Scripts

Shebang Line

Source: https://codejunction.hashnode.dev/the-advantage-of-using-usrbinenv-bash-over-binbash

This is the line that goes at the top of the script and tells the shell what to use to execute the script

#!/usr/bin/env bash

If statements

Source: https://linuxize.com/post/bash-if-else-statement/

WARNING!! I ran into syntax errors with fi if I put then on the same line with if (see error message below)

… syntax error near unexpected token `fi’

if TEST-COMMAND1
then
  STATEMENTS1
elif TEST-COMMAND2
then
  STATEMENTS2
else
  STATEMENTS3
fi

#!/bin/bash

echo -n "Enter a number: "
read VAR

if [ $VAR -gt 10 ]
then
  echo "The variable is greater than 10."
elif [ $VAR -eq 10 ]
then
  echo "The variable is equal to 10."
else
  echo "The variable is less than 10."
fi

Until

#!/bin/bash

counter=0

until [ $counter -gt 5 ]
do
  echo Counter: $counter
  ((counter++))
done

Positional arguments

Source: https://www.redhat.com/en/blog/arguments-options-bash-scripts

$0 - the name of the running script and $1 to $9 are positional arguments. The example if for a executable text file called test_script in your current directory.

#!/usr/bin/env bash

echo Running \"$0\"
echo With 1st argument in quotes is \"$1\"
echo And 2nd without quotes this time is: "$2"
echo And 3rd without quotes this time is: $3

Run with ./test_script.sh first second third and it outputs

Running "./test_script.sh"
With 1st argument in quotes is "first"
And 2nd without quotes this time is: second
And 3rd without quotes this time is: third

Case statements

Source: https://linuxize.com/post/bash-case-statement/

case EXPRESSION in

  PATTERN_1)
    STATEMENTS
    ;;

  PATTERN_2)
    STATEMENTS
    ;;

  PATTERN_N)
    STATEMENTS
    ;;

  *)
    STATEMENTS
    ;;
esac

#!/bin/bash

echo -n "Enter the name of a country: "
read COUNTRY

echo -n "The official language of $COUNTRY is "

case $COUNTRY in

  Lithuania)
    echo -n "Lithuanian"
    ;;

  Romania | Moldova)
    echo -n "Romanian"
    ;;

  Italy | "San Marino" | Switzerland | "Vatican City")
    echo -n "Italian"
    ;;

  *)
    echo -n "unknown"
    ;;
esac

Comparing Strings

Source: https://linuxize.com/post/how-to-compare-strings-in-bash/

See link for more info. One note I wanted to add is to ensure you use quotes around your strings especially if they are coming from variables.

Directory exits test (and negation)

The example below does nothing in the positive case and exits on the negative case

if [ -d "$SRC_DIR" ] 
then
    : # Do nothing (was getting trouble with negating the condition)
else
  echo "Error: Source directory does not exist"
  exit 1
fi

No-Op

Source: https://stackoverflow.com/questions/12404661/what-is-the-use-case-of-noop-in-bash

Simply use a :

while keep_waiting; do
  : # do nothing
done

Set script to exit on errors

Source: https://www.gnu.org/software/bash/manual/html_node/The-Set-Builtin.html

set -e

Set script to echo commands

Source: https://stackoverflow.com/questions/2853803/how-to-echo-shell-commands-as-they-are-executed

set -x

For loops

Source: https://opensource.com/article/18/5/you-dont-know-bash-intro-bash-arrays

Example simple application call

allThreads=(1 2 4 8 16 32 64 128)
allRuntimes=()
for t in ${allThreads[@]}; do
  runtime=$(./pipeline --threads $t)
  allRuntimes+=( $runtime )
done

Example Log Alerting

# List of logs and who should be notified of issues
logPaths=("api.log" "auth.log" "jenkins.log" "data.log")
logEmails=("jay@email" "emma@email" "jon@email" "sophia@email")

# Look for signs of trouble in each log
for i in ${!logPaths[@]};
do
  log=${logPaths[$i]}
  stakeholder=${logEmails[$i]}
  numErrors=$( tail -n 100 "$log" | grep "ERROR" | wc -l )

  # Warn stakeholders if recently saw > 5 errors
  if [[ "$numErrors" -gt 5 ]];
  then
    emailRecipient="$stakeholder"
    emailSubject="WARNING: ${log} showing unusual levels of errors"
    emailBody="${numErrors} errors found in log ${log}"
    echo "$emailBody" | mailx -s "$emailSubject" "$emailRecipient"
  fi
done