tip
NOTE In the last syntax ./ means current directory, But only . (dot) means execute given command file in current shell without starting the new copy of shell, The syntax for . (dot) command is as follows
Syntax:
. command-name
Example:$ . foo
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There are three types of quotes
|
Quotes
|
Name
|
Meaning
|
| " | Double Quotes | "Double Quotes" - Anything enclose in double quotes removed meaning of that characters (except \ and $). |
| ' | Single quotes | 'Single quotes' - Enclosed in single quotes remains unchanged. |
| ` | Back quote | `Back quote` - To execute command |
Example:
$ echo "Today is date"
Can't print message with today's date.
$ echo "Today is `date`".
It will print today's date as, Today is Tue Jan ....,Can you see that the `date` statement uses back quote?
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Wild cards (Filename Shorthand or meta Characters)
| Wild card /Shorthand | Meaning | Examples | |
| * | Matches any string or group of characters. | $ ls * | will show all files |
| $ ls a* | will show all files whose first name is starting with letter 'a' | ||
| $ ls *.c | will show all files having extension .c | ||
| $ ls ut*.c | will show all files having extension .c but file name must begin with 'ut'. | ||
| ? | Matches any single character. | $ ls ? | will show all files whose names are 1 character long |
| $ ls fo? | will show all files whose names are 3 character long and file name begin with fo | ||
| [...] | Matches any one of the enclosed characters | $ ls [abc]* | will show all files beginning with letters a,b,c |
Note:
[..-..] A pair of characters separated by a minus sign denotes a range.
Example:
$ ls /bin/[a-c]*
Will show all files name beginning with letter a,b or c like
/bin/arch /bin/awk /bin/bsh /bin/chmod /bin/cp
/bin/ash /bin/basename /bin/cat /bin/chown /bin/cpio
/bin/ash.static /bin/bash /bin/chgrp /bin/consolechars /bin/csh
But
$ ls /bin/[!a-o]
$ ls /bin/[^a-o]
If the first character following the [ is a ! or a ^ ,then any character not enclosed is matched i.e. do not show us file name that beginning with a,b,c,e...o, like
/bin/ps /bin/rvi /bin/sleep /bin/touch /bin/view
/bin/pwd /bin/rview /bin/sort /bin/true /bin/wcomp
/bin/red /bin/sayHello /bin/stty /bin/umount /bin/xconf
/bin/remadmin /bin/sed /bin/su /bin/uname /bin/ypdomainname
/bin/rm /bin/setserial /bin/sync /bin/userconf /bin/zcat
/bin/rmdir /bin/sfxload /bin/tar /bin/usleep
/bin/rpm /bin/sh /bin/tcsh /bin/vi
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NOTE:
$# holds number of arguments specified on command line. And $* or $@ refer to all arguments passed to script.
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It means when ever there is any type of comparison in Linux Shell It gives only two answer one is YES and NO is other.
| In Linux Shell Value | Meaning | Example |
| Zero Value (0) | Yes/True | 0 |
| NON-ZERO Value | No/False | -1, 32, 55 anything but not zero |
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For string Comparisons use
| Operator | Meaning |
| string1 = string2 | string1 is equal to string2 |
| string1 != string2 | string1 is NOT equal to string2 |
| string1 | string1 is NOT NULL or not defined |
| -n string1 | string1 is NOT NULL and does exist |
| -z string1 | string1 is NULL and does exist |
Shell also test for file and directory types
| Test | Meaning |
| -s file | Non empty file |
| -f file | Is File exist or normal file and not a directory |
| -d dir | Is Directory exist and not a file |
| -w file | Is writeable file |
| -r file | Is read-only file |
| -x file |
Is file is executable |
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Examples: (Assemums the file bad_file_name111 does not exists)
$ rm bad_file_name111
rm: cannot remove `bad_file_name111': No such file or directory
Above command gives error as output, since you don't have file. Now if
we try to redirect this error-output to file, it can not be send
(redirect) to file, try as follows:
$ rm bad_file_name111 > er
Still it prints output on stderr as rm: cannot remove `bad_file_name111': No such file or directory, And if you see er file as $ cat er , this file is empty, since output is send to error device and you can not redirect it to copy this error-output to your file 'er'. To overcome this problem you have to use following command:
$ rm bad_file_name111 2>er
Note that no space are allowed between 2 and >, The 2>er
directs the standard error output to file. 2 number is default number
(file descriptors number) of stderr file. To clear your idea onsider
another example by writing shell script as follows:
$ cat > demoscr |
Run it as follows:
$ chmod 755 demoscr
$ ./demoscr
Error : Number are not supplied
Usage : ./demoscr number1 number2
$ ./demoscr > er1
$ ./demoscr 5 7
Sum is 12
For first sample run , our script prints error message indicating that you have not given two number.
For
second sample run, you have redirect output of script to file er1,
since it's error we have to show it to user, It means we have to print
our error message on stderr not on stdout. To overcome this problem
replace above echo statements as follows
echo "Error : Number are not supplied" 1>&2
echo "Usage : $0 number1 number2" 1>&2
Now if you run it as follows:
$ ./demoscr > er1
Error : Number are not supplied
Usage : ./demoscr number1 number2
It will print error message on stderr and not on stdout. The 1>&2 at the end of echo statement, directs the standard output (stdout) to standard error (stderr) device.
Syntax:
from>&destination
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Now consider following shell script:
$ cat > testsign1 |
Save it and run as
$ chmod +x testsign1
$ ./testsign1
It
first ask you main database file where all appointment of the day is
stored, if no such database file found, file is created, after that it
open one temporary file in /tmp directory, and puts today's date in that
file. Then one infinite loop begins, which ask appointment title, time
and remark, if this information is correct its written to temporary
file, After that, script asks user , whether he/she wants to add next
appointment record, if yes then next record is added , otherwise all
records are copied from temporary file to database file and then loop
will be terminated. You can view your database file by using cat
command. Now problem is that while running this script, if you press
CTRL + C, your shell script gets terminated and temporary file are left
in /tmp directory. For e.g. try it as follows
$./testsign1
After given database file name and after adding at least one
appointment record to temporary file press CTRL+C, Our script get
terminated, and it left temporary file in /tmp directory, you can check
this by giving command as follows
$ ls /tmp/input*
Our script needs to detect such signal (event) when occurs; To achieve this we have to first detect Signal using trap command.
Syntax:
trap {commands} {signal number list}
| Signal Number | When occurs |
| 0 | shell exit |
| 1 | hangup |
| 2 | interrupt (CTRL+C) |
| 3 | quit |
| 9 | kill (cannot be caught) |
To catch signal in above script, put trap statement before calling Take_input1 function as trap del_file 2 ., Here trap command called del_file() when 2 number interrupt ( i.e. CTRL+C ) occurs. Open above script in editor and modify it so that at the end it will look like as follows:
|
|
Run the script as:
$ ./testsign1
After
giving database file name and after giving appointment title press
CTRL+C, Here we have already captured this CTRL + C signal (interrupt),
so first our function del_file() is called, in which it gives message as
"* * * CTRL + C Trap Occurs (removing temporary file)* * * " and then
it remove our temporary file and then exit with exit status 1. Now check
/tmp directory as follows
$ ls /tmp/input*
Now Shell will report no such temporary file exit.
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You can also move the positional parameters over more than one place by specifying a number with the shift command. The following command would shift the positional parameters two places:
shift 2
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As you can see shift command can use to parse the command line (args) option. This is useful if you have limited number of command line option. If command line options are too many then this approach works slowly as well as complex to write and maintained. You need to use another shell built in command - getopts. Next section shows the use of getopts command. You still need the shift command in conjunction with getopts and for other shell scripting work.
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Examlpe:
We have script called ani which has syntax as
ani -n -a -s -w -d
Options: These are optional argument
-n name of animal
-a age of animal
-s sex of animal
-w weight of animal
-d demo values (if any of the above options are used their values are not taken)
See because of getopts, we can pass command line argument in different style. Following are invalid options for ani script
$ ani -nLassie -a4 -sFemal -w20Kg
Animal Name: Lassie, Age: 4, Sex: Femal, Weight: 20Kg (user define mode)
No space between option and their value.
$ ani -nLassie-a4-sFemal-w20Kg
Animal Name: Lassie-a4-sFemal-w20Kg, Age: 2 Mouths, Sex: Male, Weight: 3Kg (user define mode)
$ ani -n Lassie -a 4 -s Female -w 20Kg -c Mammal
./ani: illegal option -- c Usage: ./ani -n -a -s -w -d
-c is not one of the valid options.
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Sr.No Name
11 Vivek
12 Renuka
13 Prakash
14 Ashish
15 Rani
Sr.No Mark
11 67
12 55
13 96
14 36
15 67
$cut -f2 sname
Vivek
Renuka
Prakash
Ashish
Rani
cut utility cuts out selected data from sname file. To select Sr.no. field from sname give command as follows:
$cut -f1 sname
11
12
13
14
15
| Command | Explanation |
| cut | Name of cut utility |
| -f1 | Using (-f) option, you are specifying the extraction field number. (In this example its 1 i.e. first field) |
| sname | File which is used by cut utility and which is use as input for cut utility. |
$ paste sname smark
11 Vivek 11 67
12 Renuka 12 55
13 Prakash 13 96
14 Ashish 14 36
15 Rani 15 67
$join sname smark
11 Vivek 67
12 Renuka 55
13 Prakash 96
14 Ashish 36
15 Rani 67
$ tr "h2" "3x" < sname
11 Vivek
1x Renuka
13 Prakas3
14 As3is3
15 Rani
$ tr "[a-z]" "[A-Z]"
hi i am Vivek
HI I AM VIVEK
what a magic
WHAT A MAGIC
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egg order 4
cacke good 10
cheese okay 4
pen good 12
floppy good 5
After crating file issue command
$ awk '/good/ { print $3 }' inventory
10
12
5
awk utility, select each record from file containing the word "good"
and performs the action of printing the third field (Quantity of
available goods.). Now try the following and note down its output.
$ awk '/good/ { print $1 " " $3 }' inventory
General Syntax of awk utility:
Syntax:
awk 'pattern action' {file-name}
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personame
Hello I am vivek
12333
12333
welcome
to
sai computer academy, a'bad.
what still I remeber that name.
oaky! how are u luser?
what still I remeber that name.
After creating file, issue following command at shell prompt
$ uniq personame
Hello I am vivek
12333
welcome
to
sai computer academy, a'bad.
what still I remeber that name.
oaky! how are u luser?
what still I remeber that name.
Above
command prints those lines which are unique. For e.g. our original file
contains 12333 twice, so additional copies of 12333 are deleted. But if
you examine output of uniq, you will notice that 12333 is gone
(Duplicate), and "what still I remeber that name" remains as its.
Because the uniq utility compare only adjacent lines, duplicate lines
must be next to each other in the file. To solve this problem you can
use command as follows
$ sort personame | uniq
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hello world!
cartoons are good
especially toon like tom (cat)
what
the number one song
12221
they love us
I too
After saving file, issue following command,
$ grep "too" demofile
cartoons are good
especially toon like tom (cat)
I too
grep will locate all lines for the "too" pattern and print all (matched) such line on-screen. grep prints too, as well as cartoons and toon; because grep treat "too" as expression. Expression by grep is read as the letter t followed by o and so on. So if this expression is found any where on line its printed. grep don't understand words.
Syntax:
grep "word-to-find" {file-name}
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Command like
:g/the/p
It is different from all other Os
My brother Vikrant also loves linux who also loves unix.
Will find word like theater, the, brother, other etc. What if you want to just find the word like "the" ? To find the word (Let's say Linux) you can give command like
:/\<Linux\>
Linux is cooool.
:g/\<Linux\>/p
Linux is cooool.
Linux is now 10 years old.
Rani my sister never uses Linux
The symbol \< and \> respectively match the empty string at the beginning and end of the word. To find the line which contain Linux pattern at the beginning give command
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Even you can try it as follows
:g/[0-9]
Linux is now 10 years old.
Next year linux will be 11 year old.
Here range of digit is specified by giving first digit (0-zero) and last digit (1), separated by hyphen. You can try [a-z] for lowercase character, [A-Z] for uppercase character. Not just this, there are certain named classes of characters which are predefined. They are as follows:
| Predefined classes of characters | Meaning |
| [:alnum:] | Letters and Digits (A to Z or a to z or 0 to 9) |
| [:alpha:] | Letters A to Z or a to z |
| [:cntrl:] | Delete character or ordinary control character (0x7F or 0x00 to 0x1F) |
| [:digit:] | Digit (0 to 9) |
| [:graph:] | Printing character, like print, except that a space character is excluded |
| [:lower:] | Lowercase letter (a to z) |
| [:print:] | Printing character (0x20 to 0x7E) |
| [:punct:] | Punctuation character (ctrl or space) |
| [:space:] | Space, tab, carriage return, new line, vertical tab, or form feed (0x09 to 0x0D, 0x20) |
| [:upper:] | Uppercase letter (A to Z) |
| [:xdigit:] | Hexadecimal digit (0 to 9, A to F, a to f) |
For e.g. To find digit or alphabet (Upper as well as lower) you will write
:/[0-9A-Za-Z]
Instead of writing such command you could easily use predefined classes or range as follows
:/[[:alnum:]]
The . (dot) matches any single character.
For e.g. Type following command
:g/\<.o\>
She only loves to play games and nothing else.
Do you know?
This will include lo(ves), Do, no(thing) etc.
* Matches the zero or more times
For e.g. Type following command
:g/L*
Hello World.
This is vivek from Poona.
....
....
:g/Li*
Linux is cooool.
Linux is now 10 years old.
Rani my sister never uses Linux
:g/c.*and
. (DOT) is special command of linux.
Here first c character is matched, then any single character (.) followed by n number of single character (1 or 100 times even) and finally ends with and. This can found different word as follows command or catand etc.
In
the regular expression metacharacters such . (DOT) or * loss their
special meaning if we use as \. or \*. The backslash removes the special
meaning of such meatcharacters and you can use them as ordinary
characters. For e.g. If u want to search . (DOT) character at the
beginning of line, then you can't use command as follows
:g/^.
Hello World.
This is vivek from Poona.
....
..
...
. (DOT) is special command of linux.
Okay! I will stop.
Instead of that use
:g/^\.
. (DOT) is special command of linux.
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awk reads the input from given file (or from stdin also) one line at a time, then each line is compared with pattern. If pattern is match for each line then given action is taken. Pattern can be regular expressions. Following is the summery of common awk metacharacters:
| Metacharacter | Meaning |
| . (Dot) | Match any character |
| * | Match zero or more character |
| ^ | Match beginning of line |
| $ | Match end of line |
| \ | Escape character following |
| [ ] | List |
| { } | Match range of instance |
| + | Match one more preceding |
| ? | Match zero or one preceding |
| | | Separate choices to match |
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Consider following text database file
|
Sr.No
|
Product
|
Qty
|
Unit Price
|
| 1 | Pen | 5 | 20.00 |
| 2 | Rubber | 10 | 2.00 |
| 3 | Pencil | 3 | 3.50 |
| 4 | Cock | 2 | 45.50 |
In above file fields are Sr.No,Product,Qty,Unit Price. Field is the smallest element of any record. Each fields has its own attributes. For e.g. Take Qty. field. Qty. fields attribute is its numerical (Can contain only numerical data). Collection of fields is know as record. So
1. Pen 5 20.00 ----> Is a Record.
Collection of record is know as database file. In above text database file each field is separated using space (or tab character) and record is separated using new-line character ( i.e. each record is finished at the end of line ). In the awk, fields are access using special variable. For e.g. In above database $1, $2, $3, $4 respectively represents Sr.No, Product, Qty, Unit Price fields. (Don't confuse $1,$2 etc with command line arguments of shell script)
For this part of tutorial create text datafile inven (Shown as above). Now enter following simple awk program/command at shell prompt:
$ awk '{ print $1 $2 "--> Rs." $3 * $4 }' inven
1.Pen--> Rs.100
2.Pencil--> Rs.20
3.Rubber--> Rs.10.5
4.Cock--> Rs.91
Above awk program/command can be explained as follows:
| awk program statement | Explanation |
| '{ print $1 $2 "--> Rs." $3 * $4 } ' | print command is used to print contains of variables or text enclose in " text ". Here $1, $2, $3,$4 are all the special variable. $1, $2, etc all of the variable contains value of field. Finally we can directly do the calculation using $3 * $4 i.e. multiplication of third and fourth field in database. Note that "--> Rs." is string which is printed as its. |
Note $1,$2 etc (in awk) also know as predefined variable and can assign any value found in field.
Type following awk program at shell prompt,
$ awk '{ print $2 }' inven
Pen
Pencil
Rubber
Cock
awk prints second field from file. Same way if you want to print second and fourth field from file then give following command:
$awk '{ print $2 $4}' inven
Pen20.00
Pencil2.00
Rubber3.50
Cock45.50
$0 is special variable of awk , which print entire record, you can verify this by issuing following awk command:
$ awk '{ print $0 }' inven
1. Pen 5 20.00
2. Pencil 10 2.00
3. Rubber 3 3.50
4. Cock 2 45.50
You can also create awk command (program) file as follows:
$ cat > prn_pen |
And then you can execute or run above "prn_pen" awk command file as follows
$ awk -f prn_pen inven
5
10
In above awk program /Pen/ is the search pattern, if this pattern is found on line (or record) then print the third field of record.
{ print $3 } is called Action. On shell prompt , $ awk -f prn_pen inven , -f option instruct awk, to read its command from given file, inven is the name of database file which is taken as input for awk.
Now create following awk program as follows:
$cat > comp_inv |
Run it as follows:
$ awk -f comp_inv inven
2. Pencil 10 2.00
Here third field of database is compared with 5, this the pattern. If this pattern found on any line database, then entire record is printed.
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Create awk file as follows:
$cat > def_var |
Run it as follows.
$awk -f def_var inven
Printing Rec. #1(1. Pen 5 20.00),And # of field for this record is 4
Printing Rec. #2(2. Pencil 10 2.00),And # of field for this record is 4
Printing Rec. #3(3. Rubber 3 3.50),And # of field for this record is 4
Printing Rec. #4(4. Cock 2 45.50),And # of field for this record is 4
NR and NF are predefined variables of awk which means Number of input Record, Number of Fields in input record respectively. In above example NR is changed as our input record changes, and NF is constant as there are only 4 field per record. Following table shows list of such built in awk variables.
| awk Variable | Meaning |
| FILENAME | Name of current input file |
| RS | Input record separator character (Default is new line) |
| OFS | Output field separator string (Blank is default) |
| ORS | Output record separator string (Default is new line) |
| NF | Number of input record |
| NR | Number of fields in input record |
| OFMT | Output format of number |
| FS | Field separator character (Blank & tab is default) |
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