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

• Linux
• ln

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).

-s

True if file exists and has a size greater than zero.

For a zero-length test, use:

wc -c

This also works:

if [[ -f ${file} ]] && [[ ! -s ${file} ]]; then
  echo "empty"
fi

-f

True if file exists and is a regular file.

-r

True if file exists and is readable.

-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 allocated, 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.

In case of indexed or associative arrays, declarations with "declare -a" or "declare -A" are not detected by -v. Only if an array was actually allocated -v returns true. For more details, see Indexed Array Declaration Test, Indexed Array Allocation Test, Associative Array Declaration Test and Associative Array Allocation Test.

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

-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.