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Variables are at the heart of every programming and scripting
language. They are used for arithmetic operations and manipulation of
quantities, string parsing, and working in the abstract with symbols -
tokens that represent something else. A variable is nothing more than a
location or set of locations in computer memory that holds an item of
data. - $
Enclosing a referenced value in double quotes
(" ") does not interfere with variable
substitution. This is called partial quoting, sometimes
referred to as "weak quoting". Using single
quotes (' ') causes the variable name to be
used literally, and no substitution will take place. This
is full quoting, sometimes referred to as "strong
quoting". Note that $variable is actually a
simplified alternate form of
${variable}. In contexts
where the $variable syntax
causes an error, the longer form may work (see Section 3.3.1 below). Example 3-5. Variable assignment and substitution #!/bin/bash
# Variables: assignment and substitution
a=37.5
hello=$a
# No space permitted on either side of = sign when initializing variables.
echo hello
# Not a reference.
echo $hello
echo ${hello} #Identical to above.
echo "$hello"
echo "${hello}"
echo '$hello'
# Variable referencing disabled by single quotes,
# because $ interpreted literally.
# Notice the effect of different types of quoting.
# --------------------------------------------------------------
# It is permissible to set multiple variables on the same line,
# separated by white space. Careful, this may reduce legibility.
var1=variable1 var2=variable2 var3=variable3
echo
echo "var1=$var1 var2=$var2 var3=$var3"
# --------------------------------------------------------------
echo; echo
numbers="one two three"
other_numbers="1 2 3"
# If whitespace within variables, then quotes necessary.
echo "numbers = $numbers"
echo "other_numbers = $other_numbers"
echo
echo "uninitialized variable = $uninitialized_variable"
# Uninitialized variable has null value (no value at all).
echo
exit 0 |
| Warning | An uninitialized variable has a
"null" value - no assigned value at all
(not zero!). Using a variable before assigning a value
to it will inevitably cause problems. |
- ${parameter}
Same as $parameter, i.e.,
value of the variable parameter. May be used for concatenating variables with strings. your_id=${USER}-on-${HOSTNAME}
echo "$your_id"
#
echo "Old \$PATH = $PATH"
PATH=${PATH}:/opt/bin #Add /opt/bin to $PATH for duration of script.
echo "New \$PATH = $PATH" |
- ${parameter-default}
If parameter not set, use default. echo ${username-`whoami`}
# Echoes the result of `whoami`, but variable "username" is still unset. |
Note: This is almost equivalent to
${parameter:-default}. The
extra : makes a difference only when
parameter has been declared,
but is null.
#!/bin/bash
username0=
echo "username0 = ${username0-`whoami`}"
# username0 has been declared, but is set to null.
# Will not echo.
echo "username1 = ${username1-`whoami`}"
# username1 has not been declared.
# Will echo.
username2=
echo "username2 = ${username2:-`whoami`}"
# username2 has been declared, but is set to null.
# Will echo because of :- rather than just - in condition test.
exit 0 |
- ${parameter=default}, ${parameter:=default}
If parameter not set, set it to default. Both forms nearly equivalent. The :
makes a difference only when parameter
has been declared and is null, as above. echo ${username=`whoami`}
# Variable "username" is now set to `whoami`. |
- ${parameter+otherwise}, ${parameter:+otherwise}
If parameter set, use 'otherwise", else use null string. Both forms nearly equivalent. The :
makes a difference only when parameter
has been declared and is null, as above. - ${parameter?err_msg}, ${parameter:?err_msg}
If parameter set, use it, else print err_msg. Both forms nearly equivalent. The :
makes a difference only when parameter
has been declared and is null, as above.
Example 3-6. Using param substitution and : #!/bin/bash
# Let's check some of the system's environmental variables.
# If, for example, $USER, the name of the person
# at the console, is not set, the machine will not
# recognize you.
: ${HOSTNAME?} ${USER?} ${HOME} ${MAIL?}
echo
echo "Name of the machine is $HOSTNAME."
echo "You are $USER."
echo "Your home directory is $HOME."
echo "Your mail INBOX is located in $MAIL."
echo
echo "If you are reading this message,"
echo "critical environmental variables have been set."
echo
echo
# The ':' operator seems fairly error tolerant.
# This script works even if the '$' omitted in front of
# {HOSTNAME}, {USER?}, {HOME?}, and {MAIL?}. Why?
# ------------------------------------------------------
# The ${variablename?} construction can also check
# for variables set within the script.
ThisVariable=Value-of-ThisVariable
# Note, by the way, that string variables may be set
# to characters disallowed in their names.
: ${ThisVariable?}
echo "Value of ThisVariable is $ThisVariable".
echo
echo
# If ZZXy23AB has not been set...
: ${ZZXy23AB?}
# This will give you an error message and terminate.
echo "You will not see this message."
exit 0 |
- ${var#pattern}, ${var##pattern}
Strip off shortest/longest part of
pattern if it matches the front end of
variable.
- ${var%pattern}, ${var%%pattern}
Strip off shortest/longest part of
pattern if it matches the back end of
variable.
Version 2 of bash adds additional options. Example 3-7. Renaming file extensions: #!/bin/bash
# rfe
# ---
# Renaming file extensions.
#
# rfe old_extension new_extension
#
# Example:
# To rename all *.gif files in working directory to *.jpg,
# rfe gif jpg
if [ $# -ne 2 ]
then
echo "Usage: `basename $0` old_file_suffix new_file_suffix"
exit 1
fi
for filename in *.$1
# Traverse list of files ending with 1st argument.
do
mv $filename ${filename%$1}$2
# Strip off part of filename matching 1st argument,
# then append 2nd argument.
done
exit 0 |
- ${var:pos}
Variable var expanded,
starting from offset pos.
- ${var:pos:len}
Expansion to a max of len
characters of variable var, from offset
pos. See Example A-6
for an example of the creative use of this operator.
- ${var/patt/replacement}
First match of patt,
within var replaced with
replacement. If replacement is
omitted, then the first match of
patt is replaced by
nothing, that is, deleted. - ${var//patt/replacement}
All matches of patt,
within var replaced with
replacement. Similar to above, if
replacement is omitted,
then all occurrences patt
are replaced by nothing, that
is, deleted.
Example 3-8. Using pattern matching to parse arbitrary strings #!/bin/bash
var1=abcd-1234-defg
echo "var1 = $var1"
t=${var1#*-*}
echo "var1 (with everything, up to and including first - stripped out) = $t"
t=${var1%*-*}
echo "var1 (with everything from the last - on stripped out) = $t"
echo
path_name=/home/bozo/ideas/thoughts.for.today
echo "path_name = $path_name"
t=${path_name##/*/}
# Same effect as t=`basename $path_name`
echo "path_name, stripped of prefixes = $t"
t=${path_name%/*.*}
# Same effect as t=`dirname $path_name`
echo "path_name, stripped of suffixes = $t"
echo
t=${path_name:11}
echo "$path_name, with first 11 chars stripped off = $t"
t=${path_name:11:5}
echo "$path_name, with first 11 chars stripped off, length 5 = $t"
echo
t=${path_name/bozo/clown}
echo "$path_name with \"bozo\" replaced by \"clown\" = $t"
t=${path_name/today/}
echo "$path_name with \"today\" deleted = $t"
t=${path_name//o/O}
echo "$path_name with all o's capitalized = $t"
t=${path_name//o/}
echo "$path_name with all o's deleted = $t"
exit 0 |
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