Bash Test

for portable sh(1), see here.

Contents

Bash Test

Usage

There are three interfaces to shell tests:

test(1)
the alias [ (which requires a ] following all operands, strings, and expressions)
the grammar [[ TEST ]]

The first two are equivalent, POSIX compliant, and portable. test(1) sets the exit code to 0 if the test is true, and 1 if false.

The third is an enhancement of ksh(1) that was included in multiple downstream shells, including bash(1). Word splitting and globbing are disabled within this grammar, preempting many subtle bugs.

Shell tests are frequently used with conditional logic.

if [[ TEST ]]; then
  :
fi

Variable Tests

Usage	True if...
`var1 = var2`	var1 is equivalent to var2
`var1 != var2`	var1 is not equivalent to var2
`-z var`	string length of var is 0
`-n var`	string length of var is not 0
`-v var`	var is set
`var1 -eq var2`	var1 is numerically equal to var2
`var1 -ne var2`	var1 is numerically inequal to var2
`var1 -gt var2`	var1 is numerically greater than var2
`var1 -ge var2`	var1 is numerically greater than/ equal to to var2
`var1 -lt var2`	var1 is numerically lesser than var2
`var1 -le var2`	var1 is numerically lesser than/ equal to to var2

Patterns

To match a variable against a pattern, try:

if [[ $var = a* ]]; then
  echo "var starts with an 'a'"
fi

To match a variable against a regular expression, try:

if [[ $var =~ ^[^a] ]]; then
  echo "var does not start with an 'a'"
fi

File Tests

These are probably what you are looking for.

Usage	True if...
`-d fn`	directory exists
`-e fn`	file exists

These rarely are useful.

Usage	True if...
`-b fn`	file exists and is a block special file
`-c fn`	file exists and is a character special file
`-f fn`	file exists and is a regular file
`-g fn`	file exists and its set-group-id bit is set
`-h fn`	symbolic link exists (same as `-L`)
`-p fn`	named pipe (FIFO) exists
`-r fn`	file exists and is readable
`-s fn`	file exists and its size is greater than zero
`-t fn`	file descriptor is open and refers to a terminal
`-u fn`	file exists and its set-user-id bit is set
`-w fn`	file exists and is writable
`-x fn`	file exists and is executable
`-L fn`	symbolic link exists (same as `-h`)
`-S fn`	socket exists
`fn1 -ef fn2`	fn1 has same device and inode numbers as fn2
`fn1 -nt fn2`	fn1 is newer (modification date) than fn2
`fn1 -ot fn2`	fn1 is older (modification date) than fn2

Compound Logic

bash(1) greatly simplifies compound logic constructs as compared to sh(1).

To negate a test, insert ! before the expression. Note that negation is applied to the last test only.

if [[ ! TEST ]]; then
  :
fi

The AND and OR operators can be used within or between [[ calls.

if [[ TEST && TEST ]] && [[ TEST ]]; then
  :
fi

if [[ TEST || TEST ]] || [[ TEST ]]; then
  :
fi

Use parentheses for ordering operations.

if [[ TEST && ( TEST || TEST ) ]]; then
  :
fi

Note that [[ is capable of short circuiting.

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