Advanced Bash-Scripting Guide
#!/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.
Some people very cleverly use a single ! as a limit string. But, that's not necessarily a good idea.
# This works.
cat <<!
Hello!
! Three more exclamations !!!
!
# But . . .
cat <<!
!
!
Hello!
Single exclamation point follows!
# Crashes with an error message.
# However, the following will work.
cat <<EOF
Hello!
Single exclamation point follows!
!
EOF
# It's safer to use a multi-character limit string.
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.
19.1. Here Strings
A here
string can be considered as a stripped-down form of a here
document.
Chapter 19. Here Documents 369
Advanced Bash-Scripting Guide
It consists of nothing more than
COMMAND <<< $WORD,
where $WORD is expanded and fed to the stdin of
COMMAND.
As a simple example, consider this alternative to the echo-grep construction.
# Instead of: if echo "$VAR" | grep -q txt # if [[ $VAR = *txt* ]]
# etc.
# Try: if grep -q "txt" <<< "$VAR" then # ^^^
echo "$VAR contains the substring sequence \"txt\"" fi
# Thank you, Sebastian Kaminski, for the suggestion.
Or, in combination with read:
String="This is a string of words." read -r -a Words <<< "$String"
# The -a option to "read"
#+ assigns the resulting values to successive members of an array.
echo "First word in String is: ${Words[0]}" # This echo "Second word in String is: ${Words[1]}" # is echo "Third word in String is: ${Words[2]}" # a echo "Fourth word in String is: ${Words[3]}" # string echo "Fifth word in String is: ${Words[4]}" # of echo "Sixth word in String is: ${Words[5]}" # words.
echo "Seventh word in String is: ${Words[6]}" # (null)
# Past end of $String.
# Thank you, Francisco Lobo, for the suggestion.
It is, of course, possible to feed the output of a here string into the
stdin
of a loop.
# As Seamus points out . . .
ArrayVar=( element0 element1 element2 {A..D} ) while read element ; do
echo "$element" 1>&2 done <<< $(echo ${ArrayVar[*]})
# element0 element1 element2 A B C D
Example 19-13. Prepending a line to a file
#!/bin/bash
# prepend.sh: Add text at beginning of file.
#
# Example contributed by Kenny Stauffer,
#+ and slightly modified by document author.
E_NOSUCHFILE=85
Chapter 19. Here Documents 370
Advanced Bash-Scripting Guide read -p "File: " file # -p arg to 'read' displays prompt.
if [ ! -e "$file" ] then # Bail out if no such file.
echo "File $file not found."
exit $E_NOSUCHFILE fi read -p "Title: " title cat - $file <<<$title > $file.new
echo "Modified file is $file.new" exit # Ends script execution.
from 'man bash':
Here Strings
A variant of here documents, the format is:
<<<word
The word is expanded and supplied to the command on its standard input.
Of course, the following also works:
sed -e '1i\
Title: ' $file
Example 19-14. Parsing a mailbox
#!/bin/bash
# Script by Francisco Lobo,
#+ and slightly modified and commented by ABS Guide author.
# Used in ABS Guide with permission. (Thank you!)
# This script will not run under Bash versions -lt 3.0.
E_MISSING_ARG=87 if [ -z "$1" ] then
echo "Usage: $0 mailbox-file"
exit $E_MISSING_ARG fi mbox_grep() # Parse mailbox file.
{
declare -i body=0 match=0
declare -a date sender
declare mail header value
while IFS= read -r mail
# ^^^^ Reset $IFS.
# Otherwise "read" will strip leading & trailing space from its input.
do
if [[ $mail =~ ^From ]] # Match "From" field in message.
then
(( body = 0 )) # "Zero out" variables.
(( match = 0 ))
Chapter 19. Here Documents 371
Advanced Bash-Scripting Guide
unset date
elif (( body ))
then
(( match ))
# echo "$mail"
# Uncomment above line if you want entire body
#+ of message to display.
elif [[ $mail ]]; then
IFS=: read -r header value <<< "$mail"
# ^^^ "here string"
case "$header" in
[Ff][Rr][Oo][Mm] ) [[ $value =~ "$2" ]] && (( match++ )) ;;
# Match "From" line.
[Dd][Aa][Tt][Ee] ) read -r -a date <<< "$value" ;;
# ^^^
# Match "Date" line.
[Rr][Ee][Cc][Ee][Ii][Vv][Ee][Dd] ) read -r -a sender <<< "$value" ;;
# ^^^
# Match IP Address (may be spoofed).
esac
else
(( body++ ))
(( match )) &&
echo "MESSAGE ${date:+of: ${date[*]} }"
# Entire $date array ^
echo "IP address of sender: ${sender[1]}"
# Second field of "Received" line ^
fi
done < "$1" # Redirect stdout of file into loop.
} mbox_grep "$1" # Send mailbox file to function.
exit $?
# Exercises:
# ---------
# 1) Break the single function, above, into multiple functions,
#+ for the sake of readability.
# 2) Add additional parsing to the script, checking for various keywords.
$ mailbox_grep.sh scam_mail
MESSAGE of Thu, 5 Jan 2006 08:00:56 -0500 (EST)
IP address of sender: 196.3.62.4
Exercise: Find other uses for here strings, such as, for example, feeding input to dc.
Chapter 19. Here Documents 372
