Unix interview Questions and answers: reading from a file

Unix for software developers

1.Top 17 Unix commands Java developers use frequently		2.History commands	3.Shell scripting	4. Reading from a file
5.Purging older files	6.Splitting and archiving files	7.Emulator, SSH client	8.Unix commands for developers	9.Unix basic interview Q&A

Q. If you have a shell script file or data file created in DOS, how will you convert it to Unix (especially, end of file and end of line characters)?
A. Using the dos2unix command.

dos2unix ReadWriteFile.sh ReadWriteFile.sh

Q. How do you remove the Control-M characters from a file?
A. Using the sed command that replaces Control-M with nothing

sed 's/^M//g' ReadWriteFile.sh > ReadWriteFileNew.sh

Note: The ^M is typed on the command line with ctrl+v and ctrl+M

Q. How will you go about reading a CSV file as shown below?

Portfolio,Account,PositionIndicator,Amount
AA123,CURR-AUD,CREDIT,2244.14000000
AA123,CURR-AUD,CREDIT,5.60000000
AA123,CURR-AUD,DEBIT,2249.74000000
AA123,CURR-GBP,CREDIT,0.01000000
AA123,CURR-GBP,DEBIT,0.01000000

A.

#!/bin/bash

# $# means number of arguments supplied to the command. In this case the file to to be read
if [ $# != 1 ]; then
    echo "Usage: $0 input-file"
    exit 1
else
    infile=$1
fi

# assign file descriptor 3 to input file
exec 3< $infile

# read till the end of the file
until [ $done ]
do
    read <&3 myline
 #$? is the exit code, and 0 means success and != 0 means not success. That is no line is read.
    if [ $? != 0 ]; then
        done=1
        continue
    fi

    # process file data line-by-line in here

    echo $myline
done

Q. How will you read and then write to another file?
A.

#!/bin/bash

# $# means number of arguments supplied to the command
if [ $# != 2 ]; then
    # $0 is the script name  $1 is input file and $2 is output file  
    echo "Usage: $0 input-file output-file"
    exit 1
else
    infile=$1
fi

# assign file descriptor 3 to input file
exec 3< $infile

# read till the end of the file
until [ $done ]
do
    read <&3 myline
 #$? is the exit code, and 0 means success and != 0 means not success. That is no line is read.
    if [ $? != 0 ]; then
        done=1
        continue
    fi
   
   # process file data line-by-line
   
    # append to output file. If you run it more than once, it keeps appending.
    echo $myline >> $2 
done

Q. How will you empty or clear the contents of a file?
A.

cat /dev/null > /drives/c/jpm_out/simple-out.csv

Q. How will you read the file and then write to another file with CSV data in  different order?

Account,Portfolio,PositionIndicator,Amount
CURR-AUD,AA123,CREDIT,2244.14000000
CURR-AUD,AA123,CREDIT,5.60000000
CURR-AUD,AA123,DEBIT,2249.74000000
CURR-GBP,AA123,CREDIT,0.01000000

A. Here is the revised shell script

#!/bin/bash

# $# means number of arguments supplied to the command
if [ $# != 2 ]; then
    # $0 is the script name  $1 is input file and $2 is output file  
    echo "Usage: $0 input-file output-file"
    exit 1
else
    infile=$1
fi

# assign file descriptor 3 to input file
exec 3< $infile

export IFS=","

# read till the end of the file
until [ $done ]
do
    # read 4 columns into a,b,c, and d
    read <&3 a b c d
 #$? is the exit code, and 0 means success and != 0 means not success. That is no line is read.
    if [ $? != 0 ]; then
        done=1
        continue
    fi
   
   # process file data line-by-line
   
    # append to output file. If you run it more than once, it keeps appending.
    echo $b,$a,$c,$d >> $2 
done