BASH cheat sheet-Level 3 - Control Structure

Q: How do you use an if/elif/else conditional in Bash?

Use if [[ condition1 ]]; then ... elif [[ condition2 ]]; then ... else ... fi. The corresponding command block runs based on which condition is true.

Q: What are common Bash file test operators like -f, -d, -z, and -n?

-f checks that a file exists, -d checks that a directory exists, -z checks whether a string is empty, and -n checks whether a string is non-empty.

Q: How do you iterate using for loops and while loops in this cheat sheet?

The for loop examples show iterating over lists and ranges, and looping over associative array indices. The while loop runs until the condition becomes false, using a numeric comparison inside (( ... )).

Oct 13, 2026

Upload by Sarah

BASH cheat sheet - Level 3 provides practical reference for Bash control structures and common language constructs. It explains conditional statements using if/elif/else and the case statement for matching values against patterns. The guide covers file tests, string and arithmetic comparisons, logical operators, and pattern matching. It also summarizes loop techniques (for/while), file reading patterns, and core array and hash usage, along with basic script elements, arguments, functions, and simple user menus.

LanguageEnglish

CategoryPresentations

FormatPDF

Pages2

Reading Time1 min

AccessFree to download

Control Structure

Conditional statements

File tests

String comparison and arithmetic operators

Pattern matching and logical operators

Case statement

Loops

File reading

Arrays and Hashes

Programming in bash

Arguments and Functions

User interface

BASH cheat sheet-Level 3 Control Structure Conditional statements A) The if statement: if [[ condition1 ]];then / !\ The spaces are command1 important in that syntax elif [[ condition2 ]];then command2 else command3 fi Perform command1 if condition1 is true, elseif the condition2 is true the command2 is performed, else it’s the command3 that will be performed. File tests: -‐f file True if file file exists -‐d dir True if dir dir exists -‐z string True if string is empty. -‐n string True if string is non-‐empty. file1 –nt file2 True if file1 has been changed more recently than file2, or if file1 exists and file2 does not. file1 –ot file2 True if file1 is older than file2, or if file2 exits and file1 does not. String comparison operators: string1 == string2 Compare strings equality. string1 != string2 Compare strings inequality. Arithmetic comparison operators: Generally numeric comparisons on double square brackets are obsolete, however you may still used -‐ eq, -‐ ne, -‐lt, -‐le, -‐gt, or -‐ge, meaning equal, not equal, less than, less than or equal, greater than, and greater than or equal, respectively. / !\ For arithmetic values it’s highly recommended to use the syntax (( condition )) . In that case, the following operators must be used : == , !=, < , <=, > >= , Pattern matching operator: string =~ regularExpression True if string match the pattern of the regular expresion. Logical operators : if ! [[ condition ]] ! -‐ NOT -‐ Negate the truth. Evaluate as true only if condition is false. if [[ condition1 ]] && [[ condition2 ]] && -‐ AND – Check if both conditions are true. if [[ condition1 ]] || [[ condition2 ]] || -‐ OR – Check if one of the conditions is true. B) The case statement: case $variable in pattern1) commands1 ;; pattern2|pattern3|pattern4) commands2 ;; patternN) commands3 ;; *) commands4 ;; esac It allows to check a value multiple times. If the $variable match the pattern1, the commands1 are executed. If it matches none of them, the commands4 are executed. The loops A) The for loop: for i in element1 element2 element3 ; do command done Repeat the command by assigning list’s elements to the variable i. The list can be implicit (e.g *.txt that iterates over all the txt files.) for (( i=1 ; i<=10 ; i++ )) ; do command done This is a C-‐style for loop that iterates over the integers (here from 1 to 10 namely 10 times) . for i in {1..10}; do command done This syntax allows to iterate a selected number of time (here from 1 to 10 namely 10 times) . for i in ${!array[@]} ; do echo "key :" $i echo "value:" ${array[$i]} done Iterate over an associative array. B) The while loop: The while loop continue until the condition is false. i=0 while (( $i <= 10 )); do command ((i++)) done Iterate until $i is superior to 10, namely 10 times. Jacques Dainat -‐ 2015 File reading for line in $(cat file.txt); do command done Read the file file line by line and execute the command. / !\ Here the line is defined by the IFS variable (see section’s end) . Set it to '\n'to obtain the behavior expected. while read line ;do Read the file file command line by line and done < file execute the command. IFS=$'\n' Set the Internal Field Separator (IFS) variable to '\n'. By default its value is ' \t\n'(space, tab and newline) . Arrays and Hashes A) Indexed array : array=() or declare –a array Declare an indexed array and initialize it tobe empty. In the second case an existing array is not initialized. array=(Anna Par Ulla) The array array is initialized with three values. array[N]=value Set the element N of the array array to value array+=(value1 value2 value3) Append the array with three values. ${array[N]} Expand the element referenced by the index N from array. ${\#array[N]} Size (string length) of the value ref

FAQs

How do you use an if/elif/else conditional in Bash?

Use if [[ condition1 ]]; then ... elif [[ condition2 ]]; then ... else ... fi. The corresponding command block runs based on which condition is true.

What are common Bash file test operators like -f, -d, -z, and -n?

-f checks that a file exists, -d checks that a directory exists, -z checks whether a string is empty, and -n checks whether a string is non-empty.

How do you iterate using for loops and while loops in this cheat sheet?

The for loop examples show iterating over lists and ranges, and looping over associative array indices. The while loop runs until the condition becomes false, using a numeric comparison inside (( ... )).

Oct 13, 2026

Upload by Sarah