Bourne Shell scripts
Language Dazibao:
- All variables are string variables.
- Read variable with $ prefix, write without.
- Use build-in variables $? (exit code), $#, $0..$9 (argc, argv[])
- Understand strong - '', weak - "" and redirecting - `` quotation.
- Use expr function for arithmetics and string manipulation.
- Use test for logical operations and comparison. Result is in $?.
1. Comments
#!/bin/bash
#
# first row is telling to the system which shell should be
# implemented with this script
2. Constant data in code
Strings 'this is $strong quoting' # no substitution
"this is $weak quoting" # values substituted
`expr 2 + 3` # 5 result redirected
$(expr 2 + 3) # 5, with XML flavour
3. Variable data in code
All primary types of data
- boolean,
- bit set,
- finite integer,
- floating point number
are stored as formatted strings (in human readable form).
Variables does not require declaration.
Newer versions of bash support one-dimensional arrays. (a[3]=Abcd)
Structures and Containers are not available.
% VAR = "My variable" # create / write
% echo $VAR # read
% unset $VAR # free memory allocated to $VAR
Special variables
$? return code of the last command
$# number of command line arguments
$* whole string of arguments as one string
$0 script filename
$1 .. $n command line arguments
4. Expressions
Expressions are evaluated using expr utility
Numeric expressions with integers: (+ - * / %)
+ plus
- minus
* multiplication
/ division
% modulo (returns the remainder of an integer division)
% expr 18 + 4
String manipulations: (index match length substr)
# index: position in string of first character in substring
that matches.
# substr: print substring, starting position & length specified
# length: length of string
# 'match' operations similarly to 'grep'
a=1234zipper43231
# index: position in string of first character in substring
that matches.
b=`expr index $a 23`
echo Numerical position of first 2 in $a is $b.
# substr: print substring, starting position & length specified
b=`expr substr $a 2 6`
echo Substring of $a, starting at 2 and 6 chars long is $b.
# length: length of string
b=`expr length $a`
echo Length of $a is $b.
# 'match' operations similarly to 'grep'
b=`expr match $a [0-9]*`
echo Number of digits at the beginning of $a is $b.
b=`expr match $a '\([0-9]*\)'`
echo The digits at the beginning of $a are $b.
Strings concatination:
% a="Uno"; b="Duo"; c=$a$b; echo $c # UnoDuo
% d="Just${c}Between"; echo $d # JustUnoDuoBetween
5. Flow control
Test tool
---------
Use "test" named tool to evaluate expression to boolean.
Result is returned in exit code: 0 - true, 1 - false and
is stored in $? special variable.
Logical operations with exit codes
! logical NOT
&& logical AND
|| logical OR
cp file1 $var || exit 1
if [ -e file1 ] && [ -e file2 ]
then
echo "both files exists"
fi
String comparison
-z string is "null", that is, has zero length
-n string is not "null".
= is equal to ($a = $b)
!= is not equal to ($a != $b)
\< is less than, in ASCII alphabetical order ($a \< $b)
Note that the "<" needs to be escaped.
\> is greater than, in ASCII alphabetical order ($a \> $b)
Note that the ">" needs to be escaped.
Filesystem
-e file exists
-f file is a regular file
-s file is not zero size
-d file is a directory
-b file is a block device (floppy, cdrom, etc.)
-c file is a character device (keyboard, sound card, etc.)
-p file is a pipe
-L file is a symbolic link
-S file is a socket
-r file is readable (has read permission)
-w file has write permission
-x file has execute permission
-g group-id flag set on file
-u user-id flag set on file
-k "sticky bit" set (if user does not own a directory that has
the sticky bit set, she cannot delete files in it, not even
files she owns)
-O you are owner of file
-G group-id of file same as yours
-t n file descriptor n is open. This usually refers to stdin,
stdout, and stderr (file descriptors 0 - 2).
f1 -nt f2 file f1 is newer than f2
f1 -ot f2 file f1 is older than f2
f1 -ef f2 files f1 and f2 are links to the same file
Integer comparison
-eq is equal to ($a -eq $b)
-ne is not equal to ($a -ne $b)
-gt is greater than ($a -gt $b)
-ge is greater than or equal to ($a -ge $b)
-lt is less than ($a -lt $b)
-le is less than or equal to ($a -le $b)
Logical expressions
(expression) override normal precedence of the operators
! expression logical not
-a logical and
-o logical or
Operators
---------
exit N - exit script with exit code N
if test expression
then
...
fi
while test expression until test experssion
do do
... ...
done done
break - use this operator to exit do - done loop
continue - use this operator to resume next iteration
for vVar in 1 2 4 8
do
echo $vVar
done
Note 1: Use
for vVar in *; do ... ;done
to iterate in directory of files
Note 2: Use seq tool to generate lists
seq 5 # (stop) 1 2 3 4 5
seq 3 5 # (start stop) 3 4 5
seq 3 2 7 # (start step stop) 3 5 7
Several operators on the same row should be separated by ;
for vVar in Alpha Beta Gamma; do
echo $vVar
done
case $vVar in
"$condition1" )
command...
;;
"$condition2" )
command...
;;
esac
Example
#!/bin/bash
case $1
[a-z] ) echo "Lowercase letter";;
[A-Z] ) echo "Uppercase letter";;
[0-9] ) echo "Digit";;
* ) echo "Punctuation, whitespace, or other";;
esac
# Allows ranges of characters in [square brackets].
6. Visibility and encapsulation
- Write each function in separate file.
- If only script ascript uses function foo, ascript_foo name
is recommended for this function. Otherwise anamespace_foo
name is good for functions used in more than one script.
- Also ugly and useless function implementation was introduced
in late versions of Bourne Shell (after 1979).
foo () {
# Checks if any params.
if [ -z $1 ]
then
echo "No parameters passed to function."
return 0
else
echo "Param #1 is $1."
return 1
fi
}
foo
foo abc
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
How to insert data
- from command prompt
% read a # now write the value for just created a variable
% echo $a
% echo # wait for key is pressed
- inside script source
cat > /etc/resolv.conf <".
$PS3 The tertiary prompt, displayed in a select loop.
$PS4 The quartenary prompt, shown at the beginning of each
line of output when invoking a script with the -x
option. It displays as "+".
$PWD working directory (directory you are in at the time)
$OLDPWD old working directory (previous directory you were in)
$DIRSTACK contents of the directory stack (affected by pushd and
popd)
$PPID the process id (pid) of the currently running process
This corresponds to the pidof command
$MACHTYPE machine type
$HOSTTYPE host type
$OSTYPE operating system type
$EDITOR the default editor invoked by a script,
usually vi or emacs.
$IGNOREEOF ignore EOF: how many end-of-files (control-D) the shell
will ignore before logging out.
$TMOUT If the $TMOUT environmental variable is set to a
non-zero value time, then the shell prompt will time
out after time seconds. This will cause a logout.
$SECONDS The number of seconds the script has been running.
$REPLY The default value when a variable is not supplied to
read. Also applicable to select menus, but only
supplies the item number of the variable chosen, not
the value of the variable itself.
$SHELLOPTS the list of enabled shell options, a readonly variable
$BASH the path to the bash binary itself, usually /bin/bash
$BASH_ENV an environmental variable pointing to a bash startup
file to be read when a script is invoked
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Binary tools recommended for portable Unix scripts:
echo TRUE cp touch
cat FALSE mv tar
tr ln cmp
sed expr rm diff
awk test mkdir egrep
seq rmdir grep
ls sort
pwd sleep