conditional

Conditional in Bash

There are 2 types of comparison in bash: integer and string.

Comparative Table of Comparisons

Comparison	Integer	String (using [ ] construct)	String (using [[ ]] construct)
equal	if [ ${a} -eq ${b} ]	if [ "${a}" = "${b}" ]	if [[ "${a}" = "${b}" ]]
		if [ "${a}" == "${b}" ]	if [[ "${a}" == "${b}" ]]
not equal	if [ ${a} -ne ${b} ]	if [ "${a}" != "${b}" ]	if [[ "${a}" != "${b}" ]]
greater than	if [ ${a} -gt ${b} ]	if [ "${a}" \< "${b}" ]	if [[ "${a}" < "${b}" ]]
greater than or equal to	if [ ${a} -ge ${b} ]	if [ "${a}" \> "${b}" ]	if [[ "${a}" > "${b}" ]]
less than	if [ ${a} -lt ${b} ]
less than or equal to	if [ ${a} -le ${b} ]
is empty (string)		if [ -z "${a}" ]	if [[ -z "${a}" ]]
is not empty (string)		if [ -n "${a}" ]	if [[ -n "${a}" ]]

Compound Conditionals

Comparison	Operator
or	&&
and	`

We will use an actual example to illustrate compound conditionals. Here's a list of all versions of MacOS, let's say that we are creating a script that supports MacOS Yosemite and above. This means we need to do a check on the versions from 10.10 through 12.x. We can construct our conditionals the following way.

# Check on the version.
if [[ "${major_ver}" -eq 10 ]] && [[ "${minor_ver}" -ge 10 ]] || [[ "${major_ver}" -ge 11 ]]

# We can also group all the expressions within the `[[ ]]` block.
if [[ "${major_ver}" -eq 10 && "${minor_ver}" -ge 10 || "${major_ver}" -ge 11 ]]

Notes

1. Be extra careful with spacing; if [ "${a}" = "${b}" ] is not the same as if [ "${a}"="${b}" ]. The latter is an assignment operation.

Source Code

• Example

Reference

• TLDP Advanced Bash-Scripting Guide: Other Comparison Operators