Here Doc, Here Strings

1. Here Doc

How to use Here Document in bash programming - GeeksforGeeksGeeksforGeeks

1.1 Basic syntax

The here document (<<) can be used to pass a multi-line block of text or code to an interactive command

[COMMAND] << 'DELIMITER'
    HERE-DOCUMENT
DELIMITER

1.11 Example with cat

cat << END
'THIS BUNCH OF TEXT WOULD BE SHOWN'
END

1.12 Example with python3

Explanation of Python3 features and functions used

a. python3 -

When the script name is given as '-', it refers to the standard input

2. Using the Python InterpreterPython documentation

b. sys.stdout.write

• Writes data to standard output (stdout)

sys — System-specific parameters and functionsPython documentation

python3 - << 'EOF' 
print("Hello here doc!")
EOF

• Prints Hello here doc!

python3 - << 'EOF' | cat
import sys
sys.stdout.write("12345678")
EOF

• The here-document will feed text into stdin of python3

• The pipe (|) performs the following: stdout of python3 -> stdin of cat

  • Prints 12345678

python3 - << 'EOF' >> outfile
import sys
sys.stdout.write("12345678")
EOF

• The here-document will feed text into stdin of python3

• The redirection (>>) will append the stdout of python3 to the file outfile

  • Appends 12345678 to the file

$ cat outfile
12345678

1.13 Example with while loop + read

Arr=(0 1 2 {A..D}) #  (0 1 2 A B C D)

while read element ; do
  echo "$element" 
done <<< $(echo ${Arr[*]}) 

# output: 0 1 2 A B C D

• Print each value in the array from stdout

  • echo prints to the stdout (fd 1)

Arr=(0 1 2 {A..D}) #  (0 1 2 A B C D)

while read element ; do
  echo "$element" 1>&2 # >&2 works too since `>` defaults to stdout (fd 1)
done <<< $(echo ${Arr[*]}) 

# output: 0 1 2 A B C D

• Print each value in the array from stderr

  • 1>&2 redirects stdout to stderr

Arr=(0 1 2 {A..D}) #  (0 1 2 A B C D)

while read element ; do
  echo "$element" 
done <<< $(echo ${Arr[*]}) >> outfile

# output: empty

• >> outfile append stdout to the outfile file

  • The file outfile will contain the value: 0 1 2 A B C D

Arr=(0 1 2 {A..D}) #  (0 1 2 A B C D)

while read element ; do
  echo "$element" 1>&2 # >&2 works too since `>` defaults to stdout (fd 1) 
done <<< $(echo ${Arr[*]}) >> outfile

# output: 0 1 2 A B C D

• Notice that even with the >> outfile command the data is not actually written to the file, but instead printed on the terminal

  • This is due to the 1>&2 command redirecting stdout to stderr

  • The value in stderr will be printed on the terminal

• The file outfile will be empty

# without 1>&2
while read element ; do
  echo "$element"  
done <<< $(echo ${Arr[*]}) &>/dev/null

# output: empty

while read element ; do
  echo "$element" 1>&2
done <<< $(echo ${Arr[*]}) &>/dev/null

# output: empty

• Notice that the output is empty

  • This is due to the &>/dev/null command at the end which redirect all the stout and stderr to the /dev/null file

/dev/null | Linuxjarrettgxz-sec.gitbook.io

2. Here Strings

Shell Scripting - Here Strings - GeeksforGeeksGeeksforGeeks

2.1 Basic syntax

The here string (<<<) command is used for input redirection from a text or variable

[COMMAND] <<< 'string' # from text
[COMMAND] <<< $DATA # from variable

2.12 Input redirection from a text

• Both commands shown below are equivalent

  • outputs hr

echo "here" | tr -d 'e' 
tr -d 'e' <<< "here" 

2.13 Input redirecton from a variable

DATA="here"

echo $DATA | tr -d 'e' 
tr -d 'e' <<< $DATA