What is the meaning of || after test condition in Bash Script

Question

New to Bash scripting. I came across this line in the script file which I find difficult to understand

[ "$variable" ] || echo 'variable is unset or empty!' >&2

What is || used for?

Answer 1

It is the logical OR operator. echo is executed if, and only if, test condition returns a non-zero exit status.

See here for more details: http://bash.cyberciti.biz/guide/Logical_OR

Answer 2

粗略翻译成英文，它的意思是“如果失败，请执行此操作”。

Answer 3

基本上，如果第一个命令成功执行，第二个命令将永远不会执行。

1 == 1 || echo 'math is broke'

Answer 4

The form of the expression something || something else something || something else or something && something else is called a compound command . There can be 2 or more commands and you can use { ...; ...; } { ...; ...; } { ...; ...; } to collect multiple commands together after each || or && . For example, to check that the user provides 2 valid filenames on the command line for a script:

[ -f "$1" ] && [ -f "$2" ] || {
    printf "error: invalid input. Usage: %s <file1> <file2>\n" "${0//\//}"
    exit 1
}

Which simply requires that the test of the first argument [ -f "$1" ] ( && and) the test of the second argument [ -f "$2" ] are both files making each test evaluate to true . Otherwise ( || or) print the error message and exit.