT="Jade 1.1/O'Reilly DocBook 3.0 to HTML 4.0"><LINKREV="made"HREF="mailto:online-books@oreilly.com"TITLE="Online Books Comments"><LINKREL="up"HREF="ch07_01.htm"TITLE="7. Setting Your Shell Prompt"><LINKREL="prev"HREF="ch07_12.htm"TITLE="7.12 External Commands Send Signals to Set Variables "><LINKREL="next"HREF="part02.htm"TITLE="II. Let the Computer Do the Dirty Work "></HEAD><BODYBGCOLOR="#FFFFFF"TEXT="#000000"><DIVCLASS="htmlnav"><H1><IMGSRC="gifs/smbanner.gif"ALT="UNIX Power Tools"USEMAP="#srchmap"BORDER="0"></H1><MAPNAME="srchmap"><AREASHAPE="RECT"COORDS="0,0,466,58"HREF="index.htm"ALT="UNIX Power Tools"><AREASHAPE="RECT"COORDS="467,0,514,18"HREF="jobjects/fsearch.htm"ALT="Search this book"></MAP><TABLEWIDTH="515"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch07_12.htm"TITLE="7.12 External Commands Send Signals to Set Variables "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 7.12 External Commands Send Signals to Set Variables "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1">Chapter 7<BR>Setting Your Shell Prompt</FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="PART"HREF="part02.htm"TITLE="II. Let the Computer Do the Dirty Work "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: II. Let the Computer Do the Dirty Work "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-9911">7.13 Pre-Prompt Commands in bash </A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-7341"></A><EMCLASS="emphasis">bash</EM> can run a UNIX command, or multiple commands, before it printsevery prompt.This command does not have to set the prompt; it just happens to be runbefore each prompt is printed.The command could do some system checking, reset shell variables,or almost anything that you could type at a shell prompt.Store the command(s) in the <EMCLASS="emphasis">PROMPT_COMMAND</EM> shell variable.If the commands run slowly, though, they'll delay your prompt.</P><PCLASS="para">Here's a silly example that I used to have in my <EMCLASS="emphasis">bash</EM><SPANCLASS="link">setup file (<ACLASS="linkend"HREF="ch02_02.htm"TITLE="Shell Setup Files-Which, Where, and Why ">2.2</A>)</SPAN>:</P><PCLASS="para"><TABLECLASS="screen.co"BORDER="1"><TR><THVALIGN="TOP"><PRECLASS="calloutlist"> <ACLASS="co"HREF="ch35_21.htm"TITLE="35.21 Using IFS to Split Strings ">IFS</A> <ACLASS="co"HREF="ch44_19.htm"TITLE="44.19 The Bourne Shell set Command ">set</A> <ACLASS="co"HREF="ch51_12.htm"TITLE="51.12 You Don't Have Enough Smileys? ">smiley</A> <ACLASS="co"HREF="ch45_15.htm"TITLE="45.15 How to Unset all Command-Line Parameters ">shift $#</A> </PRE></TH><TDVALIGN="TOP"><PRECLASS="screen">PROMPT_COMMAND='# Save old $IFS; set IFS to tab:OIFS="$IFS"; IFS=" "# Put x in $1, face in $2, explanation[s] in $3[, $4, ...]:set x `smiley`# Put face into $face and explanation(s) into $explan:face="$2"; shift 2; explan="$*"# Restore shell environment:shift $#; IFS="$OIFS"'# Prompt I use (includes the latest $face):PS1='\u@\h $face '</PRE></TD></TR></TABLE></P><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-7356"></A><ACLASS="indexterm"NAME="AUTOID-7358"></A>The first part is a series of shell commands that arestored in the <EMCLASS="emphasis">PROMPT_COMMAND</EM> variable;they're surrounded by a pair of single quotes (<CODECLASS="literal">' '</CODE>), one onthe first line (after the <CODECLASS="literal">=</CODE>) and the other after <CODECLASS="literal">IFS</CODE> isreset.That series of commands is executed before every prompt.It sets two shell variables, <CODECLASS="literal">$face</CODE> and <CODECLASS="literal">$explan</CODE>, withnew values before each prompt is set.The prompt is set on the last line; it includes the value of <CODECLASS="literal">$face</CODE>.</P><PCLASS="para">Here's what my screen looked like with this ridiculous setup.Notice that the prompt keeps changing as the <EMCLASS="emphasis">PROMPT_COMMAND</EM>resets <CODECLASS="literal">$face</CODE> and <CODECLASS="literal">$explan</CODE>.If I wanted the explanation of a face I saw as I went along, I could type<CODECLASS="literal">echo <">$explan<"></CODE>:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">jerry@ruby :-{) <CODECLASS="userinput"><B>echo "$explan"</B></CODE>normal smiling face with a moustachejerry@ruby +<||-) <CODECLASS="userinput"><B>vi proj.cc</B></CODE> ...jerry@ruby :-O <CODECLASS="userinput"><B>echo "$explan"</B></CODE>Mr. Bill Wow! ohh, big mouth, Mick Jagger uh ohjerry@ruby :-) < <CODECLASS="userinput"><B>g++ -Wall proj.cc</B></CODE> ... </PRE></BLOCKQUOTE></P><PCLASS="para">(It was even more useless than<SPANCLASS="link">psychoanalyze-pinhead (<ACLASS="linkend"HREF="ch32_13.htm"TITLE="An Absurd Amusement ">32.13</A>)</SPAN>,but it was fun while it lasted.)Seriously now, I'll say again:<EMCLASS="emphasis">PROMPT_COMMAND</EM> does <EMCLASS="emphasis">not</EM> have to be used to set a prompt.You can use it to run any commands.If the commands in <EMCLASS="emphasis">PROMPT_COMMAND</EM> write to standardoutput or standard error, you'll see that text before the prompt.<ACLASS="indexterm"NAME="AUTOID-7384"></A></P><DIVCLASS="sect1info"><PCLASS="SECT1INFO">- <SPANCLASS="authorinitials">JP</SPAN></P></DIV></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="515"TITLE="footer"><TABLEWIDTH="515"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch07_12.htm"TITLE="7.12 External Commands Send Signals to Set Variables "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 7.12 External Commands Send Signals to Set Variables "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><ACLASS="book"HREF="index.htm"TITLE="UNIX Power Tools"><IMGSRC="gifs/txthome.gif"SRC="gifs/txthome.gif"ALT="UNIX Power Tools"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="PART"HREF="part02.htm"TITLE="II. 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