When to use shell?
1. Shell is suitable for developing gadgets and packaging scripts
2. Act as a watering language, only calling other programs or doing very little data processing
3. If you need to use hash or nested array, it is recommended to use other programming languages.
4. For scenarios with higher performance requirements, it is recommended to use other languages.
Variable
Define variables
# Do not leave a space between a and 1, for example: it cannot be written as a = 1 a=1 echo $a
Quote variable
a=1
?
# Writing method one:
echo $a
?
# Writing method two (recommended):
echo ${a}
?
# The advantage of using ${} is that strings can be spliced
echo ${a}b # => output 1b
Quotation marks
# Try to use double quotes. Double quotes can output variables and escape, but single quotes are completely literal.
a=1
echo "${a}" # => Output 1
echo '${a}' # => output ${a}
Environment variables
# Environment variables are not required to be capitalized, but it is recommended to write them in capitals. ? export A=1 # or declare -x B=2 ? # Ordinary variables can also be exported as environment variables C=3 export C
Define constants
# Constants are generally written at the beginning of the file readonly CONST1 # or declare -r CONST2
Default global variable
# View the current directory $PWD ? # The directory where you were last located $OLDPWD ? #PATH environment variable $PATH
Special variables
# Script name $0 ? # Script parameters, $1 is the first parameter... $1 $2 $3 ? #Number of parameters $# ? # The result of the previous command, 0 means normal completion, non-0 means there is an exception $?
Basic syntax
Conditional judgment
For shell judgment, you can use [] or [[]], but it is recommended to use [[]]
if [[ $1 -eq "1" ]];then
echo "ok"
elif [[ $1 -eq "2" ]];then
echo "2333"
else
echo "no"
Judgment can be divided into number comparison, string comparison and file judgment.
Number comparison
Judgment of equality: =, ==, -eq (It is recommended to use == or -eq, the semantics are relatively clear)
The judgment is not equal: !=, -ne
Judgment is greater than: > (requires double brackets (())), -gt ([[]] i.e. Can)
Judging less than: < (double brackets are required), -lt ([[]] is enough)
nb=4 ? (( $nb > 4 )) & amp; & amp; echo "yes" || echo "no" # or [[ $nb -gt 4 ]] & amp; & amp; echo "yes" || echo "no"
Determine greater than or equal to: -ge
Determine less than or equal to: -le
String judgment
Judgment of equality: =, ==, -eq
The judgment is not equal: !=, -ne
Judgment is greater than: > (requires double brackets (())), -gt ([[]] i.e. Can)
Judging less than: < (double brackets are required), -lt ([[]] is enough)
Determine greater than or equal to: -ge
Determine less than or equal to: -le
Determine if the string is not null (the length is not 0): -n
Determine whether the string is null (the length is 0): -z
str="" # Undefined and zero-length strings are considered empty strings [[ -z $str ]] & amp; & amp; echo "yes" || echo "not" # Output yes [[ -n $str ]] & amp; & amp; echo "yes" || echo "not" # Output not
File Judgment
Determine whether it exists (no restriction on type): -e
Determine whether file exists: -f
Determine whether folder exists: -d
Logical operator
Negation: !
Or: ||
With: & amp; & amp;
a=1 [[ ! $a == 1 || 1=1 ]] & amp; & amp; echo "yes" || echo "no" # Output => yes [[ ! $a == 1 & amp; & amp; 1=1 ]] & amp; & amp; echo "yes" || echo "no" # Output => no
case statement
# | Equivalent to or,
# ;; Equivalent to break, must be present
# *) Equivalent to default
?
case $1 in
s|start)
echo "start"
;;
stop)
echo "stop"
;;
restart)
echo "restart"
;;
*)
echo "Usage:[start|stop|reload]"
exit 1
;;
esac
exit 0
Loop
for loop
# Writing method one
# Output all files in the current folder
for f in `ls`;do
echo $f
done
?
# Writing method two
# Output odd numbers from 1 to 100
for i in {1..100..2};do
echo $i
done
?
# Writing method three
# Calculate the addition from 1 to 100
sum=0
for(( i=1;i<100;i + + ));do
echo $(( sum + =i ))
done
while loop
nb=0
while [[ $nb -lt 10 ]];do
echo $(( nb + =1 ))
done
Function
Note: The return value of a function can only be a number
# Function definition
function foo() {
echo 'output'
}
?
# Function call
foo
?
# Function parameters
function sum(){
echo $1
echo $2
res=$1 + $2
if [[ $res==3 ]];then
return 0
else:
return -1
fi
}
?
# Use $? to receive the function return value. The return value of the function can only be a number!
sum 1 2
echo $? # output => 0
shell error handling
set -e: Exit when an error is reported
set -u: Detect undefined variables
set -x: debug execution, output the original content of the shell script, commonly used when writing Jenkins scripts
set -x echo "hello world" ? # Output: # + echo "hello world" # + followed by the source file content, the output format is determined by the PS4 environment variables # hello world
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