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preface

Control statements require logical expressions to branch. Bash shell logical expressions can be written in three ways: test expression, [expression], or [[expression]].

test

The format of the test command is test expression. Expression is our logical expression, which returns 0 if true or non-0 if false. Example:

#! /bin/bash
# 1 = = 1
test 1 -eq 1
echo "1 == 1: $?"
# 1 = = 2
test 1 -eq 2
echo "1 == 2: $?"
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Execution Result:

Note that the test command has a fixed way of writing value comparisons (it’s a command, after all), and falls into three main categories: numeric comparisons, string comparisons, and file type checks.

Numeric comparison

Numeric comparisons cannot be made with ==,! =, <, > these symbols compare, and have fixed arguments, as in the example above, 1==1, to be written as 1-eq 1, the following illustration:

Man test = man test = man test = man test

       INTEGER1 -eq INTEGER2

INTEGER1 is equal to INTEGER2

       INTEGER1 -ge INTEGER2

INTEGER1 is greater than or equal to INTEGER2

       INTEGER1 -gt INTEGER2

INTEGER1 is greater than INTEGER2

       INTEGER1 -le INTEGER2

INTEGER1 is less than or equal to INTEGER2

       INTEGER1 -lt INTEGER2

INTEGER1 is less than INTEGER2

       INTEGER1 -ne INTEGER2

INTEGER1 is not equal to INTEGER2

Example: test integer1-eq Integer2: Compares the value of integer1 to integer2. The same as other examples

String comparison

String comparisons can be made with = or! = these symbols, along with some non-null check options, are copied from the man test below, with some notes added:

-n STRING: The length of the STRING is greater than 0. The length of STRING is nonzero

-z STRING: The length of the STRING is 0. The length of STRING is zero

If the string is empty (it is not an empty string), both -n and -z are true

       STRING1 = STRING2

the strings are equal

STRING1 ! = STRING2 the strings are not equal

Example:

#! /bin/bash

str1="str1"
test -n $str1;echo $?
test -z $str2;echo $?
test -n $str2;echo $?
test x$str2 = x;echo $?
test $str1! ="";echo $?
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Execution result (both true) :

File comparison, type/attribute checking

Man test, man test, man test, man test

       FILE1 -ef FILE2

FILE1 and FILE2 have the same device and inode numbers

       FILE1 -nt FILE2

FILE1 is newer (modification date) than FILE2

       FILE1 -ot FILE2

FILE1 is older than FILE2

       -b FILE

FILE exists and is block special

       -c FILE

FILE exists and is character special

       -d FILE

FILE exists and is a directory

       -e FILE

FILE exists

       -f FILE

FILE exists and is a regular file

-g FILE

FILE exists and is set-group-ID

       -G FILE

FILE exists and is owned by the effective group ID

       -h FILE

FILE exists and is a symbolic link (same as -L)

       -k FILE

FILE exists and has its sticky bit set

       -L FILE

FILE exists and is a symbolic link (same as -h)

       -O FILE

FILE exists and is owned by the effective user ID

       -p FILE

FILE exists and is a named pipe

       -r FILE

FILE exists and read permission is granted

       -s FILE

FILE exists and has a size greater than zero

       -S FILE

FILE exists and is a socket

       -t FD  file descriptor FD is opened on a terminal

       -u FILE

FILE exists and its set-user-ID bit is set

       -w FILE

FILE exists and write permission is granted

       -x FILE

FILE exists and execute (or search) permission is granted

Example:

#! /bin/bash

# Whether the file exists
test -e demo.sh
echo "demo.sh exists: $?"
test -e demo1.sh
echo "demo1.sh exists: $?"
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Execution Result:

Logic and: -A, logic or: -o

Because the test is a command, so the logic and/or do not like other languages use && and | | and other symbols, these symbols in the shell but have special significance, such as the above example, use && connecting two commands, each one is successful, will perform the next article.

Example:

#! /bin/bash

#1 is 1 and 1 is 2
test 1 -eq 1 -a 1 -eq 2
echo "1 == 1 &&1 == 2: $?"
#1 is 1 or 1 is 2
test 1 -eq 1 -o 1 -eq 2
echo "1 = = 1 | | 1 = = 2: $?"
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Execution Result:

[command

Notice “[“, this is a command, usually written as [expression], notice the space before and after expression,” [” is not a symbol, it is a command, it is equivalent to the command of test, so its expression is used in the same way as test.

[,] : [,] : [,] : [,] : [,] : [,] : [,] : [,] : [,] : [

#! /bin/bash

#1 is 1 and 1 is 2
[ 1 -eq 1 -a 1 -eq 1 ] 
echo "1 == 1 &&1 == 2: $?"
#1 is 1 or 1 is 2
[ 1 -eq 1 -o 1 -eq 1 ] 
echo "1 = = 1 | | 1 = = 2: $?"
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Execution Result:

[[ expression ]]

Although “[[” than” [“, more than a “[“, but the meaning is not the same,” [” is a command, but “[[” is a keyword.

However, [[expression]] is much more powerful than [expression]. It is an enhanced version of [expression]. Such as [[expression]] can use && for logic and connector does not support – a, the same logic or | |, but does not support – o. In addition, [[expression]] is more compatible than [expression], and some other abilities are supported, such as using <,> to compare numbers, but because “[” is a command, it is not supported (need to escape).

So, using [[expression]] feels more like programming convention, as follows:

  1 #! /bin/bash2, 3,# Numerical calculation comparison,4 [[5 < $((3+3) &&4 >3]] 5echo "[[5 < 3+ 3&&4 >3]] : $?"
  6 [] : [] : [] : [] : [] : [] : [7 [5 < $((3+3) &&4 >3]Copy the code

Execution Result:

[5 -lt $((3+3)) -a 4 -gt 3]