Chapter 6 Dealing with quotes

Dealing with Quotes and slashes grep Susan phonebook grep Susan Goldberg phonebook  results in an error  grep ‘Susan Goldberg’ phonebook allows the shell to ignore spaces the single quote ignores the $ sign and * symbol

Double quotes Some things in double quotes are NOT ignored – in other words the shell WILL interpret  $ signs  back quotes (on the tilde key)  backslashes  other double quotes  newlines

Examples $x=* $echo $x addresses intro lotaspaces nu names $echo ‘$x’ $x $echo “$x” *

Hiding quotes with quotes double quotes hide single quotes and single quotes hide double quotes $x=“’Good day, ‘ she said” $echo $x ‘Good day, ‘ she said $y=‘ “Good quotes are hard to find” ‘ $echo $y “Good quotes are hard to find”

The Backslash the backslash serves as a way of putting quotes around the next character $echo \\ \ $echo ‘\’ \

The Backslash (cont’d) The backslash goes as far as causing the shell to ignore a hard return this is useful for typing in long commands $lines=command1\ >command2 $echo $lines command1command2

The Back Quote this is used to tell the shell to execute the following command $echo The date and time is `date` The date and time is Wed Jan 24 14:20:34 PST 2007 be aware that this typically is no longer the preferred method, but you may run across it

The $(…) or command substitution Construct $echo “The date and time is: $(date)” The date and time is Wed Jan 24 14:20:34 PST 2007 keep in mind that within single quotes the command substitution construct does not work, but in double quotes it does

Examples echo There are $(who | wc – l) users logged in with single quotes  echo ‘$(who | wc –l) tells how many users are logged in’ with double quotes  echo “You have $(ls | wc – l) files in your directory

more $(…) stuff to preserve newlines / hard returns use double quotes $filelist=$(ls) $echo $filelist – yields everything on one line $echo “$filelist” – yields the files on separate lines

Nifty stuff to put the contents of a file into a variable use the cat command $namelist=$(cat names1) $echo “$namelist” - will list all the names with newlines mail $(cat unames) < memo – will the memo to all users listed in unames who are on your system

Nested commands $filename=/usrs/steve/memos $firstchar=$(echo $filename | cut –c1) $filename=$(echo $filename | tr “$firstchar” “^”) $echo $filename ^usrs^steve^memos can also be written as

Nested commands (cont’d) $filename=/usrs/steve/memos $filename=$(echo $filename | tr “$(echo $filename | cut –c1)” “^”) $echo $filename

The expr operator used by older systems – only does integer arithmetic $expr $ some quirks  expr 17 * 6 - yields an error  expr “17 * 6” - yields 17 * 6  expr 17 \* 6 – yields 102