bash script conditions

https://dev.to/zakiarsyad/bash-script-conditions-1ano

Basic usage of if statement in bash scripting

if [ CONDITION ]; then FIRST_COMMANDS
elif [ CONDITION ]; then SECOND_COMMANDS
else THRID_COMMANDS
fi

Always use space between the brackets in the condition.
Always terminate the line with ; before adding a new keyword

if [ CONDITION ]; then
Use ! to invert the condition

if [ ! CONDITION ]; then
Combine conditions using -a for AND, and -o for OR

There are three syntaxes for define the condition

Single-Bracket

Support 3 types of conditions

File-based condition

[ -a file ] or [ -e file ] : if the file is exists
[ -d directory ] : if the directory is exists and it's a directory
[ -r file ] : if the file is exists and is readable to the script
[ -w file ] : if the file is exists and is writeable to the script
[ -x file ] : if the file is exists and is executable to the script
[ -s file ] : if the file is exists and has a size of more than 0 bytes
[ file1 -ef file2] : if the files refer to the same device/inode number
[ file1 -nt file2] : if the file1 was changed more recently than file2, or if file1 exists and file2 doesn't
[ file1 -nt file2] : if the file1 was changed longer ago than file2, or if file1 exists and exists
[ -N file ] : if the file is exists and was modified after the last read
[ -g file ] : if the file is exists and is a set-group-ID
[ -u file ] : if the file is exists and is a set-user-ID
[ -G file ] : if the file is exists and is owned by the effective group ID
[ -O file ] : if the file is exists and is owned by the user executing the script
[ -b file ] : if the file is exists and is a block special
[ -c file ] : if the file is exists and is a character special
[ -f file ] : if the file is exists and is a regular file
[ -h file ] or [ -L file ] : if the file is exists and is a symbolic link
[ -k file ] : if the file is exists and has its sticky bit set
[ -p file ] : if the file is exists and is a named pipe
[ -S file ] : if the file is exists and is a socket
[ -t file ] : if the file descriptor is exists and refers to an open terminal

String-based condition

[ STRING1 == STRING2 ] : if equal
[ STRING1 != STRING2 ] : if not equal
[ -n STRING ] : if has length of more than zero
[ -z STRING ] : if an empty string
[ STRING =~ PATTERN ] : if match the regex pattern

Arithmetic condition

[ NUM1 -eq NUM2 ] : if equal
[ NUM1 -ne NUM2 ] : if not equal
[ NUM1 -gt NUM2 ] : if greater than
[ NUM1 -ge NUM2 ] : if greater than equal
[ NUM1 -lt NUM2 ] : if less than
[ NUM1 -le NUM2 ] : if less than equal

Double-bracket

This is the enhanced version of single-bracket. All features in single-bracket syntax are compatible here. We can start using this syntax.

The differences :

Can use regex

[[ STRING =~ PATTERN ]] : if match the regex pattern
Can use && and || instead of -a and -o

Double-parenthesis

This is another syntax for arithmetic condition

(( NUM1 == NUM2 )) : if equal
(( NUM1 != NUM2 )) : if not equal
(( NUM1 > NUM2 )) : if grater than
(( NUM1 >= NUM2 )) : if grater than equal
(( NUM1 < NUM2 )) : if less than
(( NUM1 <= NUM2 )) : if less than equal

We also can quickly test a condition like this

# using &&
[ $foo == "bar" ] && echo true

# using ||
[ $foo == "bar" ] || echo true

tags: #terminal

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