linux : bash script conditions

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

1. 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

2. 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

3. 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

Comments

[Ben Sinclair]

You list [ STRING =~ PATTERN ] in your single-bracket conditions, but then go on to say it's different in double-bracket conditions. What did you mean?

I think the main benefit of using single-brackets is that those conditions will likely work in other shells, even if they're not as powerful.

[Zaki Arrozi Arsyad]

Nice catch man, thanks.
Updated. That regex should work with double brackets [[ STRING =~ PATTERN ]]