bhavanki/Bash.markdown

## Bash.markdown

      
    Raw
  

              Bash.markdown
            
          
    Read the lines output from a command into an array variable

readarray -t var < <(command)

var=()
while IFS= read -r line; do
  var+=( "$line" )
done < <(command)

Read the single line output from a command into an array variable

IFS=" " read -r -a var <<< "$(command)"

Read the lines of a file into an array variable

readarray -t var < /path/to/file

var=()
while IFS= read -r line; do
  var+=( "$line" )
done < /path/to/file

Feed string contents to a command (here string)

bc <<< "scale=4;$used/$available"

Parameter substitution

https://www.tldp.org/LDP/abs/html/parameter-substitution.html
${name}          # the value
${name-default}  # the value if set, or else default
${name:-default} # the value if set and non-empty, or else default
${name=default}  # the value if set, or else default and assign it
${name:=default} # the value if set and non-empty, or else default
                 # and assign it
${name+alt}      # alt if set, or else empty
${name:+alt}     # alt if set and non-empty, or else default
${name?msg}      # the value if set, or else echo msg and exit 1
${name:?msg}     # the value if set and non-empty, or else echo msg
                 # and exit 1

${#name}         # length of value or first element in array
${name#pattern}  # value with shortest pattern removed from front
${name##pattern} # value with longest pattern removed from front
${name%pattern}  # value with shortest pattern removed from back
${name%%pattern} # value with longest pattern removed from back

${name:pos}      # value starting at offset pos
${name:pos:len}  # value starting at offset pos for length len
${name/pat/rep}  # value replacing first pat with rep
${name//pat/rep} # value replacing all pat with rep
${name/#pat/rep} # value replacing prefix pat with rep
${name/%pat/rep} # value replacing suffix pat with rep

${!name}         # value of variable named ${name} (indirect)
${!n*}           # names of variables starting with n
${!n@}           # same

Fonts

Colors: https://www.shellhacks.com/bash-colors/
RED="\033[0;31m"
DEFAULT="\033[0m"
echo -e "${RED}Red text${DEFAULT}"

Font decorations:

0 = normal
1 = bold
4 = underlined
5 = blinking
7 = reverse video

Colors (fg=3x, bg=4x):

30 = black -> dark gray (bolded)
31 = red -> light red
32 = green -> light green
33 = brown -> yellow
34 = blue -> light blue
35 = purple -> light purple
36 = cyan -> light cyan
37 = light gray -> white

Define a multiline string

read -r -d '' VAR <<EOF
contents
of
string
EOF

When set -e is on, read returns 1 here and will exit the script.
Read from the command line into a variable

read -p 'Enter the thing: ' var
echo "You entered $var"


-r causes backslashes to be literal - shellcheck wants it
-s stops echoes
-n x returns after x characters automatically

Get yes/no from the command line

read -p "Did you run all relevant unit tests? [y|n] " -n 1 -r < /dev/tty
echo
if echo "$REPLY" | grep -v -E '^[Yy]$' > /dev/null; then
    echo "You said no"
else
    echo "You said yes"
fi

Write long string to a file

cat >> path/to/file <<EOF
file
contents
here
EOF

Write long string from file to file, with string interpolation

eval "cat <<EOF
$(<path/to/fromfile)
EOF
" > path/to/tofile

Associative arrays (bash 4+)

declare -A aa
aa[foo]=bar
aa=([foo]=bar ...)
aa+=([baz]=blat ...)

echo ${aa[foo]}
for i in "${!aa[@]}" # each key
do
  echo "key  : $i"
  echo "value: ${aa[$i]}"
done
if [ ${aa[foo]+_} ]; then echo foo is there; fi # param expansion
if [[ -z ${aa[foo]:+_} ]]; then echo foo is not there; fi
echo Size is "${#aa[@]}"

unset aa[foo]

Temporary files

For a directory
WORKDIR=$(mktemp -d)
trap "rm -rf "'"'"${WORKDIR}"'"' EXIT

For a file
WORKFILE=$(mktemp -t prefix)
trap "rm -f "'"'"${WORKFILE}"'"' EXIT

Regex matching

[[ $string =~ tar\.gz$ ]] && echo "It's a tarball"

man 3 regex gives you the full syntax. So you can use [[:space:]] for whitespace, for example.
Or you can use globs with ==
[[ $string == *tar.gz ]] && echo "It's a tarball"

Get directory containing script

DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd)"

Check if array contains a value

See https://stackoverflow.com/questions/3685970 for alternatives when elements have spaces in them.
if [[ " ${array[@]} " =~ " ${value} " ]]; then
    # whatever you want to do when array contains value
fi

contains() {
  local el
  local target="$1"
  shift
  for el; do
    if [[ "$el" == "$target" ]]; then
      return 0
    fi
  done
  return 1
}

Split a string into a list

IFS=',' read -ra MY_ARR <<< "this,that,other"
Sort an array

readarray -t sorted < <(printf '%s\n' "${array[@]}" | sort)
readarray -t sorted < <(printf '%s\0' "${array[@]}" | sort -z | xargs -0n1) # to cope with newlines
How to check for the presence of a command

if ! command -v xyz; then
    fail
fi
Can use hash for regular commands, type for built-ins and keywords
Conditional expressions


-a file = file exists
-b file = file exists and is block special
-c file = file exists and is character special
-d file = file exists and is directory
-e file = file exists
-f file = file exists and is regular
-g file = file exists and has set-group-id bit set
-h file = file exists and is symlink
-k file = file exists and has sticky bit set
-p file = file exists and is named pipe / fifo
-r file = file exists and is readable
-s file = file exists and has size greater than zero
-t fd = fd is open and refers to terminal
-u file = file exists and has set-user-id bit set
-w file = file exists and is writable
-x file = file exists and is executable
-G file = file exists and owned by effective group id
-L file = file exists and is symlink
-N file = file exists and was modified since last read
-O file = file exists and owned by effective user id
-S file = file exists and is socket
file1 -ef file2 = refer to same device and inode
file1 -nt file2 = file1 newer than file2, or file1 exists and file2 doesn't
file1 -ot file2 = file1 older than file2, or file2 exists and file1 doesn't
-o optname = shell option enabled
-v varname = shell variable set
-R varname = shell variable set to name reference
-z string = string has zero length
-n string = string has non-zero length
string = string has non-zero length
string1 == string 2 = strings are equal
string1 = string2 = strings are equal
string1 != string2 = strings are not equal
string1 < string2 = string1 lexicographically before
string1 > string2 = string1 lexicographically after
int1 [-eq|-ne|-lt|-le|-gt|-ge] int2 = int comparison