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I have a file named my_file.txt whose content is just the string Hello. How could I redirect its content to the command echo?

I know I have the commands less, cat, more... but I need to do it with echo.

I tried this: 

$ cat my_file.txt | echo

and also this:

$ echo < my_file.txt

But in both cases it appears only a blank in the stdout, not the content of my_file.txt.

How could I do that?

Seanny123
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danielmbcn
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    Why do you insist on using `echo`? – Kevin Feb 04 '13 at 14:53
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    @Kevin, presumably eventually he wants to expand the ANSI C escape sequences (`\n`, `b`...) in the file, which would be a valid usage of `echo` (at least a standard `echo`). Or presumably he wants to understand why it doesn't work that way. In any case, no reason to downvote IMO. – Stéphane Chazelas Apr 02 '13 at 06:38
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    I came here because I was curious about why it doesn't work when I do echo < file.txt. Seems a reasonable question to have at SE, the answer was very helpful. +1 from me. – neuronet Jun 16 '16 at 15:01
  • I have a use case for this. It looks to me like `echo` and `printf` don't handle `stdin`, unfortunately, that's why I'm looking, but I've got some text output from a command chain with embedded `"\n"` characters and I'd like to just print it properly. Is it so hard to imagine a use case where someone wants to print text to console? – NeilG Aug 23 '23 at 06:01

6 Answers6

113

You can redirect all you want to echo but it won't do anything with it. echo doesn't read its standard input. All it does is write to standard output its arguments separated by a space character and terminated by a newline character (and with some echo implementations with some escape sequences in them expanded and/or arguments starting with - possibly treated as options).

If you want echo to display the content of a file, you have to pass that content as an argument to echo. Something like:

echo "$(cat my_file.txt)"

Note that $(...) strips the trailing newline characters from the output of that cat command, and echo adds one back.

Also note that except with zsh, you can't pass NUL characters in the arguments of a command, so that above will typically not work with binary files. yash will also remove bytes that don't form part of valid characters.

If the reason for wanting to do that is because you want echo to expand the \n, \b, \0351... escape sequences in the file (as UNIX conformant echo implementations do, but not all), then you'd rather use printf instead:

printf '%b\n' "$(cat my_file.txt)"

Contrary to echo, that one is portable and won't have problems if the content of the file starts with -.

As an alternative to $(cat file), with ksh, zsh and bash, one can also do: $(<file). That's a special operator whereby the shell as opposed to cat reads the content of the file to make up the expansion. It still strips the trailing newlines and chokes on NUL bytes except in zsh. In bash, that still forks an extra process. Also note that one difference is that you won't get any error if trying to read a file of type directory that way. Also, while $(< file) is special, $(< file; other command) is not (in zsh, when not emulating other shell, that would still expand the content of the file, by running the implicit $READNULLCMD command (typically a pager)).

Stéphane Chazelas
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  • But how to repeat standard input to standard output while interpreting special characters? – NeilG Aug 23 '23 at 06:03
  • @NeilG This seems like a related but different question. If no other question on the site handles this, I would suggest you ask a brand new question about it, including example data. – Kusalananda Aug 23 '23 at 07:03
  • I was thinking about it when I found the answer among those for this question @Kusalananda. Take a look. – NeilG Aug 23 '23 at 13:08
  • @NeilG, well, just `printf '%b\n' "$(cat)"` or for more than once: `var=$(cat); printf '%b\n' "$var" "$var" "$var"...`, or use the `printf '%1$b\n%1$b...\n' "$(cat)"` supported by many `printf` implementations. – Stéphane Chazelas Aug 23 '23 at 15:56
  • Lol, @StéphaneChazelas, thanks but I think I'll just stick with `| xargs echo -e`! GNU bash, version 5.1.16(1)-release. But that's an interesting use of `cat`. I'll try some variations on that too. – NeilG Aug 24 '23 at 00:31
  • @NeilG, as already noted, you can't use `xargs` unless you use the `-0` GNU extension as `xargs` mangles quotes, whitespaces **and backslash**. It will also fail for strings like `-nene` and for long strings as there's a limit (low on some systems) on the size of the arguments that can be passed to an external command. – Stéphane Chazelas Aug 24 '23 at 05:31
  • Good to know @StéphaneChazelas . The command line length limit was what was bothering me most. I'm actually using `$(cat)` now too, as it's cooler and more conventional at the same time. – NeilG Aug 24 '23 at 06:38
17

Adding to other answers, you can also try input redirection instead of cat

echo "$(<my_file.txt)"

or

echo "`<my_file.txt`"

$(...) and `...` perform the same thing, i.e command substitution. It runs commands inside of it's scope and gives the output to the shell as if it was given as an input by the user. The only difference between these two is that $(...) can be nested simply and `...` needs to be escaped to be nested.

The quotes around command substitution preserve whitespaces

GypsyCosmonaut
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    `\`...\`` can be recursive but with a much more awkward syntax. You must also watch out for backslashes inside them (even inside single quotes), and `"` if quoted. In any case with either syntax, they should be quoted when in list context unless you want split+glob to be applied to them. Note that `$( – Stéphane Chazelas Mar 26 '21 at 07:39
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    Thank you! This is extremely helpful for debuging when your system SSD/HDD crashed and `cat` can't be loaded any more. For example on a Raspberry Pi that browned out due to insufficent voltage `cat /sys/devices/platform/soc/soc:firmware/get_throttled` yields `bash: cat: command not found` But the shell built-in `echo \` – user643011 Oct 12 '21 at 01:28
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As it was said,

If you want echo to display the content of a file, you have to pass that content as an argument to echo

For instance, you can do it like this with xargs (considering that you need to enable interpretation of backslash escapes):

$ cat my_file.txt | xargs echo -e

Or simply what you've asked (whithout interpretation of escapes sequences):

$ cat my_file.txt | xargs echo
kenichi
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    Note however that when using `xargs echo` (same as `xargs` btw as `echo` is the default command), it's the `echo` command from the file system that will be run, not the shell's builtin `echo` command. Also note that `xargs` expects its input in a very specific format (where blanks, newlines, quotes, backslashes have special meaning). `cat` doesn't make much sense here as there's only one file to concatenate (same as `< file.txt xargs echo`. Note that Unix compliant `echo`s do backslash escape expansion by default and output `-e` upon `echo -e`. – Stéphane Chazelas May 08 '17 at 08:39
  • Of course! `xargs`! Yes! `command | command | command | xargs echo -e` will take the standard output of your command chain and pipe it to xargs which will then arrange it as parameters to echo which will print the special characters! Thank you! – NeilG Aug 23 '23 at 06:06
  • @NeilG But note that `xargs` does special processing of quotes, and that not all external implementations of `echo` may have the non-standard `-e` option. – Kusalananda Aug 23 '23 at 07:19
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Simple answer: you can't. echo (be it shell built-in or regular binary) doesn't process its standard input - it is one way only.

peterph
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  • Question is, why would you actually want to do this? – peterph Feb 04 '13 at 19:14
  • please refer to multiple comments and text in the other answers to find out why. – NeilG Aug 23 '23 at 06:06
  • Yes you can! Prefix `echo` with `xargs`! Well, I know, strictly you can't; but that has a similar effect that may solve some problem scenarios that need this behaviour. – NeilG Aug 23 '23 at 06:07
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If you type

echo < my_file.txt

you are redirecting the contents of ./my_file.txt to the standard input (STDIN) of echo. It doesn't print anything out, though. It happens because echo doesn't read from the standard input, that is, it doesn't give a damn for what is within your STDIN.

If you really want that echo reads from STDIN, you must use xargs, that is,

xargs echo < my_file.txt
  • xargs forces echo to read from the STDIN rather than its arguments
  • < changes my_file.txt from an argument to the source content path of the STDIN

Now it will prints out "Hello".

Rubem Pacelli
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    Note that both facts have been mentioned in other answers already (the fact that `echo` does not read its standard input, and that you can use `xargs`). Also note that `echo` happens to be the default utility that `xargs` executes (i.e, shorter: `xargs – Kusalananda Jan 20 '23 at 15:32
  • @Kusalananda although these facts have been mentioned previously, none of answers here gave my solution (Stéphane has reached close it, though). That is right that the usage of `echo` is optional. I preferred to leave it for the sake of clarity. The issues concerning `xargs` are leave out since it is out of the scope of the question. – Rubem Pacelli Jan 20 '23 at 15:41
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In bash, also the following should work:

echo `cat my_file.txt`

The part in backticks is replaced by the output of that command, i.e. in this case replaced by the file contents.

Motlib
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    True, backticks work like `$( . . . )`. People who use `$()` usually prefer them because you can nest them. This question is old, though, it's much more satisfying to answer questions that are not already answered! – Law29 Jul 01 '16 at 09:12
  • Not quite. There is a subtle difference. When the back-tick form is used, a backslash retains its literal meaning except when followed by $, `, or \. The first back-tick not preceded by a backslash terminates the command. When using the $(,,,) form, all characters between the parentheses make up the command. – fpmurphy Jul 01 '16 at 11:29
  • As that command substitution is not quoted, the split+glob operator would be applied to it. For instance if `my_file.txt` contains `*`, that would print the list of file names in the current directory. – Stéphane Chazelas May 08 '17 at 08:41