bash scripting

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bash scripting

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Proper escaping - Everything is a string

Accessing values

everything-is-a-string.sh

#!/bin/bash

a="some string"

# this works, but I don't like it:
echo $a

# This is how I do it. Curly braces for string values:
echo "${a}"

Executing a command

run-a-command.sh

#!/bin/bash
# Running a command with backticks, but I don't like that:
fileContent=`cat 1.txt`

# Running a command with $( xxx )
fileContent="$( cat 1.txt )"
fileContent="$( echo "Hi" > 1.txt ; cat "./1.txt" ; rm ./1.txt )"

Defining variables

Creating and accessing an array in bash

array.sh

#!/bin/bash
declare releaseNames=( "api-poi" "api-trip" "api-user" "api-user-java" )

echo "The array contains ${#releaseNames[@]} elements:"

for releaseName in "${releaseNames[@]}"; do
echo "  -  ${releaseName}"
done

Creating and accessing a hashmap / dict bash

declare -A helmValues
helmValues["a"]="val a"
helmValues["b"]="val b"

echo "a: '${helmValues["a"]}'"
echo "b: '${helmValues["b"]}'"

Determine the directory of the script

show:dir.sh

#!/bin/bash
d="$( dirname "$( readlink -f "$0" )" )"
echo "Running in directory ${d}"

results in

$ pwd
/mnt/c/github/chgeuer/tips

$ ./show_dir.sh
Running in directory /mnt/c/github/chgeuer/tips

Span a command across multiple lines

Use a baskslash (\) at the end of the line (no additional whitespace), and preferably indent the next line:

multi-line.sh

#!/bin/bash
response="$( cat payload.xml | curl \
  --silent --include \
  --request POST \
  --url "${triggerURI}" \
  --header "Content-Type: application/xml" \
  --data @- )"

echo "${response}"

Lambda-style functions in bash

health-probe.sh

#!/bin/bash

#
# Define some function
#
function httpStatus {
  local url="$1"

  echo "$( curl \
  --silent \
  --output /dev/null \
  --write-out '%{http_code}' \
   "${url}" )"
}

function httpStatus2 { echo "$( curl --silent --output /dev/null --write-out '%{http_code}' $1 )" ; }

echo "Azure:    $( httpStatus "https://portal.azure.com/" )"
echo "Homepage: $( httpStatus2 "https://www.microsoft.com/" )"

results in

$ ./health-probe.sh
Azure:    200
Homepage: 200

$

Creating a text file

The cat > x <<-EOF ... EOF syntax allows to create a file in the local directory. Please not that the lines 6 and 7 below (the content) are prefixed with a tabstop (), which does not show up in the actual text file.

#!/bin/bash

SERVER_IP="127.0.0.1"

cat > somefile.ini <<-EOF
	server=${SERVER_IP}
	port=5093
EOF

base64-encode a text

The command base64 --wrap=0 converts input into a long base64-encoded string without line breaks.

#!/bin/bash

FILE_CONTENT="$( cat ./foo.bin )"
ONE_LONG_BASE64_STR=$(echo "${FILE_CONTENT}" | base64 --wrap=0)

Bash history

Put the following lines in ~/.inputrc:

## arrow up
"\e[A":history-search-backward
## arrow down
"\e[B":history-search-forward