Master Bash Conditionals and Loops: Practical Examples and Syntax Guide

Conditional statements and loops, collectively called flow control, form the most basic part of any programming language. Bash's flow control is very similar to that of familiar languages, so it is quick to pick up.

Conditional Statements

For Bash conditionals, start by reading the "Bash Test" documentation. The core of Bash conditionals is the Test command.

if

Example:

x=5;

if [ $x = 5 ]; then
    echo 'x equals 5.';
else
    echo 'x does not equal 5';
fi

# Output: x equals 5.

Abstracted syntax:

if commands; then
    commands
[elif commands; then
    commands...]
[else
    commands]
fi

A more complex example using file tests:

FILE=~/.zshrc # any path
if [ -e "$FILE" ]; then # -e unary operator
    if [ -f "$FILE" ]; then
        echo "$FILE is a regular file."
    fi
    if [ -d "$FILE" ]; then
        echo "$FILE is a directory."
    fi
    if [ -r "$FILE" ]; then
        echo "$FILE is readable."
    fi
    if [ -w "$FILE" ]; then
        echo "$FILE is writable."
    fi
    if [ -x "$FILE" ]; then
        echo "$FILE is executable/searchable."
    fi
else
    echo "$FILE does not exist"
fi

In Bash, the Test command is the core of conditional statements, used after if and elif.

case

The case statement is similar to the familiar switch construct but with different syntax.

case "$variable" in
    "$condition1" )
        command...
    ;;
    "$condition2" )
        command...
    ;;
esac

Variables are often quoted, but not required.

Each Test line must end with ).

Each condition block must end with ;;.

The entire case block ends with esac ("case" spelled backwards).

Example:

x=4

case $x in
    'a' )
        echo "x 是 a";;
    4 )
        echo "x 是 4";;
    'b' )
        echo "x 是 b"
esac

# x 是 4

In summary, Bash conditionals rely heavily on the Test command, and mastering the syntax of if and case is sufficient for most tasks.

Loops

Bash provides two common loop constructs: for and while, both of which are familiar to programmers.

for

Example: batch rename files.

Directory layout:

.
├── error_400.html
├── error_403.html
├── error_404.html
├── error_500.html
└── error_503.html

Bash code:

for $i in `ls`
do
    mv $i ${i/html/ejs};
 done

General syntax:

for variable [in words]; do
    commands
 done

do

can appear on a new line or on the same line as for (in which case for must end with ;).

The loop body must end with done. [in words] can be a wildcard, a command such as ls, or any list; it must be provided in array form.

Another simple example:

for i in *
 do
    echo $i;
 done

## prints all filenames in the current directory

while

Example:

count=1
while [ $count -le 5 ]; do
    echo $count
    count=$((count + 1))
 done
 echo "Finished."

# prints 1 - 5 and Finished.

Syntax:

while commands; do commands; done

Reference Articles

http://wiki.jikexueyuan.com/project/linux-command/chap28.html

http://tldp.org/LDP/abs/html/testbranch.html

http://wiki.jikexueyuan.com/project/linux-command/chap30.html