Trap with Examples :--

Trap:

Signals are software interrupts sent to a program to indicate that an important event has occurred. The events can vary from user requests to illegal memory access errors. Some signals, such as the interrupt signal, indicate that a user has asked the program to do something that is not in the usual flow of control.

The following are some of the more common signals you might encounter and want to use in your programs:

Signal Name
Signal Number
Description
SIGHUP
1
Hang up detected on controlling terminal or death of controlling process
SIGINT
2
Issued if the user sends an interrupt signal (Ctrl + C).
SIGQUIT
3
Issued if the user sends a quit signal (Ctrl + D).
SIGFPE
8
Issued if an illegal mathematical operation is attempted
SIGKILL
9
If a process gets this signal it must quit immediately and will not perform any clean-up operations
SIGALRM
14
Alarm Clock signal (used for timers)
SIGTERM
15
Software termination signal (sent by kill by default).

List of Signals:

There is an easy way to list down all the signals supported by your system. Just issue kill -l command and it would display all the supported signals:

The actual list of signals varies between Solaris, HP-UX, and Linux.

Sending Signals:

There are several methods of delivering signals to a program or script. One of the most common is for a user to type CONTROL-C or the INTERRUPT key while a script is executing.


When you press the Ctrl+C key a SIGINT is sent to the script and as per defined default action script terminates.


The other common method for delivering signals is to use the kill command whose syntax is as follows:

$ kill -signal pid


Here signal is either the number or name of the signal to deliver and pid is the process ID that the signal should be sent to. For Example:

$ kill -1 1001


Sends the HUP or hang-up signal to the program that is running with process ID 1001. To send a kill signal to the same process use the folloing command:

$ kill -9 1001


This would kill the process running with process ID 1001.

Cleaning Up Temporary Files:

$ trap "rm -f $WORKDIR/work1$$ $WORKDIR/dataout$$; exit" 2
$ trap "rm $WORKDIR/work1$$ $WORKDIR/dataout$$; exit" 1 2
$ trap 'rm $WORKDIR/work1$$ $WORKDIR/dataout$$; exit' 1 2





As an example of the trap command, the following shows how you can remove some files and then exit if someone tries to abort the program from the terminal:


$ trap "rm -f $WORKDIR/work1$$ $WORKDIR/dataout$$; exit" 2


From the point in the shell program that this trap is executed, the two files work1$$ and dataout$$ will be automatically removed if signal number 2 is received by the program.


So if the user interrupts execution of the program after this trap is executed, you can be assured that these two files will be cleaned up. The exit command that follows the rm is necessary because without it execution would continue in the program at the point that it left off when the signal was received.


Signal number 1 is generated for hangup: Either someone intentionally hangs up the line or the line gets accidentally disconnected.


You can modify the preceding trap to also remove the two specified files in this case by adding signal number 1 to the list of signals:

$ trap "rm $WORKDIR/work1$$ $WORKDIR/dataout$$; exit" 1 2


Now these files will be removed if the line gets hung up or if the Ctrl+C key gets pressed.


The commands specified to trap must be enclosed in quotes if they contain more than one command. Also note that the shell scans the command line at the time that the trap command gets executed and also again when one of the listed signals is received.


So in the preceding example, the value of WORKDIR and $$ will be substituted at the time that the trap command is executed. If you wanted this substitution to occur at the time that either signal 1 or 2 was received you can put the commands inside single quotes:

$ trap 'rm $WORKDIR/work1$$ $WORKDIR/dataout$$; exit' 1 2

Ignoring Signals:

If the command listed for trap is null, the specified signal will be ignored when received. For example, the command:
$ trap '' 2

Specifies that the interrupt signal is to be ignored. You might want to ignore certain signals when performing some operation that you don't want interrupted. You can specify multiple signals to be ignored as follows:
$ trap '' 1 2 3 15

Note that the first argument must be specified for a signal to be ignored and is not equivalent to writing the following, which has a separate meaning of its own:
$ trap  2

If you ignore a signal, all subshells also ignore that signal. However, if you specify an action to be taken on receipt of a signal, all subshells will still take the default action on receipt of that signal.

Resetting Traps:

After you've changed the default action to be taken on receipt of a signal, you can change it back again with trap if you simply omit the first argument; so
$ trap 1 2

resets the action to be taken on receipt of signals 1 or 2 back to the default.

SED with Exaples

<UNDER CONSTRUCTION> :)

File Viewing

Below are the commands used to view the contents of a file, soft link or of a hard link without opening

(Explanations for these links are explained in a separate post ).


i) cat

cat filename

ii) more

more filename

iii) less

 less filename

(Note: Give Enter and Q accordingly to continue and terminate the execution)

Difference B/W more and less:

more just prints the text as is, stopping for page breaks, and does not clear the screen, it can be back grounded but it doesn't clear the screen where 

less is a full-screen application that gives you a searchable, scrollable window and clears the screen after you exit.

Difference B/W more and cat:

 more can be used to work with the standard output but not cat

For example:

We can execute 

ls -lrt | more 

but not ls -lrt|cat

Cut with Examples

Cut is a command to extract portion of text from a file or from the given text.

Below are some of the examples based on a file test.txt whose contents are as below.

$ cat test.txt

cat command for file oriented operations.

cp command for copy files or directories.

ls command to list out files and directories with its attributes.

--------------------------------------------

1. Select Column of Characters

To extract only a desired column(in this case column2) from a file use -c option.

$ cut -c2 test.txt

a

p

s

--------------------------------------------

2. Select Column of Characters using Range

The following example extracts first 3 characters of each line.

$ cut -c1-3 test.txt

cat

cp

ls

----------------------------------------

3. Select Column of Characters using either Start or End Position

The following specifies only the start position before the ‘-’. This example extracts from 3rd character to end of each line from test.txt file.

$ cut -c3- test.txt

t command for file oriented operations.

 command for copy files or directories.

 command to list out files and directories with its attributes.

The following specifies only the end position after the ‘-’. This example extracts 8 characters from the beginning of each line from test.txt file.

$ cut -c-8 test.txt

cat comm

cp comma

ls comma

The entire line would get printed when you don’t specify a number before or after the ‘-’ as shown below.

$ cut -c- test.txt

cat command for file oriented operations.

cp command for copy files or directories.

ls command to list out files and directories with its attributes.

------------------------------------------

4. Select a Specific Field from a File

Instead of selecting x number of characters, if you like to extract a whole field, you can combine option -f and -d.

The following example displays only first field of each lines from /etc/passwd file using the field delimiter : (colon).

$ cut -d':' -f1 /etc/passwd

root

daemon

bin

sys

sync

games

bala

----------------------------------------

5. Select Multiple Fields from a File

$ grep "/bin/bash" /etc/passwd | cut -d':' -f1,6

root:/root

bala:/home/bala

To display the range of fields specify start field and end field as shown below. In this example, we are selecting field 1 through 4, 6 and 7

$ grep "/bin/bash" /etc/passwd | cut -d':' -f1-4,6,7

root:x:0:0:/root:/bin/bash

bala:x:1000:1000:/home/bala:/bin/bash

------------------------------------------

6. Select Fields Only When a Line Contains the Delimiter

In our /etc/passwd example, if you pass a different delimiter other than : (colon), cut will just display the whole line.

In the following example, we’ve specified the delimiter as | (pipe), and cut command simply displays the whole line, even when it doesn’t find any line that has | (pipe) as delimiter.

$ grep "/bin/bash" /etc/passwd | cut -d'|' -f1

root:x:0:0:root:/root:/bin/bash

bala:x:1000:1000:bala,,,:/home/bala:/bin/bash

But, it is possible to filter and display only the lines that contains the specified delimiter using -s option.

The following example doesn’t display any output, as the cut command didn’t find any lines that has | (pipe) as delimiter in the /etc/passwd file.

$ grep "/bin/bash" /etc/passwd | cut -d'|' -s -f1

------------------------------------------

7. Complement :

Select All Fields Except the Specified Fields

using –complement option.

The following example displays all the fields from /etc/passwd file except field 7

$ grep "/bin/bash" /etc/passwd | cut -d':' --complement -s -f7

root:x:0:0:root:/root

bala:x:1000:1000:bala,,,:/home/bala

--------------------------------------

8. Output Delimiter:

To change the output delimiter use the option –output-delimiter as shown below. In this example, the input delimiter is : (colon), but the output delimiter is # (hash).

$ grep "/bin/bash" /etc/passwd | cut -d':' -s -f1,6,7 --output-delimiter='#'

root#/root#/bin/bash

bala#/home/bala#/bin/bash

-----------------------------------------

9. Change Output Delimiter to Newline

In this example, each and every field of the cut command output is displayed in a separate line. We still used –output-delimiter, but the value is $’\n’ which indicates that we should add a newline as the output delimiter.

$ grep bala /etc/passwd | cut -d':' -f1,6,7 --output-delimiter=$'\n'

bala

/home/bala

/bin/bash

----------------------------------------

10. Example of combining Cut with Other Unix Commands

$ ps axu | grep python | sed 's/\s\+/ /g' | cut -d' ' -f2,11-

2231 /usr/bin/python /usr/lib/unity-lens-video/unity-lens-video

2311 /usr/bin/python /usr/lib/unity-scope-video-remote/unity-scope-video-remote

2414 /usr/bin/python /usr/lib/ubuntuone-client/ubuntuone-syncdaemon

2463 /usr/bin/python /usr/lib/system-service/system-service-d

3274 grep --color=auto python

AWK with Examples

BASIC s ....

Lets take an example of a file abc.txt whose contents are 

$cat abc.txt

10 100 1000 one hundred
20 200 2000 two twohundred
30 300 3000 three threehundred

To print the first column

$awk '{print $1}' abc.txt
10
20
30

To print the first and second column

$awk '{print $1 $2}' abc.txt
10 100
20 200
30 300


By default space is the delimiter.You can change by flag -F like

$awk -OFS":" '{print $1 $4}' abc.txt

10:one
20:two
30:three

FLAGS:

NR                         Line Number
IFS                         Input File Seperator
OFS                       OutPut File Seperator

To count the no of  rows of a file(to replace wc -l abc.txt)

$awk 'BEGIN {count=0} {count=count+1} END {print count}' abc.txt
3

(OR)

simply

$ awk 'END {print NR}' abc.txt
3

To count the sum of  nth clolumns of a file(say 2 in this case)

$awk 'BEGIN {sum=0} {sum=sum+$2} END {print sum}' abc.txt
600

Lets go to some Advanced ...

Reading of input and printing can be managed by file seperators which is explained in a seperate post.

Loops, control statements, file handling and varible pasing can also be done in awk as explained below

<UNDER CONSTRUCTION> :)

Redirections, Standard I/O

BasicS  ...

Most of the cases we use this redirection operators in shell script or at prompt level.

./scriptname.sh > logfile         To send the standard output of ./scriptname by replacing the existing logfile

./scriptname.sh >> logfile     To send the standard output of ./scriptname by appending the existing logfile

./scriptname.sh 2> logfile       To send the standard error while running the scriptname.sh

./scriptname.sh 2>> logfile    To send the standard error while running the scriptname.sh with append

Lets go to some advanced...

To send standard error as well as standard output to logfile

./scriptname.sh  2> logfile >&1    

(OR)

./scriptname.sh  > logfile 2>&1