My own bash cheatsheet

Bash variables

Define variables in bash

array[0]=valA                # how to define an array
array[1]=valB
array[2]=valC
array=([2]=valC [0]=valA [1]=valB)  # another way
array=(valA valB valC)              # and another

${array[i]}                  # displays array's value for this index. If no index is supplied, array element 0 is assumed
${#array[i]}                 # to find out the length of any element in the array
${#array[@]}                 # to find out how many values there are in the array

declare -a                   # the variables are treaded as arrays
declare -f                   # uses function names only
declare -F                   # displays function names without definitions
declare -i                   # the variables are treaded as integers
declare -r                   # makes the variables read-only
declare -x                   # marks the variables for export via the environment

${varname:-word}             # if varname exists and isn't null, return its value; otherwise return word
${varname:=word}             # if varname exists and isn't null, return its value; otherwise set it word and then return its value
${varname:?message}          # if varname exists and isn't null, return its value; otherwise print varname, followed by message and abort the current command or script
${varname:+word}             # if varname exists and isn't null, return word; otherwise return null
${varname:offset:length}     # performs substring expansion. It returns the substring of $varname starting at offset and up to length characters

${variable#pattern}          # if the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest
${variable##pattern}         # if the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest
${variable%pattern}          # if the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest
${variable%%pattern}         # if the pattern matches the end of the variable's value, delete the longest part that matches and return the rest
${variable/pattern/string}   # the longest match to pattern in variable is replaced by string. Only the first match is replaced
${variable//pattern/string}  # the longest match to pattern in variable is replaced by string. All matches are replaced

${#varname}                  # returns the length of the value of the variable as a character string

*(patternlist)               # matches zero or more occurrences of the given patterns
+(patternlist)               # matches one or more occurrences of the given patterns
?(patternlist)               # matches zero or one occurrence of the given patterns
@(patternlist)               # matches exactly one of the given patterns
!(patternlist)               # matches anything except one of the given patterns

$(UNIX command)              # command substitution: runs the command and returns standard output

credit belongs to awesome cheatsheets

Loop through arrays

declare -a fruits=("apple" "orange" "banana")

for fruit in "${fruits[@]}";
do
    echo "$fruit"
done

Basic if condition check

check if the line only contains 1 single character (e.g. line ending)

    if [ ${#line} -eq 1 ]; then
        continue
    else
        echo ${line}
    fi

Use regex to check if a string starts with “#”

if [[ "$var" =~ ^#.*  ]]; then
    echo "yes"
fi

Read a file and print lines not start with “#”

#!/bin/bash
input="/path/to/txt/file"
while IFS= read -r var
do
  #
  # if value of $var starts with #, ignore it
  #
  [[ $var =~ ^#.* ]] && continue
  echo "$var"
done < "$input"

Multi-line string

Ref to: Multi-line string on stackoverflow

Heredoc sounds more convenient for this purpose. It is used to send multiple commands to a command interpreter program like ex or cat

cat << EndOfMessage
This is line 1.
This is line 2.
Line 3.
EndOfMessage

The string after « indicates where to stop.

To send these lines to a file, use:

cat > $FILE <<- EOM
Line 1.
Line 2.
EOM

You could also store these lines to a variable:

read -r -d '' VAR << EOM
This is line 1.
This is line 2.
Line 3.
EOM

This stores the lines to the variable named VAR.

When printing, remember the quotes around the variable otherwise you won’t see the newline characters.

echo "$VAR"

Even better, you can use indentation to make it stand out more in your code. This time just add a - after « to stop the tabs from appearing.

read -r -d '' VAR <<- EOM
    This is line 1.
    This is line 2.
    Line 3.
EOM

But then you must use tabs, not spaces, for indentation in your code.

Frequently used bash utilities

Check the real command of an alias

type aliased-command

Check if a command exists or not

command -v kubectl >/dev/null 2>&1

Here is a very helpful explanation about the usage of redirecting to /dev/null and what 2>&1 means:

• /dev/null is a black hole where any data sent, will be discarded
• 2 is the file descriptor for Standard Error
• & is the symbol for file descriptor (without it, the following 1 would be considered a filename)
• 1 is the file descriptor for Standard Out

Therefore >/dev/null 2>&1 is redirect the output of your program to /dev/null. Include both the Standard Error and Standard Out.

To find the return code of the last executed comamnd is $?, so you can check if the value of $? equals to 0 to verify if the previous command succeeds or not.

More detail information can be found at The Linux Documentation Project’s I/O Redirection page.

Find which process listens to a port

On Linux(ubuntu/fedora)

# sudo apt install net-tools
netstat -ltnp |grep $PORT

On Mac (or other linux system)

# sudo apt-get install lsof
# sudo yum install losf
lsof  -t -i :6379

Kill all process which match with a certain name

e.g. kill all redis-server process

for pid in `pgrep -f redis-server`;do kill $pid; done

Enable sudo without password

Edit /etc/sudoers file and add a line as below for the user you want to allow nopassword sudo.

sudo cp /etc/sudoers ~/sudoers.bak
sudo visudo
# Add the following line to the sudoers file
user_name ALL=(ALL) NOPASSWD:ALL

Rename file names by adding suffixes

# add echo for test run
for f in *.mp3; do echo mv "$f" "${f%.mp3}_p192.mp3"; done

# real run
for f in *.mp3; do mv "$f" "${f%.mp3}_p192.mp3"; done

Use strict flags

set -o nounset    # error when referencing undefined variable
set -o errexit    # exit when command fails

References

• Bash check if string starts with character such as #
• Sudo without password