Always start with a Shebang :The first rule of scripting is to always start with a Shebang. The shebang is a special character sequence in a script file that specifies which program or interpreter should be called to run the script. It is always the first line in the script. Without the shebang line, the system doesn’t know what language to use to process the script.
#!/bin/bash
The sha-bang (#!) at the top of the Script created or to be created is a script that tells our Operating System that our file is a set of commands that will be fed (will be interpreted) by the command interpreter indicated after it. The character pair #! actually, it's a magic number two-byte, a special marker that designate a file type, and in our case, an executable shell script. Immediately after the sha-bang comes the name of the path where the interpreter to be executed is located plus the name of said interpreter. In other words, this is the path to the program that interprets the commands in the script, whether it be an interpreter, a programming language, or a utility.
Variables and Naming Conventions
1. variables should always have the form name=value
2. Ideally a Variable name is best identified if it consists Upper case letters, digits and '-' ( underscore )
3. Variable annotations : Bash allows for a limited form of variable annotations. The most important ones are:
local (for local variables inside a function)
readonly (for read-only variables)
Strive to annotate almost all variables in a bash script with either local or readonly.
4. Prefer local variables within functions over global variables
5. If you need global variables, make them read only
6. Make sure Variables are capitalized for better understanding
of code.if we want to use capitals, use
Environment (exported) variables: ${ALL_CAPS}
Local variables: ${lower_case}
Constants : CONSTANT_NAME
7. Variables names for loops should be similarly named for any variable you’re looping through. Check variable zone in the below loop
for zone in "${zones[@]}"; do
something_with "${zone}"
Done
8. Constants and Environment Variable Names : All caps, separated with underscores, declared at the top of the file.Constants and anything exported to the environment should be capitalized.
# Constant
readonly PATH_TO_FILES='/some/path'
# Both constant and environment
declare -xr USER_SID='PROD'
9. Define Default variables when required
VARIABLE="${1:-$DEFAULTVALUE}"
which assigns to VARIABLE the value of the 1st argument passed to the script or the value of DEFAULT VALUE if no such argument was passed. Quoting prevents globbing and word splitting.
Default values
: "${S3_HOST:="https://minio.superevil.io:9000"}"
: "${S3_BUCKET_NAME:="foo/bar"}"
: "${S3_ACCESS_KEY:-""}"
: "${S3_SECRET_KEY:-""}"
10. Declare all Variables : Bash doesn’t have a strong type system. To allow type-like behavior, it uses attributes that can be set by a command. ‘declare’ is a bash built-in command that allows you to update attributes applied to variables within the scope of your shell. In addition, it can be used to declare a variable in longhand. a simple use case looks as ,
$ declare var
$ declare -i int
$ var="1+1"
$ int="1+1"
$ echo "$var"
1+1 ## The literal "1+1"
$ echo "$int"
2 ## The result of the evaluation of 1+1
11. Don’t start Variable Name with special characters or Numbers
[root@ip-172-31-19-247 ~]# cat simple.sh
#!/bin/bash
-one="java"
123one="java"
echo $-one
echo $123one
[root@ip-172-31-19-247 ~]# sh simple.sh
simple.sh: line 4: -one=java: command not found
simple.sh: line 5: 123one=java: command not found
hBone
23one
12. Surround your variables with {}. Otherwise bash will try to access the $ENVIRONMENT_app variable in /srv/$ENVIRONMENT_app, whereas you probably intended /srv/${ENVIRONMENT}_app.
13. Surround your variable with " in if [ "${NAME}" = "java" ], because if $NAME isn't declared, bash will throw a syntax error (also see nounset).
14. Use :- if you want to test variables that could be undeclared. For instance: if [ "${NAME:-}" = "java" ] will set $NAME to be empty if it's not declared. You can also set it to noname like so if [ "${NAME:-noname}" = "java" ]
15. Set magic variables for current file, basename, and directory at the top of your script for convenience.
# Set magic variables for current file & dir
__dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
__file="${__dir}/$(basename "${BASH_SOURCE[0]}")"
__base="$(basename ${__file} .sh)"
__root="$(cd "$(dirname "${__dir}")" && pwd)" # <-- change this as it depends on your app
16. Variable substitution :
echo "${var}"
echo "Substitute the value of var."
echo "${var:-word}"
echo "If var is null or unset, word is substituted for var. The value of var does not change."
echo "${var:=word}"
echo "If var is null or unset, var is set to the value of word."
echo "${var:?message}"
echo "If var is null or unset, message is printed to standard error. This checks that variables are set correctly."
echo "${var:+word}"
echo "If var is set, word is substituted for var. The value of var does not change."
17. Global Script Variables
A number of predefined global variables are used and available to all scripts:
Global Directory Declaration:
BASE_DIR This is the base directory of the project that can be used to reference other files
SCRIPT_DIR This is the /scripts project directory
CNF_DIR This is the /etc project directory
LOG_DIR This is the /log project directory
TMP_DIR This is a temporary working directory, currently this defaults to /tmp
When creating global variables for important paths, allow for override of these by the controlling shell environment. For example.
[ ! -z “${TMP_DIR}” ] && TMP_DIR=”/tmp” # Correct definition
TMP_DIR=”/tmp” # Incorrect defintion
Global File Name Declaration :
TMP_FILE A pre-defined unique temporary file that is auto removed on completion
STOP_FILE A pre-defined file to stop script processing in loops (only if used in functions)
DEFAULT_CNF_FILE A pre-defined standard /etc config file name
DEFAULT_LOG_FILE A predefined standard /log log file name
Variables:
DATE_TIME – The date/time of the script execution
DATE_TIME_TZ – The date/time/timezone of the script execution
USER_ID – The running user id
FULL_HOSTNAME – The full and qualified hostname
SHORT_HOSTNAME – The short hostname
LOG_DATE_FORMAT – The Date Format used for all log files
Other Variables
QUIET – Quiet Logging, ERROR and WARN only
USE_DEBUG – Enable Debugging
18. Variable Usages
When using variables, they are always to be enclosed in curly brackets.
${TMP_FILE} is acceptable
$TMP_FILE is NOT acceptable
When displaying variables in stdout, they should always be included in single quotes (‘) to ensure actual value can be determined.
info “Exiting with status code of ‘${EXIT_CODE}'”
No space before or after the equal sign. Make sure no space is given when defining variables. Check the below codes language variable for reference
[root@ip-172-31-19-247 ~]# cat simple.sh
#!/bin/bash
language = java
echo $language
[root@ip-172-31-19-247 ~]# sh simple.sh
simple.sh: line 1: !#/bin/bash: No such file or directory
simple.sh: line 3: language: command not found
Double quotes around every parameter expansion : Word Splitting is the demon inside Bash that is out to get unsuspecting newcomers or even veterans who let down their guard.It's not just spaces you need to protect. Word Splitting occurs on all whitespace, including tabs, newlines, and any other characters in the IFS variable. Always double quote the variables in script as below,
[root@ip-172-31-19-247 ~]# touch "java is lang"
[root@ip-172-31-19-247 ~]# cat simple.sh
#!/bin/bash
language="java is lang"
rm $language
[root@ip-172-31-19-247 ~]# sh simple.sh
rm: cannot remove 'java': No such file or directory
rm: cannot remove 'is': No such file or directory
rm: cannot remove 'lang': No such file or directory
modified code looks as below,
[root@ip-172-31-19-247 ~]# cat simple.sh
#!/bin/bash
language="java is lang"
rm "$language"
Use Good Indentation : it is very important to make code more readable thus making it more maintainable. Where we write code with more than 1 level of logic, make sure it is always indented. It doesn't matter much whether how many spaces you indent, though most people seem to use 4 spaces or 8. Just make sure that your do's and don'ts line up and you'll be fine.
a simple indentation looks as,
#!/bin/bash
if [ $# -ge 1 ] && [ -d $1 ]; then
for file in `ls $1`
do
if [ $debug == "on" ]; then
echo working on $file
fi
wc -l $1/$file
done
else
echo "USAGE: $0 directory"
exit 1
fi
Always provide usage of the script. It is important to let executors of the script understand how to execute the code. what parameters need to be passed and how to. a simple example below, if we run the script without passing a filename, it throws a usage error
[root@ip-172-31-19-247 ~]# cat simple.sh
#!/bin/bash
if [ $# == 0 ]; then
echo "Usage: $0 filename"
exit 1
Fi
Sensible commenting : provide comments as much as you can while writing the code. explaining what the part of code does helps a lot when you revisit the code after some time. Don't explain the obvious lines, but explain every command that you're using or the important ones get lost in the mix.
user=$1
# The below logic is to check if account exists on the system or not
grep ^$user: /etc/passwd
if [ $? != 0 ]; then
echo "No such user: $username"
exit 1
Fi
Long Notation : Always use long parameter notation when available. This makes the script more readable, especially for lesser known/used commands that you don’t remember all the options for.
# Avoid:
rm -rf -- "${dir}"
# Good:
rm --recursive --force -- "${dir}"
Return an Exit Code : Always return an exit code when something goes wrong. Many of the times, we don't even care of what is returned when something exits but returning a non-zero code when something goes is not a bad idea. At some point we might need this return code when we are executing the shell script from other languages.
[root@ip-172-31-19-247 ~]# cat simple.sh
#!/bin/bash
cat file.txt
if [ $? -eq 0 ]
then
echo "The script ran ok"
exit 0
else
echo "The script failed" >&2
exit 1
fi
[root@ip-172-31-19-247 ~]# sh simple.sh
cat: file.txt: No such file or directory
The script failed
Check Argument Types : We can save a lot of our time when we make sure the arguments provided to your script are of the type expected before you start to use them. a simple check looks like below,
if ! [ "$1" -eq "$1" 2> /dev/null ]
then
echo "ERROR: $1 is not a number!"
exit 1
Fi
Use Arrays wherever Possible : Most of the times we write code where we store a collection of items or elements in a string. always use a array when there are collection of items as below,
declare -r hosts="host1 host2 host3"
for host in $hosts # not quoting $hosts here, we want word splitting
do
echo "$host"
done
# use an array instead!
declare -r -a host_array=( host1 host2 host3 )
for host in "${host_array[@]}"
do
echo "$host"
done
Avoid Unnecessary Pipelines : Most of the code that we write are dependent on the piping. Avoid unnecessary piping and use the shell builtin as below,
# instead of
cat file | command
# use
command < file
# instead of
echo text | command
# use
command <<< text
# instead of
grep pattern file | awk '{print $1}'
# use
awk '/pattern/{print $1}'
# instead of
grep pattern file | sed 's/foo/bar/g'
# use
sed -n '/pattern/{s/foo/bar/p}' file
# instead of
command | sort | uniq
# use
command | sort -u
# instead of
command | grep pattern | wc -l
# use
command | grep -c pattern
Use Process|Command Substitution wherever possible : process substitution is a form of inter-process communication that allows the input or output of a command to appear as a file. The command is substituted in-line, where a file name would normally occur, by the command shell. This allows programs that normally only accept files to directly read from or write to another program.
A simple example is the creation of temporary files
# using temp files
command1 > file1
command2 > file2
diff file1 file2
rm file1 file2
# using process substitution
diff <(command1) <(command2)
# dont use
echo 'echo “hello world”'
# use
echo $(echo “hello world)
Activate Bash Debug Mode : In many situations, bash will continue executing the script even when a specific part fails, impacting the rest of the script badly. To ensure that the script exists upon facing some fatal error, it’s recommended to have the following lines at the start.
set -o errexit
set -e : This tells the shell to exit the script as soon as any statement returns a non-zero exit code.
set -u : By default, bash will ignore any variables that don’t exist. adding "set -u" will not ignore the variables that does not exist
set -a : Using set -a, you can cause any variable or function that you create to be automatically exported so that subshells and scripts can use them.
set -x : shows each command as it’s being run as well as the output. enable xtrace option
Write error messages to stderr : Always write Error messages belonging to stderr not stdout.
echo "An error message" >&2
Comparisons : Use = instead of == for String Comparisons. Note that == is a synonym for =, therefore only use a single = for string comparisons, for instance:
value1=java.com”
value2=shell.com”
if [ "$value1" = "$value2" ]
printf over echo : For various reasons, printf is preferable to echo. printf gives more control over the output, it’s more portable and its behavior is defined better. Print error messages on stderr. E.g., I use the following function:
error() {
printf "${red}!!! %s${reset}\\n" "${*}" 1>&2
}
Trap forced exit of script : Don’t let your script exit unexpectedly, trap when someone updates press ctrl+c and exit from your script gracefully.
# trap ctrl-c and call ctrl_c()
trap ctrl_c INT
function ctrl_c() {
echo "** Trapped CTRL-C"
}
for i in `seq 1 5`; do
sleep 1
echo -n "."
done
Use $() over backticks : Avoid using backticks ““”, they are hard to read and in some fonts easily confused with single quotes. A lot of quoting needed in nesting. Use $(command) instead of `command` because it is easier to nest multiple commands and makes your code more readable.
# dont use
$ echo "one-`echo two-\`echo three-\`\`echo four\`\`\``"
one-two-three-four
# use
$ echo "one-$(echo two-$(echo three-$(echo four)))"
one-two-three-four
Logging : Logging is the most critical thing for everyone whether he is a developer, sysadmin or DevOps. Debugging seems to be impossible without logs. As we know most applications generate logs for understanding what is happening with the application, the same practice can be implemented for shell script as well. For generating logs we have a bash utility called logger.
[root@ip-172-31-19-247 ~]# cat simple.sh
#!/bin/bash
DATE=$(date)
declare DATE
check_file() {
local FILENAME="$1"
if ! ls "${FILENAME}" > /dev/null 2>&1
then
logger -s "${DATE}: ${FILENAME} doesn't exists"
else
logger -s "${DATE}: ${FILENAME} found successfully"
fi
}
check_file "/etc/passwd"
[root@ip-172-31-19-247 ~]# sh simple.sh
<13>Feb 26 12:48:19 ec2-user: Sat Feb 26 12:48:19 UTC 2022: /etc/passwd found successfully
Builtin Commands vs. External Commands : Given the choice between invoking a shell builtin and invoking a separate process, choose the builtin.. We prefer the use of builtins such as the Parameter Expansion functions in bash(1) as it’s more robust and portable (especially when compared to things like sed).
# Dont use
addition="$(expr "${X}" + "${Y}")"
substitution="$(echo "${string}" | sed -e 's/^foo/bar/')"
# Use this
addition=$(( X + Y ))
substitution="${string/#foo/bar}"
Minimize Bash Spawn Usage: Try to use bash builtins for any sort of work unless not available. If we use external commands to perform work, bash can spawn sub shell to get the work done. This can lead to other issues. For Instance a simple seq count as below,
for number in $(seq 1 10); do
Bash is able enough to do the counting for you. You do not need to spawn an external application (especially a single-platform one) to do some counting and then pass that application's output to Bash for word splitting. The above can be written as,
C-style for loops are the best method for implementing a counter for ((i=1; i<=10; i++)). For instance
# instead of dirname, use:
declare -r file_dir="{my_file%/*}"
# instead of basename, use:
declare -r file_base="{my_file##*/}"
# instead of sed 's/blah/hello', use:
declare -r new_file="${my_file/blah/hello}"
# instead of bc <<< "2+2", use:
echo $(( 2+2 ))
# instead of grepping a pattern in a string, use:
[[ $line =~ .*blah$ ]]
# instead of cut -d:, use an array:
IFS=: read -a arr <<< "one:two:three"
Use [ … ]] vs. [ … ] : Unless a script must run in a POSIX-compatible environment, use [[ ... ]] rather than [ ... ] when performing conditional tests. Unlike the [ and test bash builtins, [[ ... ]] is part of shell syntax, not a command. This means it can handle its internal elements (test conditions) in a more robust fashion, as pathname expansion and word splitting do not occur. Also, [[ ... ]] adds some additional capabilities such as =~ to perform regular expression tests.
Use shift to read function arguments : Instead of using $1, $2 etc to pick up function arguments, use shift as shown below. This makes it easier to reorder arguments, if you change your mind later.
# Processes a file.
# $1 - the name of the input file
# $2 - the name of the output file
process_file(){
local -r input_file="$1"; shift
local -r output_file="$1"; shift
}
shift is a bash built-in which kind of removes arguments from the beginning of the argument list. Given that the 3 arguments provided to the script are available in $1, $2, $3, then a call to shift will make $2 the new $1. A shift 2 will shift by two making new $1 the old $3
Use null delimited output where possible : In order to correctly handle filenames containing whitespace and newline characters, you should use null delimited output, which results in each line being terminated by a NUL (00) character instead of a newline. Most programs support this. For example, find -print0 outputs file names followed by a null character and xargs -0 reads arguments separated by null characters.
# instead of
find . -type f -mtime +5 | xargs rm -f
# use
find . -type f -mtime +5 -print0 | xargs -0 rm -f
# looping over files
find . -type f -print0 | while IFS= read -r -d $'' filename; do
echo "$filename"
done
Functions : Bash can be hard to read and interpret. Using functions can greatly improve readability. Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command
Apply the Single Responsibility Principle: a function does one thing.
function name has an underscore as a prefix. It seems like a good idea to always have a special naming convention for your bash functions to avoid any potential clashes with built-in operators or functions you might include from other files
Function Location : Put all functions together in the
file just below constants. Don’t hide executable code between functions. Doing so makes the code difficult to follow and results in nasty surprises when debugging. If you’ve got functions, put them all together near the top of the file. Only includes, set statements and setting constants may be done before declaring functions.
Document all functions that we write : it is very important to document all functions that we write. Giving meaning full explanation of the functions is very important for a better code
# Processes a file.
# $1 - the name of the input file
# $2 - the name of the output file
process_file(){
}
Function Comments : Any function that is not both obvious and short must be commented. Any function in a library must be commented regardless of length or complexity. It should be possible for someone else to learn how to use your program or to use a function in your library by reading the comments (and self-help, if provided) without reading the code.
All function comments should describe the intended API behavior using:
Description of the function.
Globals: List of global variables used and modified.
Arguments: Arguments taken.
Outputs: Output to STDOUT or STDERR.
Returns: Returned values other than the default exit status of the last command run.
Example:
#######################################
# Cleanup files from the backup directory.
# Globals:
# BACKUP_DIR
# ORACLE_SID
# Arguments:
# None
# Outputs : stdout
$ Returns : None
#######################################
function cleanup() {
…
}
Function Variable Declaration : Declare variables with a meaningful name for positional parameters of functions
happy() {
local first_arg="${1}"
local second_arg="${2}"
[...]
}
7. Create functions with a meaningful name for complex tests
# Don't do this
if [ "$#" -ge "1" ] && [ "$1" = '-h' ] || [ "$1" = '--help' ] || [ "$1" = "-?" ]; then
usage
exit 0
fi
# Do this
help_wanted() {
[ "$#" -ge "1" ] && [ "$1" = '-h' ] || [ "$1" = '--help' ] || [ "$1" = "-?" ]
}
if help_wanted "$@"; then
usage
exit 0
fi
8. Cleanup code : An idiom for tasks that need to be done before the script ends (e.g. removing temporary files, etc.). The exit status of the script is the status of the last statement before the finish function.
finish() {
result=$?
# Your cleanup code here
exit ${result}
}
9. Mandatory Script Functions :All scripts are to have the following default functions:
bootstrap : The bootstrap function is to be identical in all scripts, this is used to source necessary common functions used by all scripts
help Or Usage: The help function is to display the usage of the function and then exit. The usage needs to specify all command line arguments, and the client identifies mandatory and optional arguments.
process_args : This function is used to process the command line arguments for scripts. The getopt function is used for processing arguments however this only support single character options (e.g. -v -h -p etc). Scripts should be written for only single character options.
To improve the namespace, as well as provide a difference between operational parameters and information parameters the following two word options are used.
–help Display script help and exit
–version Display single line script version and exit
main : The main function where all the things start. the code should be minimal.
Finish : Clean up code
10 . Standard Body Template : Unless we are writing a small script, we need to use functions to modularise your code and make it more readable, reusable and maintainable. The template for the script with longer code is show below,
#! /usr/bin/env bash
#
# Author: Jagadish manchala <
#
#/ Usage: SCRIPTNAME [OPTIONS]... [ARGUMENTS]...
#/
#/
#/ OPTIONS
#/ -h, --help
#/ Print this help message
#/
#/ EXAMPLES
#/
#{{{ Bash settings
# abort on nonzero exitstatus
set -o errexit
# abort on unbound variable
set -o nounset
# don't hide errors within pipes
set -o pipefail
#}}}
#{{{ Variables
readonly script_name=$(basename "${0}")
readonly script_dir=$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )
IFS=$'\t\n' # Split on newlines and tabs (but not on spaces)
#}}}
main() {
# check_args "${@}"
:
}
#{{{ Helper functions
usage() { }
bootstrap() { }
my_function() { }
finish(){ }
#}}}
main "${@}"
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