"Echo" Output Different Answer by Sh and Bash

echo output different answer by sh and bash

echo is not very portable (even Bash's echo may behave differently on different OSes which may use different default options when compiling Bash). You can use printf. According to posix:

It is not possible to use echo portably across all POSIX systems unless both -n (as the first argument) and escape sequences are omitted.
The printf utility can be used portably to emulate any of the traditional behaviors of the echo utility [...]

Bash/SH, Same command different output?

Per POSIX, echo supports no options.

Therefore, when echo -n is run with sh, it outputs literal -n instead of interpreting -n as the no-trailing-newline option:

$ sh -c 'echo -n "apple"'
-n apple                  # !! Note the -n at the beginning.

^{Note: Not all sh implementations behave this way; some, such as on Ubuntu (where dash acts as sh), do support the -n option, but the point is that you cannot rely on that, if your code must run on multiple platforms.}

The portable POSIX-compliant way to print to stdout is to use the printf utility:

printf %s "apple" | shasum -a 256

echo $(command) gets a different result with the output of the command

The command substitution is executed by your local shell before ssh runs.

If your local system's name is here and the remote is there,

ssh there uname -n

will print there whereas

ssh there echo $(uname -n)  # should have proper quoting, too

will run uname -n locally and then send the expanded command line echo here to there to be executed.

As an additional aside, echo $(command) is a useless use of echo unless you specifically require the shell to perform wildcard expansion and whitespace tokenization on the output of command before printing it.

Also, grep x | awk { y } is a useless use of grep; it can and probably should be refactored to awk '/x/ { y }' -- but of course, here you are reinventing pidof so better just use that.

 ssh user@myserver.com pidof /srv/adih/server/app.js

If you want to capture the printed PID locally, the syntax for that is

pid=$(ssh user@myserver.com pidof /srv/adih/server/app.js)

Of course, if you only need to kill it, that's

ssh user@myserver.com pkill /srv/adih/server/app.js

Why This Command's Output Is Different Between Sh and Bash?

&> is not a valid redirection in sh. & sends the command xset q to background use if ! xset q >/dev/null 2>&1;then echo 'test'; fi;

bash vs sh script different results

Different versions of echo do different things when given -n as their first argument. Some print it as part of their output, some interpret it as a flag, who knows what's going to happen. According to the POSIX standard for echo, "If the first operand is -n, or if any of the operands contain a backslash character, the results are implementation-defined."

The most reliable way to print a string without a linefeed after it is with printf. It's slightly more complicated because you have to give it a format string as well as the string you want printed:

hex=$(printf "%s" 'betty' | xxd -p)

Why does my Bash code fail when I run it with 'sh'?

TL;DR: Since you are using Bash specific features, your script has to run with Bash and not with sh:

$ sh myscript.sh
myscript.sh: 2: myscript.sh: Bad substitution

$ bash myscript.sh
ffmpeg -i bar.mp4 bar.mp3
ffmpeg -i foo.mp4 foo.mp3

See Difference between sh and Bash. To find out which sh you are using: readlink -f $(which sh).

The best way to ensure a bash specific script always runs correctly

The best practices are to both:

1. Replace #!/bin/sh with #!/bin/bash (or whichever other shell your script depends on).
2. Run this script (and all others!) with ./myscript.sh or /path/to/myscript.sh, without a leading sh or bash.

Here's an example:

$ cat myscript.sh
#!/bin/bash
for i in *.mp4
do
  echo ffmpeg -i "$i" "${i/.mp4/.mp3}"
done

$ chmod +x myscript.sh   # Ensure script is executable

$ ./myscript.sh
ffmpeg -i bar.mp4 bar.mp3
ffmpeg -i foo.mp4 foo.mp3

(Related: Why ./ in front of scripts?)

The meaning of #!/bin/sh

The shebang suggests which shell the system should use to run a script. This allows you to specify #!/usr/bin/python or #!/bin/bash so that you don't have to remember which script is written in what language.

People use #!/bin/sh when they only use a limited set of features (defined by the POSIX standard) for maximum portability. #!/bin/bash is perfectly fine for user scripts that take advantage of useful bash extensions.

/bin/sh is usually symlinked to either a minimal POSIX compliant shell or to a standard shell (e.g. bash). Even in the latter case, #!/bin/sh may fail because bash will run in compatibility mode as explained in the man page:

If bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh as closely as possible, while conforming to the POSIX standard as well.

The meaning of sh myscript.sh

The shebang is only used when you run ./myscript.sh, /path/to/myscript.sh, or when you drop the extension, put the script in a directory in your $PATH, and just run myscript.

If you explicitly specify an interpreter, that interpreter will be used. sh myscript.sh will force it to run with sh, no matter what the shebang says. This is why changing the shebang is not enough by itself.

You should always run the script with its preferred interpreter, so prefer ./myscript.sh or similar whenever you execute any script.

Other suggested changes to your script:

• It is considered good practice to quote variables ("$i" instead of $i). Quoted variables will prevent problems if the stored file name contains white space characters.
• I like that you use advanced parameter expansion. I suggest to use "${i%.mp4}.mp3" (instead of "${i/.mp4/.mp3}"), since ${parameter%word} only substitutes at the end (for example a file named foo.mp4.backup).

How to echo shell commands as they are executed

set -x or set -o xtrace expands variables and prints a little + sign before the line.

set -v or set -o verbose does not expand the variables before printing.

Use set +x and set +v to turn off the above settings.

On the first line of the script, one can put #!/bin/sh -x (or -v) to have the same effect as set -x (or -v) later in the script.

The above also works with /bin/sh.

See the bash-hackers' wiki on set attributes, and on debugging.

$ cat shl
#!/bin/bash                                                                     

DIR=/tmp/so
ls $DIR

$ bash -x shl 
+ DIR=/tmp/so
+ ls /tmp/so
$

Redirect echo output in shell script to logfile

You can add this line on top of your script:

#!/bin/bash
# redirect stdout/stderr to a file
exec >logfile.txt 2>&1

OR else to redirect only stdout use:

exec > logfile.txt

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