Bash – Why there isn’t a new line at the end of quoting a subshell and passing the results to echo

bashcommand-substitutionechoshell

When I do ls | grep png the output of grep is:

2015-05-15-200203_1920x1080_scrot.png 
2015-05-16-025536_1920x1080_scrot.png

(filename,newline,filename,newline)

then, echo $(ls | grep png) outputs:

2015-05-15-200203_1920x1080_scrot.png 2015-05-16-025536_1920x1080_scrot.png

(filename,space from word splitting,filename,newline !!from echo!!)

That is all ok, but when I do this to prevent the word splitting: echo "$(ls | grep png)", the output is:

2015-05-15-200203_1920x1080_scrot.png 
2015-05-16-025536_1920x1080_scrot.png

And my question is, where is the second newline (one should be from grep and one from echo)?

Best Answer

That's newline from echo, you can verify by using echo -n to suppress trailing newline:

echo -n "$(ls | grep png)"

Command substitution remove all trailing newlines, last newline was added by echo, grep has nothing to do here.

Related Solutions

Unexpected Results Testing Serial Loopback Using echo and cat

Thanks to the second comment by Bruce, I was able to figure out the problem on my own.

After running stty -a -F /dev/ttyS1, there were 3 options I found to contribute to the problem: "echo", "onlcr", and "icrnl".

Since this serial port is looped back to itself, here is what happened after running echo "hi" > /dev/ttyS1:

The echo command appends a newline to the end of the message by default, so "hi" + LF is sent out to /dev/ttyS1
Because "onlcr" was set, the serial device converted the LF to CRLF so the physical message sent out the Tx line was "hi" + CRLF
Because "icrnl" was set, the physical messaged received on the Rx line converted the CR to LF. So the message outputted by 'cat' was "hi" + LFLF.
Because "echo" was set, the message received on the Rx ("hi" + LFLF), was then sent back out on the Tx line.
Because of onlcr, "hi" + LFLF became "hi" + CRLFCRLF.
Because of icrnl, "hi" + CRLFCRLF became "hi" + LFLFLFLF
Because of echo, "hi" + LFLFLFLF was then sent out the Tx

And so on...

In order to fix this problem, I ran the following command:

stty -F /dev/ttyS1 -echo -onlcr

Disabling "echo" prevents an infinite loop of messages and disabling "onlcr" prevents the serial device from converting LF to CRLF on output. Now cat receives one "hi" (with a single newline!) for each time I run echo.

CR = carriage return (ASCII 0x0D); LF = line feed or newline (ASCII 0x0A)

Shell – echo variable with content from command substitution

When referencing a variable, it is generally advisable to enclose its name in double quotes. This prevents reinterpretation of all special characters within the quoted string -- except $, ` (backquote), and \ (escape). Keeping $ as a special character within double quotes permits referencing a quoted variable ("$variable"), that is, replacing the variable with its value.

Use double quotes to prevent word splitting. An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators.

e.g.

variable1="a variable containing five words"
COMMAND This is $variable1    # Executes COMMAND with 7 arguments:
# "This" "is" "a" "variable" "containing" "five" "words"

COMMAND "This is $variable1"  # Executes COMMAND with 1 argument:
# "This is a variable containing five words"

Enclosing the arguments to an echo statement in double quotes is necessary only when word splitting or preservation of whitespace is an issue.

For more info and examples go here

Best Answer

Related Solutions

Unexpected Results Testing Serial Loopback Using echo and cat

Shell – echo variable with content from command substitution

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