Chapter 18. Here Documents

 

Here and now, boys.

--Aldous Huxley, Island

A here document is a special-purpose code block. It uses a form of I/O redirection to feed a command list to an interactive program or a command, such as ftp, cat, or the ex text editor.

COMMAND <<InputComesFromHERE ... InputComesFromHERE

A limit string delineates (frames) the command list. The special symbol << designates the limit string. This has the effect of redirecting the output of a file into the stdin of the program or command. It is similar to interactive-program < command-file, where command-file contains
command #1 command #2 ...

The here document alternative looks like this:

#!/bin/bash interactive-program <<LimitString command #1 command #2 ... LimitString

Choose a limit string sufficiently unusual that it will not occur anywhere in the command list and confuse matters.

Note that here documents may sometimes be used to good effect with non-interactive utilities and commands, such as, for example, wall.

Example 18-1. broadcast: Sends message to everyone logged in

#!/bin/bash wall <<zzz23EndOfMessagezzz23 E-mail your noontime orders for pizza to the system administrator. (Add an extra dollar for anchovy or mushroom topping.) # Additional message text goes here. # Note: 'wall' prints comment lines. zzz23EndOfMessagezzz23 # Could have been done more efficiently by # wall <message-file # However, embedding the message template in a script #+ is a quick-and-dirty one-off solution. exit 0

Even such unlikely candidates as the vi text editor lend themselves to here documents.

Example 18-2. dummyfile: Creates a 2-line dummy file

#!/bin/bash # Non-interactive use of 'vi' to edit a file. # Emulates 'sed'. E_BADARGS=65 if [ -z "$1" ] then echo "Usage: `basename $0` filename" exit $E_BADARGS fi TARGETFILE=$1 # Insert 2 lines in file, then save. #--------Begin here document-----------# vi $TARGETFILE <<x23LimitStringx23 i This is line 1 of the example file. This is line 2 of the example file. ^[ ZZ x23LimitStringx23 #----------End here document-----------# # Note that ^[ above is a literal escape #+ typed by Control-V <Esc>. # Bram Moolenaar points out that this may not work with 'vim', #+ because of possible problems with terminal interaction. exit 0

The above script could just as effectively have been implemented with ex, rather than vi. Here documents containing a list of ex commands are common enough to form their own category, known as ex scripts.
#!/bin/bash # Replace all instances of "Smith" with "Jones" #+ in files with a ".txt" filename suffix. ORIGINAL=Smith REPLACEMENT=Jones for word in $(fgrep -l $ORIGINAL *.txt) do # ------------------------------------- ex $word <<EOF :%s/$ORIGINAL/$REPLACEMENT/g :wq EOF # :%s is the "ex" substitution command. # :wq is write-and-quit. # ------------------------------------- done

Analogous to "ex scripts" are cat scripts.

Example 18-3. Multi-line message using cat

#!/bin/bash # 'echo' is fine for printing single line messages, #+ but somewhat problematic for for message blocks. # A 'cat' here document overcomes this limitation. cat <<End-of-message ------------------------------------- This is line 1 of the message. This is line 2 of the message. This is line 3 of the message. This is line 4 of the message. This is the last line of the message. ------------------------------------- End-of-message # Replacing line 7, above, with #+ cat > $Newfile <<End-of-message #+ ^^^^^^^^^^ #+ writes the output to the file $Newfile, rather than to stdout. exit 0 #-------------------------------------------- # Code below disabled, due to "exit 0" above. # S.C. points out that the following also works. echo "------------------------------------- This is line 1 of the message. This is line 2 of the message. This is line 3 of the message. This is line 4 of the message. This is the last line of the message. -------------------------------------" # However, text may not include double quotes unless they are escaped.

The - option to mark a here document limit string (<<-LimitString) suppresses leading tabs (but not spaces) in the output. This may be useful in making a script more readable.

Example 18-4. Multi-line message, with tabs suppressed

#!/bin/bash # Same as previous example, but... # The - option to a here document <<- #+ suppresses leading tabs in the body of the document, #+ but authorsnote.html endnotes.html spaces. cat <<-ENDOFMESSAGE This is line 1 of the message. This is line 2 of the message. This is line 3 of the message. This is line 4 of the message. This is the last line of the message. ENDOFMESSAGE # The output of the script will be flush left. # Leading tab in each line will not show. # Above 5 lines of "message" prefaced by a tab, not spaces. # Spaces not affected by <<- . # Note that this option has no effect on *embedded* tabs. exit 0

A here document supports parameter and command substitution. It is therefore possible to pass different parameters to the body of the here document, changing its output accordingly.

Example 18-5. Here document with parameter substitution

#!/bin/bash # Another 'cat' here document, using parameter substitution. # Try it with no command line parameters, ./scriptname # Try it with one command line parameter, ./scriptname Mortimer # Try it with one two-word quoted command line parameter, # ./scriptname "Mortimer Jones" CMDLINEPARAM=1 # Expect at least command line parameter. if [ $# -ge $CMDLINEPARAM ] then NAME=$1 # If more than one command line param, #+ then just take the first. else NAME="John Doe" # Default, if no command line parameter. fi RESPONDENT="the author of this fine script" cat <<Endofmessage Hello, there, $NAME. Greetings to you, $NAME, from $RESPONDENT. # This comment shows up in the output (why?). Endofmessage # Note that the blank lines show up in the output. # So does the "comment". exit 0

This is a useful script containing a here document with parameter substitution.

Example 18-6. Upload a file pair to Sunsite incoming directory

#!/bin/bash # upload.sh # Upload file pair (Filename.lsm, Filename.tar.gz) #+ to incoming directory at Sunsite/UNC (ibiblio.org). # Filename.tar.gz is the tarball itself. # Filename.lsm is the descriptor file. # Sunsite requires "lsm" file, otherwise will bounce contributions. E_ARGERROR=65 if [ -z "$1" ] then echo "Usage: `basename $0` Filename-to-upload" exit $E_ARGERROR fi Filename=`basename $1` # Strips pathname out of file name. Server="ibiblio.org" Directory="/incoming/Linux" # These need not be hard-coded into script, #+ but may instead be changed to command line argument. Password="your.e-mail.address" # Change above to suit. ftp -n $Server <<End-Of-Session # -n option disables auto-logon user anonymous "$Password" binary bell # Ring 'bell' after each file transfer. cd $Directory put "$Filename.lsm" put "$Filename.tar.gz" bye End-Of-Session exit 0

Quoting or escaping the "limit string" at the head of a here document disables parameter substitution within its body.

Example 18-7. Parameter substitution turned off

#!/bin/bash # A 'cat' here-document, but with parameter substitution disabled. NAME="John Doe" RESPONDENT="the author of this fine script" cat <<'Endofmessage' Hello, there, $NAME. Greetings to you, $NAME, from $RESPONDENT. Endofmessage # No parameter substitution when the "limit string" is quoted or escaped. # Either of the following at the head of the here document would have #+ the same effect. # cat <<"Endofmessage" # cat <<\Endofmessage exit 0

Disabling parameter substitution permits outputting literal text. Generating scripts or even program code is one use for this.

Example 18-8. A script that generates another script

#!/bin/bash # generate-script.sh # Based on an idea by Albert Reiner. OUTFILE=generated.sh # Name of the file to generate. # ----------------------------------------------------------- # 'Here document containing the body of the generated script. ( cat <<'EOF' #!/bin/bash echo "This is a generated shell script." # Note that since we are inside a subshell, #+ we can't access variables in the "outside" script. echo "Generated file will be named: $OUTFILE" # Above line will not work as normally expected #+ because parameter expansion has been disabled. # Instead, the result is literal output. a=7 b=3 let "c = $a aboutauthor.html aliases.html arithexp.html arrays.html asciitable.html assortedtips.html authorsnote.html awk.html bash2.html bash-options.html bashver2.html bashver3.html basic.html biblio.html colorizing.html command-line-options.html commandsub.html communications.html comparison-ops.html complexfunct.html contributed-scripts.html copyright.html credits.html dblparens.html debugging.html declareref.html devproc.html devref1.html disclaimer.html dosbatch.html endnotes.html escapingsection.html exercises.html exitcodes.html exit-status.html external.html extmisc.html filearchiv.html files.html fto.html functions.html globbingref.html gotchas.html here-docs.html histcommands.html index.html intandnonint.html internal.html internalvariables.html invoking.html io-redirection.html ioredirintro.html ivr.html list-cons.html localization.html localvar.html loopcontrol.html loops1.html loops.html mathc.html mirrorsites.html miscellany.html moreadv.html nestedifthen.html nestedloops.html numerical-constants.html operations.html opprecedence.html ops.html optimizations.html options.html othertypesv.html parameter-substitution.html part1.html part2.html part3.html part4.html part5.html portabilityissues.html prelimexer.html process-sub.html procref1.html quoting.html quotingvar.html randomvar.html recess-time.html recurnolocvar.html recursionsct.html redirapps.html redircb.html refcards.html regexp.html restricted-sh.html revisionhistory.html sample-bashrc.html scriptanalysis.html scrstyle.html securityissues.html sedawk.html sha-bang.html special-chars.html standard-options.html string-manipulation.html subshells.html sysscripts.html systemdirs.html system.html terminalccmds.html testbranch.html testconstructs.html testsandcomparisons.html tests.html testtest.html textproc.html timedate.html todolist.html toolsused.html unofficialst.html untyped.html varassignment.html variables2.html variables.html varsubn.html wherehelp.html why-shell.html winscript.html wrapper.html writingscripts.html x16044.html x16712.html x16834.html x21467.html x8885.html xrefindex.html zeros.html $b" echo "c = $c" exit 0 EOF ) > $OUTFILE # ----------------------------------------------------------- # Quoting the 'limit string' prevents variable expansion #+ within the body of the above 'here document.' # This permits outputting literal strings in the output file. if [ -f "$OUTFILE" ] then chmod 755 $OUTFILE # Make the generated file executable. else echo "Problem in creating file: \"$OUTFILE\"" fi # This method can also be used for generating #+ C programs, Perl programs, Python programs, Makefiles, #+ and the like. exit 0

It is possible to set a variable from the output of a here document. This is actually a devious form of command substitution.
variable=$(cat <<SETVAR This variable runs over multiple lines. SETVAR) echo "$variable"

A here document can supply input to a function in the same script.

Example 18-9. Here documents and functions

#!/bin/bash # here-function.sh GetPersonalData () { read firstname read lastname read address read city read state read zipcode } # This certainly looks like an interactive function, but... # Supply input to the above function. GetPersonalData <<RECORD001 Bozo Bozeman 2726 Nondescript Dr. Baltimore MD 21226 RECORD001 echo echo "$firstname $lastname" echo "$address" echo "$city, $state $zipcode" echo exit 0

It is possible to use : as a dummy command accepting output from a here document. This, in effect, creates an "anonymous" here document.

Example 18-10. "Anonymous" Here Document

#!/bin/bash : <<TESTVARIABLES ${HOSTNAME?}${USER?}${MAIL?} # Print error message if one of the variables not set. TESTVARIABLES exit 0

Tip

A variation of the above technique permits "commenting out" blocks of code.

Example 18-11. Commenting out a block of code

#!/bin/bash # commentblock.sh : <<COMMENTBLOCK echo "This line will not echo." This is a comment line missing the "#" prefix. This is another comment line missing the "#" prefix. &*@!!++= The above line will cause no error message, because the Bash interpreter will ignore it. COMMENTBLOCK echo "Exit value of above \"COMMENTBLOCK\" is $?." # 0 # No error shown. echo # The above technique also comes in useful for commenting out #+ a block of working code for debugging purposes. # This saves having to put a "#" at the beginning of each line, #+ then having to go back and delete each "#" later. echo "Just before commented-out code block." # The lines of code between the double-dashed lines will not execute. # =================================================================== : <<DEBUGXXX for file in aboutauthor.html aliases.html arithexp.html arrays.html asciitable.html assortedtips.html authorsnote.html awk.html bash2.html bash-options.html bashver2.html bashver3.html basic.html biblio.html colorizing.html command-line-options.html commandsub.html communications.html comparison-ops.html complexfunct.html contributed-scripts.html copyright.html credits.html dblparens.html debugging.html declareref.html devproc.html devref1.html disclaimer.html dosbatch.html endnotes.html escapingsection.html exercises.html exitcodes.html exit-status.html external.html extmisc.html filearchiv.html files.html fto.html functions.html globbingref.html gotchas.html here-docs.html histcommands.html index.html intandnonint.html internal.html internalvariables.html invoking.html io-redirection.html ioredirintro.html ivr.html list-cons.html localization.html localvar.html loopcontrol.html loops1.html loops.html mathc.html mirrorsites.html miscellany.html moreadv.html nestedifthen.html nestedloops.html numerical-constants.html operations.html opprecedence.html ops.html optimizations.html options.html othertypesv.html parameter-substitution.html part1.html part2.html part3.html part4.html part5.html portabilityissues.html prelimexer.html process-sub.html procref1.html quoting.html quotingvar.html randomvar.html recess-time.html recurnolocvar.html recursionsct.html redirapps.html redircb.html refcards.html regexp.html restricted-sh.html revisionhistory.html sample-bashrc.html scriptanalysis.html scrstyle.html securityissues.html sedawk.html sha-bang.html special-chars.html standard-options.html string-manipulation.html subshells.html sysscripts.html systemdirs.html system.html terminalccmds.html testbranch.html testconstructs.html testsandcomparisons.html tests.html testtest.html textproc.html timedate.html todolist.html toolsused.html unofficialst.html untyped.html varassignment.html variables2.html variables.html varsubn.html wherehelp.html why-shell.html winscript.html wrapper.html writingscripts.html x16044.html x16712.html x16834.html x21467.html x8885.html xrefindex.html zeros.html do cat "$file" done DEBUGXXX # =================================================================== echo "Just after commented-out code block." exit 0 ###################################################################### # Note, however, that if a bracketed variable is contained within #+ the commented-out code block, #+ then this could cause problems. # for example: #/!/bin/bash : <<COMMENTBLOCK echo "This line will not echo." &*@!!++= ${foo_bar_bazz?} $(rm -rf /tmp/foobar/) $(touch my_build_directory/cups/Makefile) COMMENTBLOCK $ sh commented-bad.sh commented-bad.sh: line 3: foo_bar_bazz: parameter null or not set # The remedy for this is to strong-quote the 'COMMENTBLOCK' in line 49, above. : <<'COMMENTBLOCK' # Thank you, Kurt Pfeifle, for pointing this out.

Tip

Yet another twist of this nifty trick makes "self-documenting" scripts possible.

Example 18-12. A self-documenting script

#!/bin/bash # self-document.sh: self-documenting script # Modification of "colm.sh". DOC_REQUEST=70 if [ "$1" = "-h" -o "$1" = "--help" ] # Request help. then echo; echo "Usage: $0 [directory-name]"; echo sed --silent -e '/DOCUMENTATIONXX$/,/^DOCUMENTATIONXX$/p' "$0" | sed -e '/DOCUMENTATIONXX$/d'; exit $DOC_REQUEST; fi : <<DOCUMENTATIONXX List the statistics of a specified directory in tabular format. --------------------------------------------------------------- The command line parameter gives the directory to be listed. If no directory specified or directory specified cannot be read, then list the current working directory. DOCUMENTATIONXX if [ -z "$1" -o ! -r "$1" ] then directory=. else directory="$1" fi echo "Listing of "$directory":"; echo (printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG-NAME\n" \ ; ls -l "$directory" | sed 1d) | column -t exit 0

Using a cat script is an alternate way of accomplishing this.

DOC_REQUEST=70 if [ "$1" = "-h" -o "$1" = "--help" ] # Request help. then # Use a "cat script" . . . cat <<DOCUMENTATIONXX List the statistics of a specified directory in tabular format. --------------------------------------------------------------- The command line parameter gives the directory to be listed. If no directory specified or directory specified cannot be read, then list the current working directory. DOCUMENTATIONXX exit $DOC_REQUEST fi

See also Example A-30, Example A-42, Example A-43, and Example A-44 for more examples of self-documenting scripts.

Note

Here documents create temporary files, but these files are deleted after opening and are not accessible to any other process.

bash$ bash -c 'lsof -a -p $$ -d0' << EOF > EOF lsof 1213 bozo 0r REG 3,5 0 30386 /tmp/t1213-0-sh (deleted) 

Caution

Some utilities will not work inside a here document.

Warning

The closing limit string, on the final line of a here document, must start in the first character position. There can be no leading whitespace. Trailing whitespace after the limit string likewise causes unexpected behavior. The whitespace prevents the limit string from being recognized.

#!/bin/bash echo "----------------------------------------------------------------------" cat <<LimitString echo "This is line 1 of the message inside the here document." echo "This is line 2 of the message inside the here document." echo "This is the final line of the message inside the here document." LimitString #^^^^Indented limit string. Error! This script will not behave as expected. echo "----------------------------------------------------------------------" # These comments are outside the 'here document', #+ and should not echo. echo "Outside the here document." exit 0 echo "This line had better not echo." # Follows an 'exit' command.

For those tasks too complex for a "here document", consider using the expect scripting language, which was specifically designed for feeding input into interactive programs.