Best way to test if a systemd service is running in bash script

systemctl is-active application.service
systemctl is-enabled application.service
systemctl is-active --quiet service

will exit with status zero if service is active, non-zero otherwise, making it ideal for scripts:

systemctl is-active --quiet service && echo Service is running

References
https://www.digitalocean.com/community/tutorials/how-to-use-systemctl-to-manage-systemd-services-and-units
https://unix.stackexchange.com/questions/396630/the-proper-way-to-test-if-a-service-is-running-in-a-script

Check if a File or Directory Exists in Bash

Check if File Exist

FILE=/etc/resolv.conf
if test -f "$FILE"; then
    echo "$FILE exist"
fi
FILE=/etc/resolv.conf
if [ -f "$FILE" ]; then
    echo "$FILE exist"
fi
FILE=/etc/resolv.conf
if [[ -f "$FILE" ]]; then
    echo "$FILE exist"
fi

Check if Directory Exist

FILE=/etc/docker
if [ -d "$FILE" ]; then
    echo "$FILE is a directory"
fi

Check if File does Not Exist

FILE=/etc/docker
if [ ! -f "$FILE" ]; then
    echo "$FILE does not exist"
fi

Check if Multiple Files Exist

FILE=/etc/docker
if [ -f /etc/resolv.conf -a -f /etc/hosts ]; then
    echo "$FILE is a directory"
fi
FILE=/etc/docker
if [ -f /etc/resolv.conf && -f /etc/hosts ]; then
    echo "$FILE is a directory"
fi

File test operators

  • -f FILE – True if the FILE exists and is a regular file (not a directory or device).
  • -G FILE – True if the FILE exists and has the same group as the user running the command.
  • -h FILE – True if the FILE exists and is a symbolic link.
  • -g FILE – True if the FILE exists and has set-group-id (sgid) flag set.
  • -k FILE – True if the FILE exists and has a sticky bit flag set.
  • -L FILE – True if the FILE exists and is a symbolic link.
  • -O FILE – True if the FILE exists and is owned by the user running the command.
  • -p FILE – True if the FILE exists and is a pipe.
  • -r FILE – True if the FILE exists and is readable.
  • -S FILE – True if the FILE exists and is socket.
  • -s FILE – True if the FILE exists and has nonzero size.
  • -u FILE – True if the exists and set-user-id (suid) flag is set.
  • -w FILE – True if the FILE exists and is writable.
  • -x FILE – True if the FILE exists and is executable.

References
https://linuxize.com/post/bash-check-if-file-exists/

Change the output color of echo using ANSI escape codes in Linux

Black        0;30     Dark Gray     1;30
Red          0;31     Light Red     1;31
Green        0;32     Light Green   1;32
Brown/Orange 0;33     Yellow        1;33
Blue         0;34     Light Blue    1;34
Purple       0;35     Light Purple  1;35
Cyan         0;36     Light Cyan    1;36
Light Gray   0;37     White         1;37
RED='\033[0;31m'
NC='\033[0m' # No Color
echo -e "I ${RED}love${NC} Stack Overflow"

References
https://stackoverflow.com/questions/5947742/how-to-change-the-output-color-of-echo-in-linux
https://misc.flogisoft.com/bash/tip_colors_and_formatting

Change the output color of echo using tput in Linux

tput setaf 1; echo "this is red text"

The count of colors available to tput is given by tput colors

To see the basic 8 colors (as used by setf in urxvt terminal and setaf in xterm terminal):

Color       #define       Value       RGB
black     COLOR_BLACK       0     0, 0, 0
red       COLOR_RED         1     max,0,0
green     COLOR_GREEN       2     0,max,0
yellow    COLOR_YELLOW      3     max,max,0
blue      COLOR_BLUE        4     0,0,max
magenta   COLOR_MAGENTA     5     max,0,max
cyan      COLOR_CYAN        6     0,max,max
white     COLOR_WHITE       7     max,max,max

To print all 256 colors in the terminal, try the following one-liner:

for c in {0..255}; do tput setaf $c; tput setaf $c | cat -v; echo =$c; done

References
https://stackoverflow.com/questions/5947742/how-to-change-the-output-color-of-echo-in-linux
https://unix.stackexchange.com/questions/269077/tput-setaf-color-table-how-to-determine-color-codes

Functions in Bash

#!/bin/bash
# Basic function
print_something () {
echo Hello I am a function
}
print_something
print_something

Passing Arguments

#!/bin/bash
# Passing arguments to a function
print_something () {
echo Hello $1
}
print_something Mars
print_something Jupiter

Return Values

#!/bin/bash
# Setting a return status for a function
print_something () {
echo Hello $1
return 5
}
print_something Mars
print_something Jupiter
echo The previous function has a return value of $?

Passing Arguments

#!/bin/bash
# Passing arguments to a function
print_something () {
echo Hello $1
}
print_something Mars
print_something Jupiter

Return Values

#!/bin/bash
# Setting a return status for a function
print_something () {
echo Hello $1
return 5
}
print_something Mars
print_something Jupiter
echo The previous function has a return value of $?

the variable $? contains the return status of the previously run command or function.

References
https://ryanstutorials.net/bash-scripting-tutorial/bash-functions.php

If Statements in Bash

Basic If Statements

#!/bin/bash
# Basic if statement
if [ $1 -gt 100 ]
then
echo Hey that\'s a large number.
pwd
fi
date

If Else

#!/bin/bash
# else example
if [ $# -eq 1 ]
then
nl $1
else
nl /dev/stdin
fi

If Elif Else

#!/bin/bash
# elif statements
if [ $1 -ge 18 ]
then
echo You may go to the party.
elif [ $2 == 'yes' ]
then
echo You may go to the party but be back before midnight.
else
echo You may not go to the party.
fi

Boolean Operations

#!/bin/bash
# and example
if [ -r $1 ] && [ -s $1 ]
then
echo This file is useful.
fi
#!/bin/bash
# or example
if [ $USER == 'bob' ] || [ $USER == 'andy' ]
then
ls -alh
else
ls
fi

Multiple logical operators, ((A || B) && C)

use double brackets

if [[ ($A -eq 0 || $B -ne 0) && $C -eq 0 ]]; then …

References
https://ryanstutorials.net/bash-scripting-tutorial/bash-if-statements.php
https://unix.stackexchange.com/questions/290146/multiple-logical-operators-a-b-c-and-syntax-error-near-unexpected-t