Bash Commands Cheat Sheet

 

This document serves as a non-exhaustive cheat sheet for essential Bash commands on Unix. We recommend using man <command> or <command> --help for more detailed information on each command (documentation).

Note: This document includes commands that can lead to data loss, etc. This document may also contain errors. So it's crucial to refer to the official documentation before using them.

 

1. Files and Directories

1.1 Change directory: cd

cd /home/
cd "/home/my user/"
cd /home/my\ user/ # Escaping space
cd $MY_DIRECTORY # Using a variable
cd - # Switch to the previous directory
cd # Switch to the user's home directory, same as cd ~/

Note:

• Start with "/" to begin at the root
• Start with "~" to start at the user's home directory
• ".." to go up one level, e.g., "../home/"
• If a folder name contains a space, it must be escaped with \, e.g., my\ folder. Otherwise, use quotes, e.g., "my folder".

1.2 List files/directories and their properties: ls

ls # Displays files/directories in the current directory
ls /home # Displays files/directories in the /home directory
# The "-l" option displays in long format: access rights, owners, groups, file size... The "-a" option shows files starting with "."
ls /home -la
ls /var/log/*log # Displays all files ending with "log"
ls -F # Appends a slash to directory names
ls -lr # Lists files in reverse order

Note:

• By default, the ls command displays files sorted alphabetically
• With the "-l" option, file/directory sizes are shown in bytes. For a more "human-readable" format, use it with "-h".

1.3 Display the current directory: pwd

1.4 Move or rename a file/directory: mv

mv fileA.txt fileB.txt # Rename
mv ../myFile.txt ./ # Move the file "myFile.txt" to the current directory
mv ../myFile.txt myFolder/ # Move the file "myFile.txt" to the folder myFolder
mv myFolder myFolder2 # Rename myFolder to myFolder2
mv myFolder2/* ../ # Move all contents (*) of the folder myFolder2 to the parent folder (../)

1.5 Copy a file: cp

cp myFile myFolder/ # Simple file copy
cp myFolder/* myFolder2/ # Copy folder contents to another
cp -r myFolder/ myFolder3/ # Copy myFolder to myFolder3 using the -r (recursive) option

1.6 Create a file: >, >> and touch

The touch command updates the access and modification time of one or more files to the current time. If the file does not exist, it creates an empty file.

touch myFile.txt

Also, with stream redirection:

> myFile.txt # Creates the file if it doesn't exist. Warning, it empties it if it exists!
true > myFile.txt # same as above

>> myFile.txt # Creates the file if it doesn't exist, does not modify its content or date if it exists.

echo "AAA" > myFile.txt # Creates the file if it doesn't exist. Puts "AAA" inside.

1.7 Delete a file/directory: rm

rm myFile.txt # Delete a file in the current directory
rm *.log # Delete all files ending with .log in the current directory

rm -r myFolder/ # Delete a folder (recursive)
rm -rf myFolder/ # Delete a folder with the "force" option. Therefore, no confirmation will be asked for certain folders (use with caution)

1.8 Display file contents: cat

cat myFile.txt # Display file contents
cat -n myFile.txt # Display file contents with line numbers
cat *.log # Display all contents of files ending with log

1.9 Display the first lines: head

Displays the first 10 lines of the file

head myFile.txt # Display the first 10 lines
head -n 5 myFile.txt # Display the first 5 lines
head -n -5 myFile.txt # Display all except the last 5 lines
head -n -5 myFile1.txt myFile2.txt # Display all except the last 5 lines of myFile1.txt, then myFile2.txt

1.10 Display the last lines: tail

tail myFile.txt # Display the last 10 lines
tail -n 5 myFile.txt # Display the last 5 lines
tail -n +5 myFile.txt # Display all except the first 5 lines
tail -n +5 myFile1.txt myFile2.txt # Display all except the first 5 lines of myFile1.txt, then myFile2.txt

1.11 Other display methods: more and less

more myFile.txt # Paginated display
less myFile.txt # For large files

 

2. Command Management

2.1 Execute a command

myCommand
myCommand & # Command executed in the background
myCommand1; myCommand2; # Commands executed sequentially

Logical operators && (and) and || (or) allow conditional execution of commands

# myCommand1 && myCommand2;
# Execute myCommand1 then myCommand2 if myCommand1 succeeds
mkdir myFolder && touch myFolder/myFile.txt

# myCommand1 || myCommand2;
# Execute myCommand1 then myCommand2 if myCommand1 fails.
mkdir myFolder || echo "KO"

# myCommand1 && myCommand2 || myCommand3;
# Execute myCommand1 then myCommand2 if myCommand1 succeeds.
# If myCommand1 or myCommand2 fails, then execute myCommand3.
mkdir myFolder && echo "OK" || echo "KO"

Note: To check if a command fails, you can display the content of the $? variable after executing the command. It returns 0 if the command executed successfully, otherwise another value.

mkdir myFolder
echo $? # Output: 0 => success
mkdir myFolder # Output: mkdir: cannot create directory ‘myFolder ’: File exists
echo $? # Output: 1 => error because the folder already exists

2.2 Stream Management

Each process has by default:

• An input (stdin)
• A standard output (stdout: 1)
• An error output (stderr: 2)

Called "pipe", the pipe mode ("|") redirects the output of one command to the input of another command. Here, from myCommand1 to the input of myCommand2:

myCommand1 | myCommand2

Redirecting to a file:

# Only redirect the standard output (stdout 1) to myFile.txt
myCommand > myFile.txt

# Read the file myFile and send the initial read content to the standard output to the file myFile2.txt (creates or empties myFile2.txt if it exists)
cat myFile > myFile2.txt

# Similar to the previous command, but appends to the file (does not empty the file before reading).
myCommand >> myFile.txt

# Redirect errors to the file (creates or empties the errorFile.log if it exists)
myCommand 2> errorFile.log
mkdir myFolder 2> errorFile.log

# Similar to the previous command, but appends to the file (does not empty the file before reading).
myCommand 2>> errorFile.log

Tools:

# Never use or see the error output, so redirect it to dev/null
myCommand 2>/dev/null

# Have no output
myCommand > /dev/null 2>&1

# Redirect the error output to the same file as the standard output
myCommand > myFile.log 2>&1

Redirecting the standard input with < or <<. The keyword following "<<" must appear on a new line containing only that keyword to signify the end of the standard input for the specified command.

myCommand << KEYWORD TEXT
TEXT
KEYWORD

echo << END "AAA"
END # Output: AAA


# Redirect the input contained in myFile.txt to myCommand
myCommand < myFile.txt

2.3 Process Management

ps # Lists user processes
ps -A # Lists all processes
top # Lists all processes and their load, more visual than ps
htop # Like top, but more readable (package to install)
kill PID # Stop a process by its PID
kill -9 PID # Sends the SIGKILL signal to the process (force stop)
killall NAME # Stop a process by its name

 

3. Variables

3.1 Creation

Basic declaration: VARIABLE_NAME="VALUE" or VARIABLE_NAME='VALUE':

age=30
firstName="Alex"
firstName='Alex'

Usage example:

echo "My name is $firstName, I am ${age} years old."
# Output: My name is Alex, I am 30 years old.

# Also works to display only the variable content
echo $firstName
echo ${age}

To display a variable, you can use the $VARIABLE_NAME syntax, or ${VARIABLE_NAME} (best practices)

Note:

• You can use ' ' instead of " ", but variables inside will not be interpreted.
• Be careful, no space around the equal sign.

Other useful declaration forms:

# Read-only
readonly lastName="Brown" # or declare -r

# Declare a variable as an integer
declare -i age=30

3.2 Deletion: unset

To delete a variable, you need to use the unset command.

unset firstName;
echo "My name is $firstName, I am ${age} years old."
# Output: My name is , I am 30 years old.

3.3 Visibility: local / export

When declared, by default, a variable is only visible in its current environment. It can also be local (scoped within a function) using the keyword "local", or have a more global visibility (visible in the entire environment) using the keyword "export":

firstName="Alex" # visible in the current exec environment
local firstName; # Locally visible (for example in a function)
export firstName; # Globally visible (in the entire environment)

Example:
function test() {
  local firstName;
  firstName="Alex";
  echo $firstName;
}
firstName="Claire";
echo $firstName; # Output: Claire
test; # Output: Alex
echo $firstName; # Output: Claire

3.4 Performing calculations

Generally, note that there are spaces at the beginning and end of the operation closures. Without them, you will get an error.

result=$(( (10 * 5) + 3 ))
result=$[ (10 * 5) + 3 ]

Also see the let function.

3.5 Environment variables

Useful in scripts:

• $SECONDS: The number of seconds the script has been running.
• $LINENO: Returns the current line number of the script (useful for debugging)
• $RANDOM: Returns a random number

Useful at the end of a command/script execution:

• $?: The exit code of the last executed program

Generally useful:

• $USER: The username
• $HOSTNAME: The machine name
• $HOME: User directory
• $PWD: Current directory
• $PATH: Set of directories containing the executable commands for the user
• $SHELL: Name of the login shell
• $HITSIZE: Number of lines or commands stored in memory in a history list during your bash session.
• $HISTFILESIZE: Number of lines or commands allowed in the history file at the start of a session and stored in the history file at the end of your bash session.

Also see the env function.

4. Strings

4.1 Escaping special characters

As seen previously, to list a file containing a space, we had to put the character \ before the space. Without it, we would have had an error. Several characters called "special" are not interpreted in the same way. The space would have meant that there was a second argument (cd /home/my\ user/). Similarly, the *, which means "all" (cat *.log).

Examples with the star:

echo He saw a * # Output: He saw a fileA.txt fileB.txt fileC.txt

The special characters that need to be escaped are generally the following: ", ', `, \, $, !, ?, ~, (), [], {}, @, *

They can be escaped with a \, be enclosed in ' or ":

echo "He saw a *" # Output: He saw a *
echo 'It works!' # Output: It works!
echo "It works too!" # Output: It works too!
echo This\ also\ works\ \! # Output: This also works!

# Escaping also within " "
echo "My path is $PATH and my shell is $SHELL" # Will replace variables $...
echo "My path is \$PATH and my shell is \$SHELL" # Will not replace variables $
echo 'My path is $PATH and my shell is $SHELL' # Will not replace variables $, same as the previous command

4.2 Creating a string

myString="Alex" # the variable contains "Alex"
myString=$(ls /) # the variable contains the listing of the root directory (/)
myString=`ls /` # the variable contains the listing of the root directory (/). Obsolete.

4.3 String manipulation

String manipulation for assignment:

 Examples:

# Help to retrieve a default value if not defined => ":-"
echo ${VARIABLE1-$VARIABLE2} => if VARIABLE1 is not defined or empty, $VARIABLE2 returned
${VARIABLE1-'text'} => if VARIABLE1 is not defined or empty, 'text' will be returned

# Help to retrieve a default value if not defined and not empty => ":-"
echo ${VARIABLE1:-$VARIABLE2} => if VARIABLE1 is not defined and is not empty, $VARIABLE2 will be returned
${VARIABLE1:-'text'} => if VARIABLE1 is not defined or empty and is not empty, 'text' will be returned

# Help to assign a variable if not defined and not empty => ":="
echo ${VARIABLE1=$VARIABLE2} => if VARIABLE1 is not defined, $VARIABLE2 will be returned and VARIABLE1 will have the value of $VARIABLE2
${VARIABLE1='text'} => if VARIABLE1 is not defined or empty, 'text' will be returned and assigned to VARIABLE1

# Help to assign a variable if not defined and not empty => ":="
echo ${VARIABLE1:=$VARIABLE2} => if VARIABLE1 is not defined and is not empty, $VARIABLE2 will be returned and VARIABLE1 will have the value of VARIABLE2
${VARIABLE1:='text'} => if VARIABLE1 is not defined or empty and is not empty, 'text' will be used

# Error if the variable is not defined => "?" and ":?"
VARIABLE1=""
${VARIABLE1?"Variable is not set"} # bash: VARIABLE1: Variable is not set
unset VARIABLE1
${VARIABLE1:?"Variable is not set or empty"} # bash: VARIABLE1: Variable is not set or empty

 

String extraction

 Examples:

firstName="Alexandre"

echo "String length ${#firstName}" # Output: String length 9

echo "Cut the string: ${firstName:4}" # Cut the string: andre

echo "${firstName: -3}" # dre

echo "Cut the string to a fixed length: ${firstName:4:3}" # Cut the string to a fixed length: and

echo "Cut the string to a fixed length: ${firstName:4:30}" # Cut the string to a fixed length: andre

echo "${firstName::3}" # Ale

echo "${firstName::-3}" # Alexan

myText="The development was completed correctly this morning. This development took 30 minutes."

echo ${myText#The*o} # Output: pment was completed correctly this morning. This development took 30 minutes.

echo ${myText##The*o} # Output: k 30 minutes.

echo ${PWD%/*}

echo ${myText%% *} # Output: The

# Convert to lowercase
echo ${myText,,}

# Convert to uppercase
echo ${myText^^}

 

String replacement

Examples:

myText="The development was completed correctly this morning. This development took 30 minutes."

# Replace development with integration (first occurrence)
echo ${myText/development/integration}
# Output: The integration was completed correctly this morning. This development took 30 minutes.

# Replace development with integration (all occurrences)
echo ${myText//development/integration}
# Output: The integration was completed correctly this morning. This integration took 30 minutes.

Concrete example:

file="/etc/log/log.txt"
fileName="log.txt"

# Extract the path
echo ${file%/*} # /etc/log/
# Extract the file
echo ${file##*/} # log.txt
# Extract the file without the extension
echo ${fileName%.*} # log
# Extract the file extension
echo ${fileName##*.} # txt

 

5. Bash scripts

Bash scripts have a .sh file extension (although this is optional). These scripts have the first line #!/bin/bash (shebang), which allows defining with what the script will be interpreted (here bash).

List of argument variables:

• $0 : Script name
• $1, $2, $3 .. : argument 1, argument 2, argument 3 ....
• $#: Number of parameters provided as script input
• $* or $@ (they do not behave the same within quotes): String containing all commands provided as script input

 To add parameters by automatically updating $1 etc, use the set command, example:

set alex 21

 

Useful standard variables:

• $$: The current process ID (PID)
• $? : Return code of the last executed command
• $! : Process number of the last command
• $SECONDS: The number of seconds the script has been running.
• $LINENO: Returns the current line number of the script (useful for debugging)

A bash script has a default exit value of 0, which is conventionally a success exit. It is possible to exit with a failure, using the exit command. The exit code can range from 0 to a maximum of 255 inclusive.

exit 25;

The recovery of this value is done via the $? variable. Let's take for example "myScript.sh which does an "exit 25;" only:

./myScript.sh
echo "Error code: $?"; # Output: Error code: 25
echo "Error code: $?"; # Output: Error code: 0

Note: We can see that the second "echo" returns "0" as the error code, because "$?" returns the error code of the last executed command => here the previous "echo" which correctly displayed "Error code: 25".

To include a bash script in another, it must be executed. Example:

my_dir="/your/path"
"$my_dir/other_script.sh"

 

6. Functions

To avoid having everything in the same block, bash allows creating functions.

# Function without arguments
function example {
  echo "Hello";
}

# Function with 2 arguments
function example2 {
  echo "Hello $1 and $2, I have $# arguments";
  echo "Here are all my arguments: $*";
  echo "Or: $@";
}

myVariable=$(example) # You can store the output of a function in a variable
echo $myVariable # Output: Hello

example2 A B # Note: Calls are made without parentheses or commas to separate arguments. A space is enough.
# Output:
# Hello A and B, I have 2 arguments
# Here are all my arguments: A B
# Or: A B

# Get the function declaration
declare -f example2
#Output
#example2 ()
#{
#    echo "Hello $1 and $2, I have $# arguments";
#    echo "Here are all my arguments: $*";
#    echo "Or: $@"
#}

 

7. Arrays and Dictionaries (map)

Arrays (indexed arrays => integer keys) work almost the same as dictionaries (like maps: associative arrays => integer/string keys).

Note * : [@] or [*] : prefer [@] within quotes to keep the true values (distinguish spaces from another element). Without quotes, they behave the same, but with quotes, [*] will return everything as a single word and [@] will return each element as a distinct word.

8. Control Structures

8.1 Tests and the if command

There are several ways to implement a condition (spaces are important):

if [ $MY_VARIABLE -gt 2 ] ; then # If MY_VARIABLE greater than > 2
  echo "YES"
fi

if [ $# -lt 2 ] ; then
  echo "YES"
else
  echo "NO"
fi


if [ $# -gt 2 ] ; then
  echo "YES"
elif [ $# -eq 0 ] ; then # else if
  echo "MAYBE"
else
  echo "NO"
fi

# It is possible to perform a test with the test command and []
if test $MY_VARIABLE -gt 2  ; then # If MY_VARIABLE greater than > 2
  echo "YES"
fi

 

Comparators:

To combine tests, you can use the operators && and || with double brackets, -a (for and) and -o (for or) with single brackets.

8.2 Case

It is generally used to manage program parameters.

Example:

while [[ $# -gt 0 ]]
do
  key="$1"
  case $key in
    -a1|--argument1)
      ARGUMENT1="$2"
      shift # past argument
      shift # past value
    ;;
    -a2|--argument2)
      ARGUMENT2="$2"
      shift 2 # past 2 argument
    ;;
    --a3)
      a3=YES
      shift # past argument
    ;;
    --help)
      echo "how to use : ....";
      //exit ;
      shift # past argument
    ;;
    *)    # unknown option
    echo "Error in argument with $1"
    //exit 1
    shift # past argument
    ;;
  esac
done

Note: The shift command shifts the arguments to the left. The value of $1 <= $2, $2 <= $3, and so on. Therefore, $# is also decremented.

8.3 For loop

firstName1="Alex";
firstName2="Claire";
firstName3="Angel";

# With a list
for firstName in $firstName1 $firstName2 $firstName3; do
  echo $firstName;
done

# Output:
# Alex
# Claire
# Angel

# With a command output
for item in $(ls); do
  echo item: $item;
done

# Output:
# item: fileA.txt
# item: fileB.txt
# item: fileC.txt
# item: folderA

# With a command output #2 (number sequence)
for i in $(seq 1 5); do
  echo $i;
done

# Output:
# 1
# 2
# 3
# 4
# 5

# Loop with iteration
for ((it=0; it<= 5; it++)); do
  echo $it;
done

# Output:
# 0
# 1
# 2
# 3
# 4
# 5

declare -a myArray;
myArray=( Alex Claire Angel );

for firstName in "${myArray[@]}"
do
  echo "${firstName}"
done

# Output:
# Alex
# Claire
# Angel

8.4 While loop

it=0;
while [ $it -lt 5 ] ; do
  echo "it : ${it}";
  it=$(( $it + 1 )) # equivalent to ((it++))
done

# Output:
# it : 0
# it : 1
# it : 2
# it : 3
# it : 4

while :
do
  read -p "Enter your age: " age # Reads from standard output
  echo "You are ${age} years old";

  if [ ${age} -ge 100 ] ; then
    echo "You are old";
    break;
  else
    echo "You are not old enough"
  fi
  echo "I won't leave until I meet someone who is 100 years old or older.";
done

# Output:
# Enter your age: 30
# You are 30 years old
# You are not old enough
# I won't leave until I meet someone who is 100 years old or older.
# Enter your age: 101
# You are 101 years old
# You are old

# Read a file line by line
while read LINE; do
    echo $LINE;
done < file.txt

# Output:
# Content of the first line
# Content of the second line
# Content of the third and last line of the file.

9. Useful commands