bash_basic.rst

Bash Basic cheatsheet

Table of Contents

• :
• Special Parameters
• Set Positional Parameters
• Brace Expansion
• Variable Expansion
• String length
• String Slice
• Delete Match String
• Here Documents
• Here Strings
• Logger
• Check Command Exist
• Read a File Line by Line
• Read a File field wise
• Dictionary
• Check if a Key Exists in a Dictionary
• Remove a Key-Value from a Dictionary
• Append Elements to an Array
• Prompt
• Parse Arguments

:

true was instead simply aliased to :, and false like let 0.

$ while :; do sleep 1; date; done

Special Parameters

#!/bin/bash

foo() {
  # expand to the position params, equal to "$1$2..."
  echo $*
  # expand to the position params, equal to "$1" "$2" ...
  echo $@
  # expand to the number of position params
  echo $#
  # expand to the pid
  echo $$
  # expand to the exit status
  echo $?
  # expand to the name of shell script
  echo $0
}

foo "a" "b" "c"

Set Positional Parameters

$ set -- a b c
$ echo $@
a b c
$ echo "$1" "$2" "$3"
a b c

Brace Expansion

$ echo foo.{pdf,txt,png,jpg}
foo.pdf foo.txt foo.png foo.jpg

Variable Expansion

$ foo1="foo1"
$ foo2="foo2"

# expand to "$foo1 foo2"
$ echo "${!foo*}"

# expand to "$foo1" "$foo2"
$ echo "${!foo@}"

String length

echo ${#foo}
7

String Slice

$ foo="01234567890abcdefg"

# ${param:offset}
$ echo ${foo:7}
7890abcdefg

$ echo ${foo: -7}
abcdefg
$ echo ${foo: -7:2}
ab

# ${param:offset:length}
$ echo ${foo:7:3}
789

Delete Match String

$ foo="123,456,789"
# ${p##substring} delete longest match of substring from front
$ echo ${foo##*,}
789

# ${p#substring} delete shortest match of substring from front
echo ${foo#*,}
456,789

# ${p%%substring} delete longest match of substring from back
$ echo ${foo%%,*}
123

$ echo ${foo%,*}
123,456

Other examples

disk="/dev/sda"
$ echo ${disk##*/}
sda

$ disk="/dev/sda3"
echo ${disk%%[0-9]*}
/dev/sda

Here Documents

cat <<EOF
    Hello Document
EOF

Here Strings

# CMD <<< $w, where $w is expanded to the stdin of CMD

bc <<< "1 + 2 * 3"

Logger

REST='\e[0m'
RED='\e[1;31m'
GREEN='\e[1;32m'
YELLOW='\e[1;33m'
CYAN='\e[1;36m'

info() {
  echo -e "[$(date +'%Y-%m-%dT%H:%M:%S%z')][${GREEN}info${REST}] $*"
}

debug() {
  echo -e "[$(date +'%Y-%m-%dT%H:%M:%S%z')][${CYAN}debug${REST}] $*"
}

warn() {
  echo -e "[$(date +'%Y-%m-%dT%H:%M:%S%z')][${YELLOW}warn${REST}] $*" >&2
}

err() {
  echo -e "[$(date +'%Y-%m-%dT%H:%M:%S%z')][${RED}error${REST}] $*" >&2
}

Check Command Exist

cmd="tput"
if command -v "${tput}" > /dev/null; then
  echo "$cmd exist"
else
  echo "$cmd does not exist"
fi

Read a File Line by Line

#!/bin/bash

file="file.txt"
while IFS= read -r l; do echo $l; done < "$file"

Read a File field wise

#!/bin/bash

file="/etc/passwd"
while IFS=: read -r n _ _ _ _ _ _; do echo $n; done < "$file"

Dictionary

#!/bin/bash

declare -A d
d=( ["foo"]="FOO" ["bar"]="BAR" )
d["baz"]="BAZ"

for k in "${!d[@]}"; do
  echo "${d[$k]}"
done

Check if a Key Exists in a Dictionary

#!/bin/bash

declare -A d
d["foo"]="FOO"
if [ -v "d[foo]" ]; then
  echo "foo exists in d"
else
  echo "foo does exists in d"
fi

Remove a Key-Value from a Dictionary

$ declare -A d
$ d["foo"]="FOO"
$ unset d["foo"]

Append Elements to an Array

#!/bin/bash

arr=()

for i in "a b c d e"; do
  arr+=($i)
done

echo "${arr[@]}"

Prompt

#!/bin/bash

read -p "Continue (y/n)? " c
case "$c" in
  y|Y|yes) echo "yes" ;;
  n|N|no) echo "no" ;;
  *) echo "invalid" ;;
esac

Parse Arguments

#!/bin/bash

program="$1"

usage() {
  cat <<EOF

Usage:  $program [OPTIONS] params

Options:

  -h,--help                show this help
  -a,--argument string     set an argument

EOF
}

arg=""
params=""
while (( "$#" )); do
  case "$1" in
        -h|-\?|--help) usage; exit 0 ;;
        -a|--argument) args="$2"; shift 2 ;;
        # stop parsing
        --) shift; break ;;
        # unsupport options
        -*|--*=) echo "unsupported option $1" >&2; exit 1 ;;
        # positional arguments
        *) params="$params $1"; shift ;;
  esac
done