Shell – Linux command line that will fail if any standard out is produced

shell-scripttext processing

Is there a simple utility which I can pipe output to on Linux that will:

Return a success code if there is no output on standard out (and / or standard error).
Return a failure code if output is produced on standard out (and / or standard error).

To provide some context, the command I'm running is:

svn mergeinfo --show-revs eligible
http://mysvnserver.example.com/SVF/repos/common/abc/branches/abc-1.7
http://mysvnserver.example.com/SVF/repos/common/abc/trunk

If there are any unmerged entries on the branch, the command will return a list of revision numbers on standard out. Ideally, the additional command that I'm talking about would:

Detect entries on standard out and return an error condition to Linux.
Pass on the standard out so that it does end up appearing on the terminal. I'd rather not suppress it.

Best Answer

That's grep you're looking for:

if svn ... 2>&1 | grep '^'; then
  echo "there was some output"
else
  echo "there wasn't"
fi

You can replace grep '^' with grep . or grep '[^[:blank:]]' to check for non-empty or non-blank lines (but that will remove the empty/blank ones from the output).

(note the behaviour will vary across grep implementations if the input contains non-text data like NUL bytes or too long or non-terminated lines (which wouldn't happen for svn though)).

Related Solutions

Bash – How to create a bash script that sums any number of command line arguments

No need for bash, plain sh will do as well:

#! /bin/sh - 
IFS=+; echo "$(($*))"

$* in POSIX shells, expands to the list of positional parameters (in this case, the arguments to the script) separated by the first character of $IFS (or space if $IFS is unset or nothing if $IFS is empty). $((...)) is the shell internal arithmetic expansion operator (note that it supports decimal, octal and hexadecimal numbers)

If you need floating point support, that's where you'll need a different shell like ksh93 or zsh (not bash as bash only supports integer arithmetic), though you could also use awk:

#! /usr/bin/awk -f
BEGIN {t=0; for (i in ARGV) t+=ARGV[i]; print t}

That will use long (for integer) and double (for floating point) type numbers as implemented by your system. The input numbers must be decimal floating point or engineering notation in the English style (floating point delimiter is the period character regardless of the locale). With some awk implementations, it will fail if the first number is negative as awk would try to interpret it as an option.

Some awk implementations like GNU awk when POSIXLY_CORRECT is in the environment also support hexadecimals including with binary exponent notations. Or with --non-decimal-data, it understands octals and hexadecimals:

$ POSIXLY_CORRECT=1 ./sum  0xap3 0xa
90 # (0xa * 2^3) + 0xa
$ awk --non-decimal-data -f ./sum  010
8

Shell – A command that generates both standard error and output

A simple approach would be to use ls to list actual and imaginary files:

ls . *.blah

This assumes that there are visible files in the working directory and that you don't have any files that end in .blah¹

_{1. ...and if you do, we won't judge you.}

Best Answer

Related Solutions

Bash – How to create a bash script that sums any number of command line arguments

Shell – A command that generates both standard error and output

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