Programing - Paiet/Tech-Journal-for-Everything GitHub Wiki

If/Then Statements

---

Script with user input

#! /bin/bash
read -p "Enter Firstname: " FNAME
read -p "Enter Lastname: " LNAME
echo "Full Name: $FNAME $LNAME"

Script modified to include arguments

#! /bin/bash
FNAME=$1
LNAME=$2
if [ "$FNAME" = "" ]; then
  read -p "Enter Firstname: " FNAME
  read -p "Enter Lastname: " LNAME
fi
echo "Full Name: $FNAME $LNAME"

If/Then/Else

#! /bin/bash
FNAME=$1
LNAME=$2
if [ "$FNAME" = "" ]; then
  read -p "Enter Firstname: " FNAME
  read -p "Enter Lastname: " LNAME
else
  echo "Inline variables detected"
fi
echo "Full Name: $FNAME $LNAME"

• Test
  • Performs testing logic
    • Logic is placed between [ and ]
  • Tons of options
    • -a and -e Does a file exist
    • -d Does a directory exist
    • -w Is the file writable
    • -x Is the file executable
  • Comparisons
    • -a And
    • -o Or
    • -nt Newer than
    • -ot Older than
    • -eq and = Equal
    • -ne and != Not equal
    • -gt / -ge Greater than (or equal)
    • -lt / -le Less than (or equal)

Shorthand Notation

dirname="/tmp/testdir"
[ -e $dirname ] && echo $dirname already exists || mkdir $dirname

• && represents "then"
• || represents "else"

Case

---

• Combines multiple if statements

case `date +%a` in
  "Sat")
    tar -czf ~/Backups/weekly.tgz ~/Documents
    ;;
  "Mon" | "Wed" | "Fri")
    tar -czf ~/Backups/daily.tgz ~/Documents
    ;;
  # No Backups any other day
  *)
    ;;
esac

For/Do Loops

---

• Performs an action repeatedly

for NAME in John Paul Ringo George ; do
  echo $NAME is my favorite Beatle
done

for FILE in `/bin/ls`
do
  echo $FILE
done

for FILE in ~/Backups/*
do
  tar -czf $FILE.tgz $FILE
done

While/Do and Until/Do

---

• While/Do executes while a statement is true
• Until/Do executes while a statement is false

N=0
while [ $N -lt 10 ] ; do
  echo -n $N
  let N=$N+1
done

N=0
until [ $N -eq 10 ] ; do
  echo -n $N
  let N=$N+1
done