Bash test
External
- http://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html
- http://tldp.org/LDP/abs/html/testconstructs.html
Internal
Overview
'test' is a bash built-in command which tests the expression following it, according to the syntax described below. 'test' and [...] are synonymous and can be used interchangeably.
Negation
[ ! -f ${file} ] && echo "we're not a file" || echo "we're a file"
-e
True if file exists (regardless of type).
-f
True if file exists and is a regular file.
-w
True if file exists and is writable.
-h
Tests whether the file exists and it is a symbolic link.
[ -h $0 ] && echo "we're a link" || echo "we're not a link"
-v
True if the variable specified after -v has been declared, even if it has an empty value (blank string):
For the following cases -v some_var evaluates to true:
some_var=
some_var=""
some_var="something"
If the variable is not mentioned at all -v some_var evaluates to false.
Integer Comparison
[arg1 -eq|-ne|-lt|-le|-gt|-ge arg2]
String Comparison
< and > can be used for string comparison, in ASCII alphabetical order, but the extended test command [[ ... ]]:
if [[ "${a}" > "${b}" ]]; them
...
fi
File Time Comparison
-nt
if [[ ${file1} -nt ${file2} ]]; then
...
fi
-ot
-z, -n
-z: true if the length of the string is zero.
-n: true if the length of string is non-zero.
Can be used to check for undeclared variables
[ -z ${var} ] && echo" 'var' not declared"
- Quotation marks around the variables are needed, otherwise the expression will fail syntactically when the variable is declared and it contains a space:
var="a b" ... line 177: [: 900: binary operator expected
Combining test and Command Conditionals
Not sure if this belongs here, refactor with http://www.gnu.org/software/bash/manual/html_node/Conditional-Constructs.html#Conditional-Constructs
verbose=false
if ! ${verbose} && [ "$1" = "--verbose" -o "$1" = "-v" ]; then
echo "setting verbose to ON"
fi
Must have a space between ! and the command that evaluates to true or false.