Assignment1

Exercise Sheet 1

In this exercise you will be asked to work with a shell. The idea is to get familiar with some Unix commands and bash programming.

Task 1

Consider the following scripts. Explain what each line means within these scripts. Explain what happens when you execute these scripts with and without input arguments.

Script 1

#!/bin/bash
for FN in "$@"
do
    chmod 0750 "$FN"
done

Script 2

#!/bin/bash
function usage {
    echo "$0: <TODO: fill me in>"
    exit 1
}

ARG1=$1; shift || usage
ARG2=$1; shift || usage
ARG3=$1; shift || usage

grep -n "$ARG3" "$ARG1" > "$ARG2"

Hint: shift is a shell buit-in, use help shift to get more information.

Task 2

What does this shell script do? Improve its usability by adding error handling for the following cases:

• Print a help message when the number of provided arguments is not two.
• Log an error message to a file error.log when the file $OUTFILE is not writable.

#!/bin/bash
INFILE=$1
OUTFILE=$2
if [ -e "$INFILE" ]
then
    cat "$INFILE" >> "$OUTFILE"
fi

Hint: Take a look at the man page for the "test" command: man "[".

Task 3

Write a script my_backup.sh to perform a backup of the current directory (containing only files) into a given folder. The script receives a single parameter which is the path of the folder where the backup is to be stored. If the backup folder already exists then a file must be copied if and only if it does not exist within the backup folder or if it is newer than the existing one in the backup folder. The script will be used as follows

bash my_backup.sh /path/to/backup-folder # creates a backup of . into backup-folder

Hint: Use stat --format %Y <file> to get the modification time of a file.