Chapter 9 — Bash Scripting Basics — Solutions

do the assignment first. reading the answer is not the same as doing it, and u will feel like u learned it when u didn’t.

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1 and 2 and 3 — hello.sh

#!/bin/bash
# hello.sh -- say hello to someone
 
if [ -z "$1" ]; then                 # -z means "is it empty?", so: was no name given?
    echo "usage: ./hello.sh <name>"  # tell them how to use it
    exit 1                           # stop here. 1 means it did not work
fi                                   # closes the if
 
echo "hello $1"                      # only gets here if a name was given

chmod u+x hello.sh
./hello.sh ati

hello ati

./hello.sh
echo $?

usage: ./hello.sh <name>
1

• check that exit code. that 1 is what lets someone else write ./hello.sh ati && echo "it worked".

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4 — count.sh

#!/bin/bash
 
count=1                      # a counter to keep track of where we are
while [ $count -le 5 ]; do   # -le is "less than or equal". keep going while it is 1..5
    echo $count              # print whatever the counter holds right now
    count=$((count + 1))     # add 1. $(( )) is how u do maths in bash.
                             # forget this line and the loop never ends
done                         # closes the while

or the shorter way with a for, both are correct:

#!/bin/bash
 
for count in 1 2 3 4 5; do   # go through the list, one number at a time
    echo $count              # `count` holds a different one each time round
done

1
2
3
4
5

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5 — check.sh

#!/bin/bash
# check.sh -- is this a file, a directory, or nothing
 
if [ -f "$1" ]; then           # -f asks: does it exist AND is it a normal file?
    echo "$1 is a file"
elif [ -d "$1" ]; then         # elif is "else if". -d asks: is it a directory?
    echo "$1 is a directory"
else                           # neither, so it is not there at all
    echo "$1 does not exist"
fi

./check.sh notes.txt
./check.sh backup
./check.sh nothing

notes.txt is a file
backup is a directory
nothing does not exist

• elif is else if written short. u can have as many as u want between the if and the else.
• the order matters. -f has to be checked before -d, otherwise nothing breaks here, but in general the first branch that matches is the one that runs and the rest are never looked at.

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6 — backup.sh

#!/bin/bash
# backup.sh -- copy every txt file into backup/
 
mkdir -p backup              # -p means: do not complain if it is already there
 
count=0                      # count how many we actually copied
for file in *.txt; do        # the shell turns *.txt into the real names first
 
    if [ -f "$file" ]; then  # is this a real file? see the note below
        cp "$file" backup/   # copy it in
        count=$((count + 1)) # and add one to the tally
    fi
 
done
 
echo "copied $count files into backup/"

./backup.sh

copied 3 files into backup/

• mkdir -p does not complain if the folder is already there, which is exactly what u want in a script u run more than once.
• the if [ -f "$file" ] inside the loop looks pointless but it is not. if there is no .txt file at all, bash hands the loop the text *.txt itself, and without that check u would try to copy a file with that name and get an error.

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7 — ask.sh

#!/bin/bash
 
read -p "what is ur name? " name       # -p prints the question, then waits.
read -p "what city do u live in? " city # whatever they type lands in the variable
 
echo "$name lives in $city"

what is ur name? ati
what city do u live in? Istanbul
ati lives in Istanbul

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8 — watching it run

bash -x hello.sh ati

+ '[' -z ati ']'
+ echo 'hello ati'
hello ati

every line starting with + is a command about to run, with the variables already replaced. so u are not guessing what the script saw, u are looking at it.

try it on the one that fails too:

bash -x hello.sh

+ '[' -z '' ']'
+ echo 'usage: ./hello.sh <name>'
usage: ./hello.sh <name>
+ exit 1

• there it is: -z '' was true because there was no argument, so it went into the if and exited. this is how u debug a script, and it beats putting echo everywhere.

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