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shell script

explainshell.com

general

  • To use a variable: ${VARIABLE}, $VARIABLE
  • By convention variables names use uppercase snake case (EXAMPLE_VARIABLE). You should not use dash ('-') because it leads to errors when executing the script.
  • quotes:
    • single quotes ' mark literal values. (echo '$path' #print $path)
    • In double quotes " varriables will be replaced. (echo "$path" # prints the content of the path variable)
  • To print commands executed in a shell script add set -o xtrace to the script or run the script with bash -x <scriptname.sh>. Really helpful for debugging.
  • to save the return value of a command suraound the command in a dollar and braces e.g result=$(l -al)
  • if you want to print a variable add quotes around it: echo "$VARIABLE"
  • you can access the command-arguments/positional-parameter of a script by (incomplete list)
    • $1, $2, ... ($0 is the script name)
    • $argv[1],$argv[2]
    • "$@" array of all positional parameters, {$1, $2, $3 ...}.
    • $# number of positional parameters.
    • $$ pid of the current shell (not subshell).
    • $0 name of the shell or shell script.

handy

set relative paths

Sometimes it should be irrelevant from where you start you script. If the script uses relative references to files you first need to find out in which directory the script is saved and add this directory as a prefix to every relative file reference (which makes it an absolute path). Otherwise the working directory of the shell is used as reference.

#!/bin/bash

# to debug script
set -o xtrace

BASEDIR="$(dirname "$0")"

Filter lists and pipe input

# delete pods which contains example in name
kubectl get pod | grep "example" | awk '{print $1}' | xargs kubectl delete pod

JSON parsing

  • parse a field from a json via grep, awk oder sed
    • grep -Po '"${JSON_FIELD_NAME}": *"\K[^"]*' ${JSON_FILE_PATH}
  • or better yet use jq

curl

  • To send a json file via curl you need
    • to set content type: -H 'Content-Type:application/json'
    • add the file -d @RELATIVE/OR/FULLPATH/example.json or just a string variable -d @"${CONTENT}". Under windows you need to escape the double quotes when using a string variable.
    • complete exmaple: curl -X POST -H 'Content-Type:application/json' -d @RELATIVE/OR/FULLPATH/example.json $URL
  • to show error but no progress bar run curl: curl -sS http://. -s for silent mode (suppress progress bar and error) and -S for explicitly showing error.
  • to download stuff follow redirects and set output name: curl -L https://github.com/stedolan/jq/releases/download/jq-1.6/jq-linux64 -o /usr/bin/jq
  • Check firewall: curl --connect-timeout 10 --show-error bla.de. Connection error 28 indicates that a firewall blocks the call

string replacement

  • to replace the first occurrence of a pattern in a string use: "${stringVarible/patternValue/$replaceValueVariable}"
  • to replace all occurrences of a pattern in a string use: "${stringVarible//patternValue/$replaceValueVariable}"