python -c vs python -<< heredoc
Question:
I am trying to run some piece of Python code in a Bash script, so i wanted to understand what is the difference between:
#!/bin/bash
#your bash code
python -c "
#your py code
"
vs
python - <<DOC
#your py code
DOC
I checked the web but couldn’t compile the bits around the topic. Do you think one is better over the other?
If you wanted to return a value from Python code block to your Bash script then is a heredoc the only way?
Answers:
The main flaw of using a here document is that the script’s standard input will be the here document. So if you have a script which wants to process its standard input, python -c is pretty much your only option.
On the other hand, using python -c '...' ties up the single-quote for the shell’s needs, so you can only use double-quoted strings in your Python script; using double-quotes instead to protect the script from the shell introduces additional problems (strings in double-quotes undergo various substitutions, whereas single-quoted strings are literal in the shell).
As an aside, notice that you probably want to single-quote the here-doc delimiter, too, otherwise the Python script is subject to similar substitutions.
python - <<'____HERE'
print("""Look, we can have double quotes!""")
print('And single quotes! And `back ticks`!')
print("$(and what looks to the shell like process substitutions and $variables!)")
____HERE
As an alternative, escaping the delimiter works identically, if you prefer that (python - <<____HERE)
If you prefer to use python -c '...' without having to escape with the double-quotes you can first load the code in a bash variable using here-documents:
read -r -d '' CMD << '--END'
print ("'quoted'")
--END
python -c "$CMD"
The python code is loaded verbatim into the CMD variable and there’s no need to escape double quotes.
If you are using bash, you can avoid heredoc problems if you apply a little bit more of boilerplate:
python <(cat <<EoF
name = input()
print(f'hello, {name}!')
EoF
)
This will let you run your embedded Python script without you giving up the standard input. The overhead is mostly the same of using cmda | cmdb. This technique is known as Process Substitution.
If want to be able to somehow validate the script, I suggest that you dump it to a temporary file:
#!/bin/bash
temp_file=$(mktemp my_generated_python_script.XXXXXX.py)
cat > $temp_file <<EoF
# embedded python script
EoF
python3 $temp_file && rm $temp_file
This will keep the script if it fails to run.
How to use here-docs with input
tripleee’s answer has all the details, but there’s Unix tricks to work around this limitation:
So if you have a script which wants to process its standard input, python -c is pretty much your only option.
This trick applies to all programs that want to read from a redirected stdin (e.g., ./script.py < myinputs) and also take user input:
python - <<'____HERE'
import os
os.dup2(1, 0)
print(input("--> "))
____HERE
Running this works:
$ bash heredocpy.sh
--> Hello World!
Hello World!
If you want to get the original stdin, run os.dup(0) first. Here is a real-world example.
This works because as long as either stdout or stderr are a tty, one can read from them as well as write to them. (Otherwise, you could just open /dev/tty. This is what less does.)
In case you want to process inputs from a file instead, that’s possible too — you just have to use a new fd:
Example with a file
cat <<'____HERE' > file.txt
With software there are only two possibilites:
either the users control the programme
or the programme controls the users.
____HERE
python - <<'____HERE' 4< file.txt
import os
for line in os.fdopen(4):
print(line.rstrip().upper())
____HERE
Example with a command
Unfortunately, pipelines don’t work here — but process substitution does:
python - <<'____HERE' 4< <(fortune)
import os
for line in os.fdopen(4):
print(line.rstrip().upper())
____HERE
I am trying to run some piece of Python code in a Bash script, so i wanted to understand what is the difference between:
#!/bin/bash
#your bash code
python -c "
#your py code
"
vs
python - <<DOC
#your py code
DOC
I checked the web but couldn’t compile the bits around the topic. Do you think one is better over the other?
If you wanted to return a value from Python code block to your Bash script then is a heredoc the only way?
The main flaw of using a here document is that the script’s standard input will be the here document. So if you have a script which wants to process its standard input, python -c is pretty much your only option.
On the other hand, using python -c '...' ties up the single-quote for the shell’s needs, so you can only use double-quoted strings in your Python script; using double-quotes instead to protect the script from the shell introduces additional problems (strings in double-quotes undergo various substitutions, whereas single-quoted strings are literal in the shell).
As an aside, notice that you probably want to single-quote the here-doc delimiter, too, otherwise the Python script is subject to similar substitutions.
python - <<'____HERE'
print("""Look, we can have double quotes!""")
print('And single quotes! And `back ticks`!')
print("$(and what looks to the shell like process substitutions and $variables!)")
____HERE
As an alternative, escaping the delimiter works identically, if you prefer that (python - <<____HERE)
If you prefer to use python -c '...' without having to escape with the double-quotes you can first load the code in a bash variable using here-documents:
read -r -d '' CMD << '--END'
print ("'quoted'")
--END
python -c "$CMD"
The python code is loaded verbatim into the CMD variable and there’s no need to escape double quotes.
If you are using bash, you can avoid heredoc problems if you apply a little bit more of boilerplate:
python <(cat <<EoF
name = input()
print(f'hello, {name}!')
EoF
)
This will let you run your embedded Python script without you giving up the standard input. The overhead is mostly the same of using cmda | cmdb. This technique is known as Process Substitution.
If want to be able to somehow validate the script, I suggest that you dump it to a temporary file:
#!/bin/bash
temp_file=$(mktemp my_generated_python_script.XXXXXX.py)
cat > $temp_file <<EoF
# embedded python script
EoF
python3 $temp_file && rm $temp_file
This will keep the script if it fails to run.
How to use here-docs with input
tripleee’s answer has all the details, but there’s Unix tricks to work around this limitation:
So if you have a script which wants to process its standard input,
python -cis pretty much your only option.
This trick applies to all programs that want to read from a redirected stdin (e.g., ./script.py < myinputs) and also take user input:
python - <<'____HERE'
import os
os.dup2(1, 0)
print(input("--> "))
____HERE
Running this works:
$ bash heredocpy.sh
--> Hello World!
Hello World!
If you want to get the original stdin, run os.dup(0) first. Here is a real-world example.
This works because as long as either stdout or stderr are a tty, one can read from them as well as write to them. (Otherwise, you could just open /dev/tty. This is what less does.)
In case you want to process inputs from a file instead, that’s possible too — you just have to use a new fd:
Example with a file
cat <<'____HERE' > file.txt
With software there are only two possibilites:
either the users control the programme
or the programme controls the users.
____HERE
python - <<'____HERE' 4< file.txt
import os
for line in os.fdopen(4):
print(line.rstrip().upper())
____HERE
Example with a command
Unfortunately, pipelines don’t work here — but process substitution does:
python - <<'____HERE' 4< <(fortune)
import os
for line in os.fdopen(4):
print(line.rstrip().upper())
____HERE