LPI Linux Certification in a Nutshell (45 page)

break

break [
n
]

Exit from the innermost (most deeply nested)
for
,
while
, or
until
loop or from the
n
innermost levels of the loop.

case

case
string
in
pattern1
)
commands1
;;
pattern2
)
commands2
;;
...
esac

Choose
string
from among
a series of possible patterns. These patterns use the same form as
file globs (wildcards). If
string
matches pattern
pattern1
, perform the
subsequent
commands1
. If string matches
pattern2
, perform
commands2
. Proceed down the list of
patterns until one is found. To catch all remaining strings, use
*)
at the end.

continue

continue [
n
]

Skip remaining commands in a
for
,
while
, or
until
loop, resuming with the next
iteration of the loop (or skipping
n
loops).

echo

echo [
options
] [
string
]

Write
string
to standard
output, terminated by a newline. If no string is supplied, echo
only a newline. Some Linux distributions have a version of
echo
at
/bin/echo
. If
that is the case, the built-in
bash
version
of
echo
will usually take precedence.

exit

exit [
n
]

Exit a shell script with status
n
. The value for
n
can be 0 (success) or nonzero
(failure). If
n
is not given, the exit
status is that of the most recent command.

if ! test -f somefile
then
echo "Error: Missing file somefile"
exit 1
fi

for

for
x
in
list
do
commands
done

Assign each word in
list
to
x
in turn and execute
commands
. If
list
is omitted, it is assumed that
positional parameters from the command line, which are stored in
$@
, are to be used.

for filename in bigfile* ; do
echo "Compressing $filename"
gzip $filename
done

function

function
name
{
commands
}

Define function
name
.
Positional parameters (
$1
,
$2
, ...) can be used within
commands
.

#
function myfunc
{
echo "parameter is $1"
}
#
myfunc 1
parameter is 1
#
myfunc two
parameter is two

getopts

getopts
string name
[
args
]

Process command-line arguments (or
args
, if specified) and check for legal
options. The
getopts
is used
in shell script loops and is intended to ensure standard syntax
for command-line options. The
string
contains the option letters to be recognized by
getopts
when running the script. Valid
options are processed in turn and stored in the shell variable
name
. If an option letter is followed
by a colon, the option must be followed by one or more arguments
when the command is entered by the user.

if

if
expression1
then
commands1
elif
expression2
then
commands2
else
commands
fi

The
if
command is used to
define a conditional statement. There are three possible formats
for using the
if
command:

if-then-fi
if-then-else-fi
if-then-elif-then-...fi

kill

kill [
options] IDs

Send signals to each specified process or job ID,
which you must own unless you are a privileged user. The default
signal sent with the
kill
command is
TERM
, instructing processes to shut
down.

read

read [
options] variable1
[
variable2
...]

Read one line of standard input, and assign each
word to the corresponding variable, with all remaining words
assigned to the last variable.

echo -n "Enter last-name, age, height, and weight > "
read lastname everythingelse
echo $lastname
echo $everythingelese

The name entered is placed in variable
$lastname
; all of the other values,
including the spaces between them, are placed in
$everythingelse
.

return

return [
n
]

This command is used inside a function definition to
exit the function with status
n
. If
n
is omitted, the exit status of the
previously executed command is returned.

seq

seq [
OPTION
]... LAST
seq [
OPTION
]... FIRST LAST
seq [
OPTION
]... FIRST INCREMENT LAST

Print a sequence of numbers. This is useful in
for
and
while
loops.

year=$(date +%Y) # get current year
for month in $(seq -w 1 12)
{
monthname=$(date -d "${year}-${month}-01" +%B)
echo "Month $month is $monthname"
}
Month 01 is January
Month 02 is February
Month 03 is March
Month 04 is April
Month 05 is May
Month 06 is June
Month 07 is July
Month 08 is August
Month 09 is September
Month 10 is October
Month 11 is November
Month 12 is December

shift

shift [
n
]

Shift positional parameters down
n
elements. If
n
is omitted, the default is 1, so
$2
becomes
$1
,
$3
becomes
$2
, and so on.

source

source
file
[
arguments
]
.
file
[
arguments
]

Read and execute lines in
file
. The
file
does not need to be executable but
must be in a directory listed in
PATH
. The dot syntax is equivalent to
stating
source
.

test

test
expression
[
expression
]

Evaluate the conditional expression and return a
status of 0 (true) or 1 (false). The first form explicitly calls
out the
test
command. The second form implies
the
test
command. The spaces around
expression
are required in the second
form.
expression
is constructed using
options. Some Linux distributions have a version of
test
at
/usr/bin/test
.
If that is the case, the built-in
bash
version of
test
will usually take
precedence.

To determine if a file exists and is readable, use the
-r
option:

if test -r file
then
echo "file exists"
fi

Using the
[ ]
form
instead, the same test looks like this:

if [ -r file ]
then
echo "file exists"
fi

until

until
test-commands
do
commands
done

Execute
test-commands
(usually a
test
command), and if the exit
status is nonzero (that is, the test fails), perform
commands
and repeat. Opposite of
while
.