Bash Cheatsheet

bash_cheatsheet.md

Bash Cheatsheet

Header

use bash

#!/bin/bash

Safety

# -e          - exit when a command fails (b/c e.g. it doesn't exist). Use ` || true` to override this.
# -u          - catch unset variables
# -o pipefail - piping bad commands into other commands will now fail
# -E          - trap
set -Eeuo pipefail

Control Flow

If

if [[ <some test> ]]
then
  <commands>
else
  <other commands>
fi

Case

case $my_var in
  ecom) echo "$ZV_ECOM";;
  cg)   echo "$ZV_CG"  ;;
  jp)   echo "$ZV_JP"  ;;
  *)    return 1
esac

Parse Args

getopts

# single-character flags only
# flags that take an arg have a `:` after then
# Begin the opts string with a colon (e.g. ':habs') in order to suppress error message
#
# If using this in a function, you need to add `OPTIND=1` before this
while getopts "habs:" opt; do
  case $opt in
    h)
      $COMOTO_CLI_LIB/render_help
      return 0
      ;;
    a)
      do_ansible=1
      ;;
    b)
      do_remove_redline_build=1
      ;;
    s)
      what_to_bounce=$OPTARG
      ;;
    *) echo "$0: error" >&2; return 1
  esac
done

• If sourcing a script (i.e. not an executable), you'll have to set OPTIND=1.

Manual Arg Parsing

while :; do
  case "${1-}" in
    -h|--help) $COMOTO_CLI_LIB/render_help; return 0;;
    --opt-with-val) my_val=$2; shift;;
    -t) text_only='true' ;;
    *) break ;;
  esac
  shift
done

getopt https://www.baeldung.com/linux/bash-parse-command-line-arguments#parsing-long-command-line-options-with-getopt

If the args are all flags (i.e. they don't take params), do this:

for arg in "$@"
do
  case $arg in
    -h|--help) $COMOTO_CLI_LIB/render_help; return 0;;
    -c) confirmation_option='-c';;
    -d) _add_db_migrate_task 'down';;
    -l) _add_db_migrate_task 'lock';;
    -u) _add_db_migrate_task 'up';;
    *)
      >&2 echo invalid input! "($1)"
      _comoto_cli_help
      return 1
      ;;
  esac
done

Executable

chmod u+x <filename>

Test

if [[ $# -eq 0 ]]; then
  # do something if no args
fi

User Input

Confirmation

while true; do
  read -p "Do you want to proceed? [Y/n] " yn
  case $yn in
      [Yy]* ) return 0;;
      [Nn]* ) return 1;;
      * ) echo "Please answer yes or no.";;
  esac
done

Arrays

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

# initialize
myArray=(string1 string2 three 4)
emptyArray=()

# Index access
# - zero-indexed
# - curly braces required
echo ${myArray[1]}

# print all elements
echo ${myArray[@]}

# loop over elements
for e in ${myArray[@]}
do
  echo $e
done

# Add elements
myArray+=(five 6)

List all commands

compgen -c