script:
#! /bin/bash
var_1=${1:-0} # if not specified, assign 0 by default
var_2=${2:-0} # if not specified, assign 0 by default
#note: keyword "then" can't be ignored after "if" and "elif".
if [ $var_1 -gt $var_2 ] # if var_1 is greater than var_2
then
echo "$var_1 is max"
elif [ $var_1 -eq $var_2 ] # if var_1 is equal to var_2
then
echo "$var_1 = $var_2"
else
echo "$var_2 is max"
fi
7 terminal:
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script
0 = 0
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script 1 3
3 is max
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script 2 3
3 is max
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script 12 3
12 is max
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script 12
12 is max
======================================
exit status of structure "if-elif-else-fi":
script:
This is trying to output the exit status of "if-elif-else-fi"
#! /bin/bash
var1=${1:-0}
var2=${2:-0}
if [ $var1 -lt $var2 ]
then
echo "var2 is max"
elif [ $var1 -gt $var2 ]
then
: #do nothing
else
Hello
fi
echo $?
terminal:
structure "if-elif-else-fi"'s exit status is the status of last command got executed inside the structure.
1) First Command, inside the "if-elif-else" structure, the last command got executed is "Hello", which is invalid, the and exit status of entire "if-elif-else" is the exit status of "Hello".
2) Second command, inside the "if-elif-else" structure, the last command got executed is "echo", whose exit status is 0. So the entire "if-elif-else"structure's exit status is also 0.
3) Third command, inside the "if-elif-else" structure, the last command got executed is the comparison operation after "elif", and then the exit status is 0, since the comparison succeeded.
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script 2 2
./script: line 12: Hello: command not found
127
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script 1 2
var2 is max
0
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script 2 1
0
2. Logical Operations
world:
#! /bin/bash
echo "Hello world!"
script:
#! /bin/bash
if ./world
then
echo $?
echo "world script can be executed"
else
echo $?
echo "world script can't be executed"
fi
terminal:
The result of "./world" after keyword "if" does depend on the exit status of "./world". If the exit status is 0, it means success and then goes to the first block, outputting "world script can be executed"
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ chmod +x ./world
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script
Hello world!
0
world script can be executed
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ chmod -x ./world
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script
./script: line 3: ./world: Permission denied
126
world script can't be executed
=======================================
text:
Hello world!
Hello Hello my great world!
script:
#! /bin/bash
pattern=${1:-}
echo $pattern
if grep $pattern ./text >/dev/null
then
echo $?
echo "Found $pattern"
else
echo $?
echo "Failed to find $pattern"
fi
terminal:
The result after keyword "if" does depend on the exit status of "grep". If exit status is 0, it means success and then goes to the first block outputting the "Found $pattern".
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script Hello
Hello
0
Found Hello
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script amazing
amazing
1
Failed to find amazing
=======================================
Logical NOT:
script:
#! /bin/bash
var1=${1:-0}
var2=${2:-0}
if [ ! $var1 -le 0 ]
then
echo "$var1 is positive"
else
echo "$var1 is negative or 0"
fi
terminal:
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script -10
-10 is negative or 0
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script 10
10 is positive
Logical And:
script:
#! /bin/bash
var1=${1:-0}
#note: [] is another expression of "test command", we can't use expression like:
# [ ! $var -lt 0 && ! $var1 -gt 0 ], this is not ok to be converted to "test command"
# expression
#note: we have to put in a space between "!" and "$var1", otherwise ! is working on $var1
#with a space, ! is working on expression "$var1 -lt 0"
if [ ! $var1 -lt 0 ] && [ ! $var1 -gt 0 ]
then
echo "$var1 is 0"
else
echo "$var1 is non-zero"
fi
terminal:
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script 1
1 is non-zero
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script 0
0 is 0
Logical OR:
script:
#! /bin/bash
var1=${1:-0}
#note: with expression like:
#if $var1 - lt 0 || $var1 -gt 0, $var1 will be taken as one command, normally an invalid
#command, again, we need to convert expression to a test command format: [ $var1 -lt 0 ],
#we need to make sure that statements after "if" are "commands"
if [ $var1 -lt 0 ] || [ $var1 -gt 0 ]
then
echo "$var1 is non-zero"
else
echo "$var1 is zero"
fi
terminal:
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script 0
0 is zero
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script 2
2 is non-zero
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script -2
-2 is non-zero
3. Short Circuit of Logical Operations
text:
Hello world!
Hello Hello my great world!
script_1:
#! /bin/bash
pattern=${1:-}
grep $pattern ./text > /dev/null &&
{
cat ./text
echo $pattern found in text
}
script_2:
#! /bin/bash
pattern=${1:-}
if grep $pattern ./text >/dev/null
then
cat ./text
echo $pattern found in text
fi
terminal:
for script_1, only when grep get the text successfully, the next part is then executed. So actually, script_1 is equal to script_2, but script_2 is more clear.
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script_1 Hello
Hello world!
Hello Hello my great world!
Hello found in text
aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script_2 Hello
Hello world!
Hello Hello my great world!
Hello found in text
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