1

Shell Programming – Extra Slides

2

Counting the number of lines in a file#!/bin/sh

#countLines1

filename=$1 #Should check if arguments are given

count=0

while read aline

do

count=`expr $count + 1`

done < $filename

echo "$filename has $count lines“

From the command line:

$wc –l data.txt

11 data.txt

$countLines1 data.txt

Data.txt has 0 lines

3

Counting the number of lines in a file#!/bin/sh

#countLines2

filename=$1 #Should check if arguments are given

count=0

Cat $filename | while read aline

do

count=`expr $count + 1`

done

echo "$filename has $count lines“

From the command line:

$wc –l data.txt

11 data.txt

$countLines2 data.txt

Data.txt has 0 lines

WHY? Subshell execution takes place if input is piped into a for, while, until, if or case command or if the output is piped out.

4

Counting the number of lines in a fileSolution

#!/bin/sh

#countLines3

filename=$1 #Should check if arguments are given

count=0

exec < $filename #any commands that will read from stand in

# will read from $filename

while read aline

do

count=`expr $count + 1`

done

echo "$filename has $count lines“

From the command line:

$countLines3 data.txt

Data.txt has 11 lines

exec < /dev/tty--- Reassign the standard input back to terminal

5

trap

The pressing of DELETE key at the terminal, when a program is in execution, sends a signal to the executing program.

Using the trap command, the program can specify the action to be taken on receiving the signal.

Usage: trap commands signals, where commands are the actions to be taken on receiving the signals.

Some Commonly used signal numbers 0 Exit from Shell 1 Hangup 2 Interrupt (eg: Delete key) 15 Software termination (sent by kill, for example)

6

ExampleExample 1:

#!/bin/sh

i=1

JUNK=junkfile

trap ‘rm $JUNK$$;exit’ 2

while [ $i -le 100 ]

Do

# remove the file when interrupt is received

echo $i >> $JUNK$$

i=`expr $i + 1`

done

7

ExamplesExample 2:

trap 'echo Caught SIGINT - exiting; exit' 2

X=0

while : #loop forever

do

echo "X=$X"

X=`expr ${X} + 1`

sleep 1

Done

Example 3

#!/bin/sh

trap 'echo `pwd` $$ >>./errdir' 2 3 15

while (true)

do

echo 'Hi'

done

8

Example#!/bin/sh#What does this program do? trap 'increment' 2increment() { echo "Caught SIGINT ..." X=`expr ${X} + 500` if [ "${X}" -gt "2000" ] then echo "Okay, I'll quit ..." exit 1 fi}### main scriptX=0while :do echo "X=$X" X=`expr ${X} + 1` sleep 1done

9

Input and Output Redirection We know that Unix shell allows us to redirect the input and output of programs

using redirection (> and <) and piping(|). When the Unix kernel starts any process, for example, grep, ls and so on, it

sets up several places, called open files, for that process t read from and write to. Each of these files is given a number to identify with, called a file descriptor.

A file descriptor (also known as a file handle) is a non-negative digit that points at a file.  The file descriptors for stdin, stdout, and stderr are 0, 1, and 2, respectively.  Any of these

may be redirected to a file or to each other.  In other words, it is quite possible to send the output from stdin and stderr to the same

file.  This is quite useful when a user would rather check a script's results after it has completed processing. 

By default, the file that is opened for stdin, stdout and sterr is /dev/tty (your terminal).

But, when the shell starts a process, you can tell the shell what file to connect to any of those file descriptors.

For example in the following command,

grep aPattern somefile > output, the file descriptor 1 is connected to the file, output.

10

More On I/O What if you want to send the standard out to screen and capture the standard error in a

pipe or a file? It is easy to redirect any file descriptor to any file. Eg: command 2>errorFile

command 2> file – redirects the standard error from any command

cd JUNK 2>>out #the directory JUNK does not exist

cat out

sh: JUNK: not found.

11

More On I/O What if you want to send the standard out to screen and capture the standard error in a

pipe or a file? It is easy to redirect any file descriptor to any file. Eg: command 2>errorFile

command 2> file – redirects the standard error from any command

cd JUNK 2>>out #the directory JUNK does not exist

cat out

sh: JUNK: not found. Let us take a look at a few cases: Sending both standard output and errors to the pipe or

backquotes.

command 2>&1 |…

or

var=`command 2>&1`

This means that send standard error (with file descriptor 2) to the same place standard output is going (down the pipe or backquotes)

12

More On I/O Sending stderr go down a pipe and stdout to the screen.

command 2>&1 1>&2|… Will Not Work

We should use file descriptors 3 to 9,as holding places, to accomplish this.

command 3>&2 2>&1 1>&3 |…

or

var=` command 3>&2 2>&1 1>&3`

13

More On I/O Example:For the following grep command, assume that afile contains one matching line for “unix”

and bfile does not exist.

var=`grep “unix” afile bfile`echo $var #What is the output?

var=`grep “unix” afile bfile 3>&2`echo $var #What is the output?

var=`grep “unix” afile bfile 3>&2 2>&1`echo $var #What is the output?

var=`grep “unix” afile bfile 3>&2 2>&1 1>&3`echo $var #What is the output?