/if

Snippets for if-else sentence

Primary LanguageShell

Snippets for if-sentence

  1. if-else
  2. Evaluate $? with if statement, using if [[ ]] and if (( ))
  3. Evaluate using if [ ]
  4. Using if (( )) and elif statement
  5. Comparing strings inside if statement
  6. Using only [ ]
  7. Using if and test (then you can omit [ ])
  8. Use a command as sentence of if
  9. Use double square bracket [[ ]] to test strings
  10. Use test inside and if statement
  11. Equivalence using if-else statement and command && command || command
  12. Multiple conditions on if statement

1. if-else

if echo "hello" && echo "World"; then echo "OK"; fi
hello
World
OK

2. Evaluate $? with if statement, using if [[ ]] and if (( ))

❯ echo "Hello World"
Hello World
❯ if [[ $? ]]; then echo "successful execution $?"; else echo "execution failed"; fi
successful execution 0
❯ echo "Hello World"
Hello World
❯ if (( $? )); then echo "execution failed $?"; else echo "successfull execution, but \$? = $?"; fi
successfull execution, but $? = 1

3. Evaluate using if [ ]

if [ $# -lt 3 ]; then
  printf "%b" "Error. Not enough arguments.\n"
  printf "%b" "usage: myscript file1 op file2\n"
  # exit 1 # uncomment on a script
fi

Alternatively

if (( $# < 3 )); then
  printf "%b" "Error. Not enough arguments.\n"
  printf "%b" "usage: myscript file1 op file2\n"
  # exit 1 # uncomment on a script
fi

4. Using if (( )) and elif statement

if (( $# < 3 )); then
  printf "%b" "Error. Not enough arguments.\n"
  printf "%b" "usage: myscript file1 op file2\n"
  # exit 1 # uncomment on a script
elif (( $# > 3 )); then
  printf "%b" "Error. Too many arguments.\n"
  printf "%b" "usage: myscript file1 op file2\n"
  # exit 2 # uncomment on a script
else
  printf "%b" "Argument count correct.  Proceeding...\n"
fi

5. Comparing strings inside if statement

choice="yes"
if [ "$choice" = "yes" ]; then
  echo "Your choice is yes"
fi

Output:

Your choice is yes

6. Using only [ ]

result=1
[ $result = 1 ] && { echo "Result is 1" }
Result is 1

7. Using if and test (then you can omit [ ])

if test $# -lt 3; then
  echo try again.
fi

Output

try again

8. Use a command as sentence of if

mkdir directory
if [ $? = 0 ]; then
  echo "directory was created successfully"
else 
  echo "directory was already created"
fi

move the command into the if statement

if mkdir directory; then
  echo "directory was created successfully"
else 
  echo "directory was already created"
fi

9. Use double square bracket [[ ]] to test strings

if [[ "${var}" == "some_string" ]]; then
  echo "do something"
fi
# -z (string length is zero) and -n (string length is not zero) are
# preferred over testing for an empty string
if [[ -z "${my_var}" ]]; then
  echo "do something"
fi
# This is OK (ensure quotes on the empty side), but not preferred:
if [[ "${my_var}" == "" ]]; then
  echo "do something"
fi

10. Use test inside and if statement

touch ~/file.txt
if test -f ~/file.txt; then
  echo "file exist"
else 
  echo "file does not exist"
fi

console output

file exist"

11. Equivalence using if-else statement and command && command || command

if cp file.txt $HOME/iOS-Projects; then
  echo "Done"
else 
  echo "failed"
  exit 1
fi

Equivalence:

cp file.txt $HOME/iOS-Projects && { echo "Done" } || { echo "Failed"; exit 1 }

In this case we should use the exit command in order to point out that the command was not successfully executed"

12. Multiple conditions on if statement

#!/bin/bash

var_one=1

# Only one var might be set or none of them
var_two=
var_three=1

# Only one flag might be set or none of them
flag_one=
flag_two=
flag_three=

# draw table of thruth

if [[ "${var_one}" ]] \
  && { [[ "${var_two}" ]] || [[ "${var_three}" ]]; } \
  && { [[ -z "${flag_one}" ]] && [[ -z "${flag_two}" ]] && [[ -z "${flag_three}" ]]; }; then
  echo "flag_one=$flag_one"
  echo "flag_two=$flag_two"
  echo "flag_three=$flag_three"
  echo "all flags are length zero"
  exit 1
fi
./if-statement.sh

Console output:

flag_one=
flag_two=
flag_three=
all flags are length zero