Number is not greater than 10<\/code><\/pre>\n\n\n\nExplanation:<\/p>\n\n\n\n
Since the condition 5 -gt 10<\/code> is false, the code block inside the else<\/code> branch is executed.<\/p>\n\n\n\n<\/span>The if-elif-else<\/code> Statement<\/span><\/h2>\n\n\n\nThe if-elif-else<\/code> statement allows you to test multiple conditions in sequence and execute different code blocks based on the first true condition<\/strong> encountered.<\/p>\n\n\n\nSyntax:<\/p>\n\n\n\n
if [[ condition1 ]]; then\n # Code to execute if condition1 is true\nelif [[ condition2 ]]; then\n # Code to execute if condition2 is true\nelse\n # Code to execute if no conditions are true\nfi<\/code><\/pre>\n\n\n\nExample:<\/p>\n\n\n\n
if [[ 18 -lt 18 ]]; then\n echo \"You are a minor\"\nelif [[ 18 -eq 18 ]]; then\n echo \"You just turned 18\"\nelse\n echo \"You are an adult\"\nfi<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
You just turned 18<\/code><\/pre>\n\n\n\nIn this case, the first condition (18 -lt 18<\/code>) is false, but the second condition (18 -eq 18<\/code>) is true, so the code block corresponding to the elif<\/code> branch is executed.<\/p>\n\n\n\n-gt<\/code>: greater than
-ge<\/code>: greater than or equal to
-lt<\/code>: less than
-le<\/code>: less than or equal to<\/p>\n\n\n\n<\/span>The case<\/code> Statement<\/span><\/h2>\n\n\n\nThe case<\/code> statement provides a way to test multiple conditions against a single value. It is particularly useful when you want to compare a variable against multiple possible values. Here’s the basic structure:<\/p>\n\n\n\ncase \"$variable\" in\n \"value1\")\n # Code1 to execute for value1\n ;;\n \"value2\")\n # Code2 to execute for value2\n ;;\n *)\n # Code3 to execute for all other cases\n ;;\nesac<\/code><\/pre>\n\n\n\nLet’s consider an example:<\/p>\n\n\n\n
fruit=\"apple\"\ncase $fruit in\n \"apple\")\n echo \"It's an apple\";;\n \"banana\")\n echo \"It's a banana\";;\n *)\n echo \"Unknown fruit\";;\nesac<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
It's an apple<\/code><\/pre>\n\n\n\nIn this case, since the value of the fruit<\/code> variable is “apple”, the corresponding code block is executed.<\/p>\n\n\n\nNote<\/strong>: You need to use *<\/code> not \"*\"<\/code> since *<\/code> is not a string here.<\/p>\n\n\n\n<\/span>Numeric Comparison<\/span><\/h2>\n\n\n\nYou can use numeric comparisons to check relationships between numbers. For example:<\/p>\n\n\n\n
if [[ 15 -lt 20 ]]; then\n echo \"15 is less than 20\"\nfi<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
15 is less than 20<\/code><\/pre>\n\n\n\nIn this example, the condition 15 -lt 20<\/code> is true, so the code block is executed.<\/p>\n\n\n\n<\/span>The [<\/code> Command (aka test<\/code>)<\/span><\/h2>\n\n\n\nThe [<\/code> command and test<\/code> are basically the same. For example, test -f <file_name><\/code> and [ -f <file_name>]<\/code> are the same.<\/p>\n\n\n\nThe [<\/code> command is a program that evaluates conditional expressions (-n<\/code>, -z<\/code>, -r<\/code>, etc. given below). It is commonly used in conditional statements. For examples see the headings given below.<\/p>\n\n\n\nHowever, please note that for more advanced conditional tests, it would be more beneficial to use [[]]<\/code> and (())<\/code>. In my opinion, stick with double brackets because they work most of the time<\/strong>.<\/p>\n\n\n\n<\/span>-f<\/code> and -d<\/code> for ‘file’ or ‘directory’ checks <\/span><\/h2>\n\n\n\nYou can use the -f<\/code> and -d<\/code> flags with the [<\/code> command to check if a file exists (-f<\/code>) or if a directory exists (-d<\/code>). For instance:<\/p>\n\n\n\n[ajay@legion bash]$ ls -1\nbash_articles.md\nbash_brace.md\nbash_conditionals.md\nbash_loops_article.md\nbash_loops.md\nbash_parameter_substitution.md\ntemp.md\ntestdir<\/code><\/pre>\n\n\n\nif [ -f \"bash_brace.md\" ]; then\n echo \"File exists: bash_brace.md\"\nfi\n\nif [ -d \"testdir\" ]; then\n echo \"Directory exists: testdir\"\nfi<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
File exists: bash_brace.md\nDirectory exists: testdir<\/code><\/pre>\n\n\n\nIn the first example, the -f<\/code> flag checks if the file “bash_brace.md” exists, while in the second example, the -d<\/code> flag checks for the existence of the directory “testdir”.<\/p>\n\n\n\n<\/span>String Comparison<\/span><\/h2>\n\n\n\nString comparisons allow you to compare two strings for equality or inequality. For example:<\/p>\n\n\n\n
if [ \"smart\" = \"tech101\" ]; then\n echo \"Strings are equal\"\nelse\n echo \"Strings are not equal\"\nfi<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
Strings are not equal<\/code><\/pre>\n\n\n\nIn this case, the strings “smart” and “tech101” are not equal, so the code block in the else<\/code> branch is executed.<\/p>\n\n\n\n<\/span>Check if a File Exists and Is Readable<\/span><\/h2>\n\n\n\nYou can use the -r<\/code> flag with the [[<\/code> command to check if a file exists and is readable:<\/p>\n\n\n\n[ajay@legion bash]$ ls -1\nbash_articles.md\nbash_brace.md\nbash_conditionals.md\nbash_loops_article.md\nbash_loops.md\nbash_parameter_substitution.md\ntemp.md\ntestdir<\/code><\/pre>\n\n\n\nif [[ -r \"bash_brace.md\" ]]; then\n echo \"File exists and is readable\"\nfi<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
File exists and is readable<\/code><\/pre>\n\n\n\nThe -r<\/code> flag checks if the file “bash_brace.md” exists and has read permission.<\/p>\n\n\n\n<\/span>Check if a Directory Exists and Is Writable<\/span><\/h2>\n\n\n\nYou can combine the -d<\/code> and -w<\/code> flags to check if a directory exists and is writable:<\/p>\n\n\n\n$ ls -al \ntotal 96\ndrwxr-xr-x 3 ajay ajay 4096 Aug 8 14:05 .\ndrwxr-xr-x 28 ajay ajay 16384 Aug 9 09:24 ..\n-rw-r--r-- 1 ajay ajay 11024 Aug 6 15:31 bash_articles.md\n-rw-r--r-- 1 ajay ajay 7087 Aug 3 14:19 bash_brace.md\ndrwxr-xr-x 2 ajay ajay 4096 Aug 8 14:05 testdir<\/code><\/pre>\n\n\n\nif [[ -d \"testdir\" && -w \"testdir\" ]]; then\n echo \"Directory exists and is writable\"\nfi<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
Directory exists and is writable<\/code><\/pre>\n\n\n\nIn this example, the script checks if both the directory “testdir” exists (-d<\/code> flag) and has write permission (-w<\/code> flag).<\/p>\n\n\n\n<\/span>Check if a String Is Empty or non-empty<\/span><\/h2>\n\n\n\nUse -z<\/code> and -n<\/code>. Both are exactly opposite of each other.<\/p>\n\n\n\nJust remember the following to avoid the confusion:
-z<\/code> means “null”
-n<\/code> means “not null”<\/p>\n\n\n\nfoo=\"bar\"\nif [[ -n \"$foo\" ]]; then\n echo \"foo is not null\"\nfi<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
foo is not null<\/code><\/pre>\n\n\n\nfoo=\"bar\"\nif [[ -z \"$foo\" ]]; then\n echo \"foo is null\"\nfi<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
foo=\"\"\nif [[ -n \"$foo\" ]]; then\n echo \"foo is not null\"\nfi<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
foo=\"\"\nif [[ -z \"$foo\" ]]; then\n echo \"foo is null\"\nfi<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
foo is null<\/code><\/pre>\n\n\n\n<\/span>Compare Strings for Equality<\/span><\/h2>\n\n\n\nYou can compare two strings for equality using the ==<\/code> operator:<\/p>\n\n\n\nstr1=\"smarttech101\"\nstr2=\"smarttech101\"\nif [[ \"$str1\" == \"$str2\" ]]; then\n echo \"Strings are equal\"\nfi<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
Strings are equal<\/code><\/pre>\n\n\n\nIn this example, the string variables “str1” and “str2” give equal strings, so the code block is executed.<\/p>\n\n\n\n
For variable expansion in bash, look over here<\/a><\/p>\n\n\n\n<\/span>Check if a Variable Is Numeric<\/span><\/h2>\n\n\n\n