Ji Geng 43 / 90

Random thoughts on programming

Bash conditional

if sentence

if commands; then
commands
[elif commands; then  commands...]
[else  commands]
fi

if The judging condition of structure , In general use test command , There are three forms .

#  Writing a 
test expression
#  Write two 
[ expression ]
#  Write three 
[[ expression ]]
// The above three forms are equivalent , But the third form also supports regular judgment , The first two don't support .
// Be careful , The second and third way of writing ,[ and ] There must be a space between and the inner expression .

File to judge

[ -a file ]： If  file  There is , Then for true.
[ -d file ]： If  file  Exists and is a directory , Then for true.
[ -e file ]： If  file  There is , Then for true.
[ -w file ]： If  file  Being and writable （ The current user has writable permissions ）, Then for true.
[ -x file ]： If  file  Exists and is executable （ Effective users have execution ／ Search rights ）, Then for true.

String judgment

[ string ]： If string Not empty （ Longer than 0）, Then the judgment is true .
[ -n string ]： If the string string The length of is greater than zero , Then the judgment is true .
[ -z string ]： If the string string The length of is zero , Then the judgment is true 
[ string1 = string2 ]： If string1 and string2 identical , Then the judgment is true .
[ string1 == string2 ]  Equate to [ string1 = string2 ].
[ string1 != string2 ]： If string1 and string2 inequality , Then the judgment is true .
[ string1 '>' string2 ]： If you follow the dictionary order string1 Arrange in string2 after , Then the judgment is true .
[ string1 '<' string2 ]： If you follow the dictionary order string1 Arrange in string2 Before , Then the judgment is true .
// Be careful ,test Inside the command > and <, Must be enclosed in quotation marks （ Or it's a backslash ）. otherwise , They will be  shell  Interpreted as a redirection operator .

Integer judgment

[ integer1 -eq integer2 ]： If integer1 be equal to integer2, Then for true.
[ integer1 -ne integer2 ]： If integer1 It's not equal to integer2, Then for true.
[ integer1 -le integer2 ]： If integer1 Less than or equal to integer2, Then for true.
[ integer1 -lt integer2 ]： If integer1 Less than integer2, Then for true.
[ integer1 -ge integer2 ]： If integer1 Greater than or equal to integer2, Then for true.
[ integer1 -gt integer2 ]： If integer1 Greater than integer2, Then for true.

Regular judgment

[[ string1 =~ regex ]] //regex It's a regular expression ,=~ It's a regular comparison operator .

Logical operations

AND operation ： Symbol &&, You can also use parameters -a.
OR operation ： Symbol ||, You can also use parameters -o.
NOT operation ： Symbol !.

Arithmetic judgment

((...)) As an arithmetic condition , The judgment of arithmetic operations .
if ((3 > 2)); then
  echo "true"
fi
// There is no need to use test command , But directly ((...)) structure . The return value of this structure , Determines the authenticity of the judgment .
// If the arithmetic result is nonzero , It means that the judgment is true . This is the opposite of the return value of the command , Need to be careful .

case structure

case expression in
  pattern )
    commands ;;
  pattern )
    commands ;;
  ...
esac