Condition

If Statement#

If the exit code of command is 0, then execute the content of then.

if command
then
    ...
fi

Another form.

if command; then
    ...
fi

Else#

if command; then
    ...
else
    ...
fi

Elif#

if command; then
    ...
elif command; then
    ...
fi

Check User#

Check if the soda user exists.

#!/usr/bin/env bash

if grep soda /etc/passwd
then
 echo "soda exists"
fi

soda:x:1001:1001:,,,:/home/soda:/bin/bash
soda exists

grep has an exit code of 0 when there is data, and 1 when there is no data.

Test Command#

Test conditions, if true, the return code is 0, otherwise the return code is 1.

test - check file types and compare values

Basic syntax.

test EXPRESSION

Short syntax.

[ EXPRESSION ]

After executing on the command line, you can use echo $? to see the return code.

Check File#

• [ -e file ]：Check if the file exists.
• [ -d file ]：Check if the file exists and is a directory.
• [ -f file ]：Check if the file exists and is a file.
• [ -s file ]：Check if the file exists and is not empty.
• [ -r file ]：Check if the file exists and is readable.
• [ -w file ]：Check if the file exists and is writable.
• [ -x file ]：Check if the file exists and is executable.
• [ -O file ]：Check if the file exists and belongs to the current user.
• [ -G file ]：Check if the file exists and belongs to the user group.
• [ a -nt b ]：Check if file a is newer than file b.
• [ a -ot b ]：Check if file a is older than file b.

If file or $file variable contains spaces, use double quotes.

[ -e "file" ]

[ -e "$file" ]

Check String#

• [ -z str ]：Check if the string is empty (length is 0).
• [ -n str ]：Check if the string is not empty (length is not 0).
• [ s1 = s2 ]：Check if the strings are equal.
• [ s1 != s2 ]：Check if the strings are not equal.

Check Number#

• [ a -eq b ]：Check if the two numbers are equal.
• [ a -ne b ]：Check if the two numbers are not equal.
• [ a -gt b ]：Check if a is greater than b.
• [ a -ge b ]：Check if a is greater than or equal to b.
• [ a -lt b ]：Check if a is less than b.
• [ a -le b ]：Check if a is less than or equal to b.

Compound Conditions#

Consistent with traditional programming languages.

[ cond1 ] && [ cond2 ]

[ cond1 ] || [ cond2 ]

Double Parentheses#

Double parentheses can be used for advanced mathematical expressions without escaping.

if (( 2**10 > 1000 ))
then
    ...
fi

Double Square Brackets#

Provide advanced matching patterns for strings.

if [[ $BASH_VERSION == 5.* ]]
then
    ...
fi

Case Statement#

#!/usr/bin/env bash

# The first parameter of the script
case "$1" in
  start)
    echo "Starting the service..."
    # Add the command to start the service here
    ;;
  stop)
    echo "Stopping the service..."
    # Add the command to stop the service here
    ;;
  *)
    echo "Usage: $0 {start|stop}"
    exit 1
    ;;
esac

• $0：The name of the script.
• $1：The first parameter of the script
• )：Branch condition end tag.
• ;;：Branch command end tag.
• *)：Default branch, executed when no branch matches.