Test whether stdout has been written to - bash

I have a script that prints in a loop. I want the loop to print differently the first time from all other times (i.e., it should print differently if anything has been printed at all). I am thinking a simple way would be to check whether anything has been printed yet (i.e., stdout has been written to). Is there any way to determine that?
I know I could also write to a variable and test whether it's empty, but I'd like to avoid a variable if I can.

I think that will do what you need. If you echo something between # THE SCRIPT ITSELF and # END, THE FOLLOWING DATA HAS BEEN WRITTEN TO STDOUT will be printed STDOUT HAS NOT BEEN TOUCHED else...
#!/bin/bash
readonly TMP=$(mktemp /tmp/test_XXXXXX)
exec 3<> "$TMP" # open tmp file as fd 3
exec 4>&1 # save current value of stdout as fd 4
exec >&3 # redirect stdout to fd 3 (tmp file)
# THE SCRIPT ITSELF
echo Hello World
# END
exec >&4 # restore save stdout
exec 3>&- # close tmp file
TMP_SIZE=$(stat -f %z "$TMP")
if [ $TMP_SIZE -gt 0 ]; then
echo "THE FOLLOWING DATA HAS BEEN WRITTEN TO STDOUT"
echo
cat "$TMP"
else
echo "STDOUT HAS NOT BEEN TOUCHED"
fi
rm "$TMP"
So, output of the script as is:
THE FOLLOWING DATA HAS BEEN WRITTEN TO STDOUT
Hello World
and if you remove the echo Hello World line:
STDOUT HAS NOT BEEN TOUCHED
And if you really want to test that while running the script itself, you can do that, too :-)
#!/bin/bash
#FIRST ELSE
function echo_fl() {
TMP_SIZE=$(stat -f %z "$TMP")
if [ $TMP_SIZE -gt 0 ]; then
echo $2
else
echo $1
fi
}
TMP=$(mktemp /tmp/test_XXXXXX)
exec 3 "$TMP" # open tmp file as fd 3
exec 4>&1 # save current value of stdout as fd 4
exec >&3 # redirect stdout to fd 3 (tmp file)
# THE SCRIPT ITSELF
for f in fst snd trd; do
echo_fl "$(echo $f | tr a-z A-Z)" "$f"
done
# END
exec >&4 # restore save stdout
exec 3>&- # close tmp file
TMP_SIZE=$(stat -f %z "$TMP")
if [ $TMP_SIZE -gt 0 ]; then
echo "THE FOLLOWING DATA HAS BEEN WRITTEN TO STDOUT"
echo
cat "$TMP"
else
echo "STDOUT HAS NOT BEEN TOUCHED"
fi
rm "$TMP"
output is:
THE FOLLOWING DATA HAS BEEN WRITTEN TO STDOUT
FST
snd
trd
as you can see: Only the first line (FST) has caps on. That's what the echo_fl function does for you: If it's the first line of output, if echoes the first argument, if it's not it echoes the second argument :-)

It's hard to tell what you are trying to do here, but if your script is printing to stdout, you could simply pipe it to perl:
yourcommand | perl -pe 'if ($. == 1) { print "First line is: $_" }'
It all depends on what kind of changes you are attempting to do.

You cannot use the -f option with %z. The line TMP_SIZE=$(stat -f %z "$TMP") produces a long string that fails the test in if [ $TMP_SIZE -gt 0 ].

Related

/bin/sh: capture stderr into a variable

I am assigning the output of a command to variable A:
A=$(some_command)
How can I "capture" stderr into a variable B ?
I have tried some variations with 2>&1 and read but that does not work:
A=$(some_command) 2>&1 | read B
echo $B
Here's a code snippet that might help you
# capture stderr into a variable and print it
echo "capture stderr into a variable and print it"
var=$(lt -l /tmp 2>&1)
echo $var
capture stderr into a variable and print it
zsh: command not found: lt
# capture stdout into a variable and print it
echo "capture stdout into a variable and print it"
var=$(ls -l /tmp)
echo $var
# capture both stderr and stdout into a variable and print it
echo "capture both stderr and stdout into a variable and print it"
var=$(ls -l /tmp 2>&1)
echo $var
# more classic way of executing a command which I always follow is as follows. This way I am always in control of what is going on and can act accordingly
if somecommand ; then
echo "command succeeded"
else
echo "command failed"
fi
If you have to capture the output and stderr in different variables, then the following might help as well
## create a file using file descriptor for stdout
exec 3> stdout.txt
# create a file using file descriptor for stderr
exec 4> stderr.txt
A=$($1 /tmp 2>&4 >&3);
## close file descriptor
exec 3>&-
exec 4>&-
## open file descriptor for reading
exec 3< stdout.txt
exec 4< stderr.txt
## read from file using file descriptor
read line <&3
read line2 <&4
## close file descriptor
exec 3<&-
exec 4<&-
## print line read from file
echo "stdout: $line"
echo "stderr: $line2"
## delete file
rm stdout.txt
rm stderr.txt
You can try running it with the following
╰─ bash test.sh pwd
stdout: /tmp/somedir
stderr:
╰─ bash test.sh pwdd
stdout:
stderr: test.sh: line 8: pwdd: command not found
As noted in a comment your use case may be better served in other scripting languages. An example: in Perl you can achieve what you want quite simple:
#!/usr/bin/env perl
use v5.26; # or earlier versions
use Capture::Tiny 'capture'; # library is not in core
my $cmd = 'date';
my #arg = ('-R', '-u');
my ($stdout, $stderr, $exit) = capture {
system( $cmd, #arg );
};
say "STDOUT: $stdout";
say "STDERR: $stderr";
say "EXIT: $exit";
I'm sure similar solutions are available in python, ruby, and all the rest.
I gave it another try using process substitution and came up with this:
# command with no error
date +%b > >(read A; if [ "$A" = 'Sep' ]; then echo 'September'; fi ) 2> >(read B; if [ ! -z "$B" ]; then echo "$B"; fi >&2)
September
# command with error
date b > >(read A; if [ "$A" = 'Sep' ]; then echo 'September'; fi ) 2> >(read B; if [ ! -z "$B" ]; then echo "$B"; fi >&2)
date: invalid date “b“
# command with both at the same time should work too
I had no success "exporting" the variables from the subprocesses back to the original script. It might be possible though. I just couldn't figure it out.
But this gives you at least access to stdout and stderr as a variable. This means you can do whatever processing you want on them as variables. It depends on your use case if this is of any help to you. Good luck :-)

How can I use process substitution strings in BASH?

I know I can do something like
cat <(cat somefile)
But I want to build up a string of <().
So:
for file in *.file; do
mySubs="${mySubs} <(cat ${file})"
done
cat ${mySubs} #cat <(cat 1.file) <(cat 2.file) ... <(cat something.file)
Without having to use eval.
Use named pipes directly. Use mktemp to create temporary file names for each pipe so that you can remove them after you are done.
fifos=()
for f in file1 file2 file3; do
t=$(mktemp)
mkfifo "$t"
pipes+=("$t")
someCommand "$f" > "$t" &
done
someOtherCommand "${pipes[#]}"
rm "${pipes[#]}"
I'm assuming cat is a standin for a more complicated command. Here, I'm explicitly wrapping it to show that:
#!/usr/bin/env bash
someCommand() { echo "Starting file $1"; cat "$1"; echo "Ending file $1"; }
wrap_all() {
## STAGE 1: Assemble the actual command we want to run
local fd cmd_len retval
local -a cmd fds fd_args
cmd_len=$1; shift
while (( cmd_len > 0 )); do
cmd+=( "$1" )
cmd_len=$((cmd_len - 1))
shift
done
## STAGE 2: Open an instance of someCommand for each remaining argument
local fd; local -a fds
fds=( )
for arg; do
exec {fd}< <(someCommand "$arg")
fds+=( "$fd" )
fd_args+=( "/dev/fd/$fd" )
done
## STAGE 3: Actually run the command
"${cmd[#]}" "${fd_args[#]}"; retval=$?
## STAGE 4: Close all the file descriptors
for fd in "${fds[#]}"; do
exec {fd}>&-
done
return "$retval"
}
Invocation as:
echo "one" >one.txt; echo "two" >two.txt
wrap_all 1 cat one.txt two.txt
...which outputs:
Starting file one.txt
one
Ending file one.txt
Starting file two.txt
two
Ending file two.txt
Note that this requires bash 4.1 for automatic FD allocation support (letting us avoid the need for named pipes).

Shell script calling an awk program - Assign the output to a variable [duplicate]

Let's say I have a script like the following:
useless.sh
echo "This Is Error" 1>&2
echo "This Is Output"
And I have another shell script:
alsoUseless.sh
./useless.sh | sed 's/Output/Useless/'
I want to capture "This Is Error", or any other stderr from useless.sh, into a variable.
Let's call it ERROR.
Notice that I am using stdout for something. I want to continue using stdout, so redirecting stderr into stdout is not helpful, in this case.
So, basically, I want to do
./useless.sh 2> $ERROR | ...
but that obviously doesn't work.
I also know that I could do
./useless.sh 2> /tmp/Error
ERROR=`cat /tmp/Error`
but that's ugly and unnecessary.
Unfortunately, if no answers turn up here that's what I'm going to have to do.
I'm hoping there's another way.
Anyone have any better ideas?
It would be neater to capture the error file thus:
ERROR=$(</tmp/Error)
The shell recognizes this and doesn't have to run 'cat' to get the data.
The bigger question is hard. I don't think there's an easy way to do it. You'd have to build the entire pipeline into the sub-shell, eventually sending its final standard output to a file, so that you can redirect the errors to standard output.
ERROR=$( { ./useless.sh | sed s/Output/Useless/ > outfile; } 2>&1 )
Note that the semi-colon is needed (in classic shells - Bourne, Korn - for sure; probably in Bash too). The '{}' does I/O redirection over the enclosed commands. As written, it would capture errors from sed too.
WARNING: Formally untested code - use at own risk.
Redirected stderr to stdout, stdout to /dev/null, and then use the backticks or $() to capture the redirected stderr:
ERROR=$(./useless.sh 2>&1 >/dev/null)
alsoUseless.sh
This will allow you to pipe the output of your useless.sh script through a command such as sed and save the stderr in a variable named error. The result of the pipe is sent to stdout for display or to be piped into another command.
It sets up a couple of extra file descriptors to manage the redirections needed in order to do this.
#!/bin/bash
exec 3>&1 4>&2 #set up extra file descriptors
error=$( { ./useless.sh | sed 's/Output/Useless/' 2>&4 1>&3; } 2>&1 )
echo "The message is \"${error}.\""
exec 3>&- 4>&- # release the extra file descriptors
There are a lot of duplicates for this question, many of which have a slightly simpler usage scenario where you don't want to capture stderr and stdout and the exit code all at the same time.
if result=$(useless.sh 2>&1); then
stdout=$result
else
rc=$?
stderr=$result
fi
works for the common scenario where you expect either proper output in the case of success, or a diagnostic message on stderr in the case of failure.
Note that the shell's control statements already examine $? under the hood; so anything which looks like
cmd
if [ $? -eq 0 ], then ...
is just a clumsy, unidiomatic way of saying
if cmd; then ...
For the benefit of the reader, this recipe here
can be re-used as oneliner to catch stderr into a variable
still gives access to the return code of the command
Sacrifices a temporary file descriptor 3 (which can be changed by you of course)
And does not expose this temporary file descriptors to the inner command
If you want to catch stderr of some command into var you can do
{ var="$( { command; } 2>&1 1>&3 3>&- )"; } 3>&1;
Afterwards you have it all:
echo "command gives $? and stderr '$var'";
If command is simple (not something like a | b) you can leave the inner {} away:
{ var="$(command 2>&1 1>&3 3>&-)"; } 3>&1;
Wrapped into an easy reusable bash-function (probably needs version 3 and above for local -n):
: catch-stderr var cmd [args..]
catch-stderr() { local -n v="$1"; shift && { v="$("$#" 2>&1 1>&3 3>&-)"; } 3>&1; }
Explained:
local -n aliases "$1" (which is the variable for catch-stderr)
3>&1 uses file descriptor 3 to save there stdout points
{ command; } (or "$#") then executes the command within the output capturing $(..)
Please note that the exact order is important here (doing it the wrong way shuffles the file descriptors wrongly):
2>&1 redirects stderr to the output capturing $(..)
1>&3 redirects stdout away from the output capturing $(..) back to the "outer" stdout which was saved in file descriptor 3. Note that stderr still refers to where FD 1 pointed before: To the output capturing $(..)
3>&- then closes the file descriptor 3 as it is no more needed, such that command does not suddenly has some unknown open file descriptor showing up. Note that the outer shell still has FD 3 open, but command will not see it.
The latter is important, because some programs like lvm complain about unexpected file descriptors. And lvm complains to stderr - just what we are going to capture!
You can catch any other file descriptor with this recipe, if you adapt accordingly. Except file descriptor 1 of course (here the redirection logic would be wrong, but for file descriptor 1 you can just use var=$(command) as usual).
Note that this sacrifices file descriptor 3. If you happen to need that file descriptor, feel free to change the number. But be aware, that some shells (from the 1980s) might understand 99>&1 as argument 9 followed by 9>&1 (this is no problem for bash).
Also note that it is not particluar easy to make this FD 3 configurable through a variable. This makes things very unreadable:
: catch-var-from-fd-by-fd variable fd-to-catch fd-to-sacrifice command [args..]
catch-var-from-fd-by-fd()
{
local -n v="$1";
local fd1="$2" fd2="$3";
shift 3 || return;
eval exec "$fd2>&1";
v="$(eval '"$#"' "$fd1>&1" "1>&$fd2" "$fd2>&-")";
eval exec "$fd2>&-";
}
Security note: The first 3 arguments to catch-var-from-fd-by-fd must not be taken from a 3rd party. Always give them explicitly in a "static" fashion.
So no-no-no catch-var-from-fd-by-fd $var $fda $fdb $command, never do this!
If you happen to pass in a variable variable name, at least do it as follows:
local -n var="$var"; catch-var-from-fd-by-fd var 3 5 $command
This still will not protect you against every exploit, but at least helps to detect and avoid common scripting errors.
Notes:
catch-var-from-fd-by-fd var 2 3 cmd.. is the same as catch-stderr var cmd..
shift || return is just some way to prevent ugly errors in case you forget to give the correct number of arguments. Perhaps terminating the shell would be another way (but this makes it hard to test from commandline).
The routine was written such, that it is more easy to understand. One can rewrite the function such that it does not need exec, but then it gets really ugly.
This routine can be rewritten for non-bash as well such that there is no need for local -n. However then you cannot use local variables and it gets extremely ugly!
Also note that the evals are used in a safe fashion. Usually eval is considerered dangerous. However in this case it is no more evil than using "$#" (to execute arbitrary commands). However please be sure to use the exact and correct quoting as shown here (else it becomes very very dangerous).
# command receives its input from stdin.
# command sends its output to stdout.
exec 3>&1
stderr="$(command </dev/stdin 2>&1 1>&3)"
exitcode="${?}"
echo "STDERR: $stderr"
exit ${exitcode}
POSIX
STDERR can be captured with some redirection magic:
$ { error=$( { { ls -ld /XXXX /bin | tr o Z ; } 1>&3 ; } 2>&1); } 3>&1
lrwxrwxrwx 1 rZZt rZZt 7 Aug 22 15:44 /bin -> usr/bin/
$ echo $error
ls: cannot access '/XXXX': No such file or directory
Note that piping of STDOUT of the command (here ls) is done inside the innermost { }. If you're executing a simple command (eg, not a pipe), you could remove these inner braces.
You can't pipe outside the command as piping makes a subshell in bash and zsh, and the assignment to the variable in the subshell wouldn't be available to the current shell.
bash
In bash, it would be better not to assume that file descriptor 3 is unused:
{ error=$( { { ls -ld /XXXX /bin | tr o Z ; } 1>&$tmp ; } 2>&1); } {tmp}>&1;
exec {tmp}>&- # With this syntax the FD stays open
Note that this doesn't work in zsh.
Thanks to this answer for the general idea.
A simple solution
{ ERROR=$(./useless.sh 2>&1 1>&$out); } {out}>&1
echo "-"
echo $ERROR
Will produce:
This Is Output
-
This Is Error
Iterating a bit on Tom Hale's answer, I've found it possible to wrap the redirection yoga into a function for easier reuse. For example:
#!/bin/sh
capture () {
{ captured=$( { { "$#" ; } 1>&3 ; } 2>&1); } 3>&1
}
# Example usage; capturing dialog's output without resorting to temp files
# was what motivated me to search for this particular SO question
capture dialog --menu "Pick one!" 0 0 0 \
"FOO" "Foo" \
"BAR" "Bar" \
"BAZ" "Baz"
choice=$captured
clear; echo $choice
It's almost certainly possible to simplify this further. Haven't tested especially-thoroughly, but it does appear to work with both bash and ksh.
EDIT: an alternative version of the capture function which stores the captured STDERR output into a user-specified variable (instead of relying on a global $captured), taking inspiration from Léa Gris's answer while preserving the ksh (and zsh) compatibility of the above implementation:
capture () {
if [ "$#" -lt 2 ]; then
echo "Usage: capture varname command [arg ...]"
return 1
fi
typeset var captured; captured="$1"; shift
{ read $captured <<<$( { { "$#" ; } 1>&3 ; } 2>&1); } 3>&1
}
And usage:
capture choice dialog --menu "Pick one!" 0 0 0 \
"FOO" "Foo" \
"BAR" "Bar" \
"BAZ" "Baz"
clear; echo $choice
Here's how I did it :
#
# $1 - name of the (global) variable where the contents of stderr will be stored
# $2 - command to be executed
#
captureStderr()
{
local tmpFile=$(mktemp)
$2 2> $tmpFile
eval "$1=$(< $tmpFile)"
rm $tmpFile
}
Usage example :
captureStderr err "./useless.sh"
echo -$err-
It does use a temporary file. But at least the ugly stuff is wrapped in a function.
This is an interesting problem to which I hoped there was an elegant solution. Sadly, I end up with a solution similar to Mr. Leffler, but I'll add that you can call useless from inside a Bash function for improved readability:
#!/bin/bash
function useless {
/tmp/useless.sh | sed 's/Output/Useless/'
}
ERROR=$(useless)
echo $ERROR
All other kind of output redirection must be backed by a temporary file.
I think you want to capture stderr, stdout and exitcode if that is your intention you can use this code:
## Capture error when 'some_command() is executed
some_command_with_err() {
echo 'this is the stdout'
echo 'this is the stderr' >&2
exit 1
}
run_command() {
{
IFS=$'\n' read -r -d '' stderr;
IFS=$'\n' read -r -d '' stdout;
IFS=$'\n' read -r -d '' stdexit;
} < <((printf '\0%s\0%d\0' "$(some_command_with_err)" "${?}" 1>&2) 2>&1)
stdexit=${stdexit:-0};
}
echo 'Run command:'
if ! run_command; then
## Show the values
typeset -p stdout stderr stdexit
else
typeset -p stdout stderr stdexit
fi
This scripts capture the stderr, stdout as well as the exitcode.
But Teo how it works?
First, we capture the stdout as well as the exitcode using printf '\0%s\0%d\0'. They are separated by the \0 aka 'null byte'.
After that, we redirect the printf to stderr by doing: 1>&2 and then we redirect all back to stdout using 2>&1. Therefore, the stdout will look like:
"<stderr>\0<stdout>\0<exitcode>\0"
Enclosing the printf command in <( ... ) performs process substitution. Process substitution allows a process’s input or output to be referred to using a filename. This means <( ... ) will pipe the stdout of (printf '\0%s\0%d\0' "$(some_command_with_err)" "${?}" 1>&2) 2>&1into the stdin of the command group using the first <.
Then, we can capture the piped stdout from the stdin of the command group with read. This command reads a line from the file descriptor stdin and split it into fields. Only the characters found in $IFS are recognized as word delimiters. $IFS or Internal Field Separator is a variable that determines how Bash recognizes fields, or word boundaries, when it interprets character strings. $IFS defaults to whitespace (space, tab, and newline), but may be changed, for example, to parse a comma-separated data file. Note that $* uses the first character held in $IFS.
## Shows whitespace as a single space, ^I(horizontal tab), and newline, and display "$" at end-of-line.
echo "$IFS" | cat -vte
# Output:
# ^I$
# $
## Reads commands from string and assign any arguments to pos params
bash -c 'set w x y z; IFS=":-;"; echo "$*"'
# Output:
# w:x:y:z
for l in $(printf %b 'a b\nc'); do echo "$l"; done
# Output:
# a
# b
# c
IFS=$'\n'; for l in $(printf %b 'a b\nc'); do echo "$l"; done
# Output:
# a b
# c
That is why we defined IFS=$'\n' (newline) as delimiter.
Our script uses read -r -d '', where read -r does not allow backslashes to escape any characters, and -d '' continues until the first character '' is read, rather than newline.
Finally, replace some_command_with_err with your script file and you can capture and handle the stderr, stdout as well as the exitcode as your will.
This post helped me come up with a similar solution for my own purposes:
MESSAGE=`{ echo $ERROR_MESSAGE | format_logs.py --level=ERROR; } 2>&1`
Then as long as our MESSAGE is not an empty string, we pass it on to other stuff. This will let us know if our format_logs.py failed with some kind of python exception.
In zsh:
{ . ./useless.sh > /dev/tty } 2>&1 | read ERROR
$ echo $ERROR
( your message )
Capture AND Print stderr
ERROR=$( ./useless.sh 3>&1 1>&2 2>&3 | tee /dev/fd/2 )
Breakdown
You can use $() to capture stdout, but you want to capture stderr instead. So you swap stdout and stderr. Using fd 3 as the temporary storage in the standard swap algorithm.
If you want to capture AND print use tee to make a duplicate. In this case the output of tee will be captured by $() rather than go to the console, but stderr(of tee) will still go to the console so we use that as the second output for tee via the special file /dev/fd/2 since tee expects a file path rather than a fd number.
NOTE: That is an awful lot of redirections in a single line and the order matters. $() is grabbing the stdout of tee at the end of the pipeline and the pipeline itself routes stdout of ./useless.sh to the stdin of tee AFTER we swapped stdin and stdout for ./useless.sh.
Using stdout of ./useless.sh
The OP said he still wanted to use (not just print) stdout, like ./useless.sh | sed 's/Output/Useless/'.
No problem just do it BEFORE swapping stdout and stderr. I recommend moving it into a function or file (also-useless.sh) and calling that in place of ./useless.sh in the line above.
However, if you want to CAPTURE stdout AND stderr, then I think you have to fall back on temporary files because $() will only do one at a time and it makes a subshell from which you cannot return variables.
Improving on YellowApple's answer:
This is a Bash function to capture stderr into any variable
stderr_capture_example.sh:
#!/usr/bin/env bash
# Capture stderr from a command to a variable while maintaining stdout
# #Args:
# $1: The variable name to store the stderr output
# $2: Vararg command and arguments
# #Return:
# The Command's Returnn-Code or 2 if missing arguments
function capture_stderr {
[ $# -lt 2 ] && return 2
local stderr="$1"
shift
{
printf -v "$stderr" '%s' "$({ "$#" 1>&3; } 2>&1)"
} 3>&1
}
# Testing with a call to erroring ls
LANG=C capture_stderr my_stderr ls "$0" ''
printf '\nmy_stderr contains:\n%s' "$my_stderr"
Testing:
bash stderr_capture_example.sh
Output:
stderr_capture_example.sh
my_stderr contains:
ls: cannot access '': No such file or directory
This function can be used to capture the returned choice of a dialog command.
If you want to bypass the use of a temporary file you may be able to use process substitution. I haven't quite gotten it to work yet. This was my first attempt:
$ .useless.sh 2> >( ERROR=$(<) )
-bash: command substitution: line 42: syntax error near unexpected token `)'
-bash: command substitution: line 42: `<)'
Then I tried
$ ./useless.sh 2> >( ERROR=$( cat <() ) )
This Is Output
$ echo $ERROR # $ERROR is empty
However
$ ./useless.sh 2> >( cat <() > asdf.txt )
This Is Output
$ cat asdf.txt
This Is Error
So the process substitution is doing generally the right thing... unfortunately, whenever I wrap STDIN inside >( ) with something in $() in an attempt to capture that to a variable, I lose the contents of $(). I think that this is because $() launches a sub process which no longer has access to the file descriptor in /dev/fd which is owned by the parent process.
Process substitution has bought me the ability to work with a data stream which is no longer in STDERR, unfortunately I don't seem to be able to manipulate it the way that I want.
$ b=$( ( a=$( (echo stdout;echo stderr >&2) ) ) 2>&1 )
$ echo "a=>$a b=>$b"
a=>stdout b=>stderr
For error proofing your commands:
execute [INVOKING-FUNCTION] [COMMAND]
execute () {
function="${1}"
command="${2}"
error=$(eval "${command}" 2>&1 >"/dev/null")
if [ ${?} -ne 0 ]; then
echo "${function}: ${error}"
exit 1
fi
}
Inspired in Lean manufacturing:
Make errors impossible by design
Make steps the smallest
Finish items one by one
Make it obvious to anyone
I'll use find command
find / -maxdepth 2 -iname 'tmp' -type d
as non superuser for the demo. It should complain 'Permission denied' when acessing / dir.
#!/bin/bash
echo "terminal:"
{ err="$(find / -maxdepth 2 -iname 'tmp' -type d 2>&1 1>&3 3>&- | tee /dev/stderr)"; } 3>&1 | tee /dev/fd/4 2>&1; out=$(cat /dev/fd/4)
echo "stdout:" && echo "$out"
echo "stderr:" && echo "$err"
that gives output:
terminal:
find: ‘/root’: Permission denied
/tmp
/var/tmp
find: ‘/lost+found’: Permission denied
stdout:
/tmp
/var/tmp
stderr:
find: ‘/root’: Permission denied
find: ‘/lost+found’: Permission denied
The terminal output has also /dev/stderr content the same way as if you were running that find command without any script. $out has /dev/stdout and $err has /dev/stderr content.
use:
#!/bin/bash
echo "terminal:"
{ err="$(find / -maxdepth 2 -iname 'tmp' -type d 2>&1 1>&3 3>&-)"; } 3>&1 | tee /dev/fd/4; out=$(cat /dev/fd/4)
echo "stdout:" && echo "$out"
echo "stderr:" && echo "$err"
if you don't want to see /dev/stderr in the terminal output.
terminal:
/tmp
/var/tmp
stdout:
/tmp
/var/tmp
stderr:
find: ‘/root’: Permission denied
find: ‘/lost+found’: Permission denied

Write and read from a fifo from two different script

I have two bash script.
One script write in a fifo. The second one read from the fifo, but AFTER the first one end to write.
But something does not work. I do not understand where the problem is. Here the code.
The first script is (the writer):
#!/bin/bash
fifo_name="myfifo";
# Se non esiste, crea la fifo;
[ -p $fifo_name ] || mkfifo $fifo_name;
exec 3<> $fifo_name;
echo "foo" > $fifo_name;
echo "bar" > $fifo_name;
The second script is (the reader):
#!/bin/bash
fifo_name="myfifo";
while true
do
if read line <$fifo_name; then
# if [[ "$line" == 'ar' ]]; then
# break
#fi
echo $line
fi
done
Can anyone help me please?
Thank you
Replace the second script with:
#!/bin/bash
fifo_name="myfifo"
while true
do
if read line; then
echo $line
fi
done <"$fifo_name"
This opens the fifo only once and reads every line from it.
The problem with your setup is that you have fifo creation in the wrong script if you wish to control fifo access to time when the reader is actually running. In order to correct the problem you will need to do something like this:
reader: fifo_read.sh
#!/bin/bash
fifo_name="/tmp/myfifo" # fifo name
trap "rm -f $fifo_name" EXIT # set trap to rm fifo_name at exit
[ -p "$fifo_name" ] || mkfifo "$fifo_name" # if fifo not found, create
exec 3< $fifo_name # redirect fifo_name to fd 3
# (not required, but makes read clearer)
while :; do
if read -r -u 3 line; then # read line from fifo_name
if [ "$line" = 'quit' ]; then # if line is quit, quit
printf "%s: 'quit' command received\n" "$fifo_name"
break
fi
printf "%s: %s\n" "$fifo_name" "$line" # print line read
fi
done
exec 3<&- # reset fd 3 redirection
exit 0
writer: fifo_write.sh
#!/bin/bash
fifo_name="/tmp/myfifo"
# Se non esiste, exit :);
[ -p "$fifo_name" ] || {
printf "\n Error fifo '%s' not found.\n\n" "$fifo_name"
exit 1
}
[ -n "$1" ] &&
printf "%s\n" "$1" > "$fifo_name" ||
printf "pid: '%s' writing to fifo\n" "$$" > "$fifo_name"
exit 0
operation: (start reader in 1st terminal)
$ ./fifo_read.sh # you can background with & at end
(launch writer in second terminal)
$ ./fifo_write.sh "message from writer" # second terminal
$ ./fifo_write.sh
$ ./fifo_write.sh quit
output in 1st terminal:
$ ./fifo_read.sh
/tmp/myfifo: message from writer
/tmp/myfifo: pid: '28698' writing to fifo
/tmp/myfifo: 'quit' command received
The following script should do the job:
#!/bin/bash
FIFO="/tmp/fifo"
if [ ! -e "$FIFO" ]; then
mkfifo "$FIFO"
fi
for script in "$#"; do
echo $script > $FIFO &
done
while read script; do
/bin/bash -c $script
done < $FIFO
Given two script a.sh and b.sh where both scripts pass "a" and "b" to stdout, respectively, one will get the following result (given that the script above is called test.sh):
./test.sh /tmp/a.sh /tmp/b.sh
a
b
Best,
Julian

logging blocks of code to log files in bash

I have a huge bash script and I want to log specific blocks of code to a specific & small log files (instead of just one huge log file).
I have the following two methods:
# in this case, 'log' is a bash function
# Using code block & piping
{
# ... bash code ...
} | log "file name"
# Using Process Substitution
log "file name" < <(
# ... bash code ...
)
Both methods may interfere with the proper execution of the bash script, e.g. when assigning values to a variable (like the problem presented here).
How do you suggest to log the output of commands to log files?
Edit:
This is what I tried to do (besides many other variations), but doesn't work as expected:
function log()
{
if [ -z "$counter" ]; then
counter=1
echo "" >> "./General_Log_File" # Create the summary log file
else
(( ++counter ))
fi
echo "" > "./${counter}_log_file" # Create specific log file
# Display text-to-be-logged on screen & add it to the summary log file
# & write text-to-be-logged to it's corresponding log file
exec 1> >(tee "./${counter}_log_file" | tee -a "./General_Log_File") 2>&1
}
log # Logs the following code block
{
# ... Many bash commands ...
}
log # Logs the following code block
{
# ... Many bash commands ...
}
The results of executions varies: sometimes the log files are created and sometimes they don't (which raise an error).
You could try something like this:
function log()
{
local logfile=$1
local errfile=$2
exec > $logfile
exec 2> $errfile # if $errfile is not an empty string
}
log $fileA $errfileA
echo stuff
log $fileB $errfileB
echo more stuff
This would redirect all stdout/stderr from current process to a file without any subprocesses.
Edit: The below might be a good solution then, but not tested:
pipe=$(mktemp)
mknod $pipe p
exec 1>$pipe
function log()
{
if ! [[ -z "$teepid2" ]]; then
kill $teepid2
else
tee <$pipe general_log_file &
teepid1=$!
count=1
fi
tee <$pipe ${count}_logfile &
teepid2=$!
(( ++count ))
}
log
echo stuff
log
echo stuff2
if ! [[ -z "$teepid1" ]]; then kill $teepid1; fi
Thanks to Sahas, I managed to achieve the following solution:
function log()
{
[ -z "$counter" ] && counter=1 || (( ++counter ))
if [ -n "$teepid" ]; then
exec 1>&- 2>&- # close file descriptors to signal EOF to the `tee`
# command in the bg process
wait $teepid # wait for bg process to exit
fi
# Display text-to-be-logged on screen and
# write it to the summary log & to it's corresponding log file
( tee "${counter}.log" < "$pipe" | tee -a "Summary.log" 1>&4 ) &
teepid=$!
exec 1>"$pipe" 2>&1 # redirect stdout & stderr to the pipe
}
# Create temporary FIFO/pipe
pipe_dir=$(mktemp -d)
pipe="${pipe_dir}/cmds_output"
mkfifo "$pipe"
exec 4<&1 # save value of FD1 to FD4
log # Logs the following code block
{
# ... Many bash commands ...
}
log # Logs the following code block
{
# ... Many bash commands ...
}
if [ -n "$teepid" ]; then
exec 1>&- 2>&- # close file descriptors to signal EOF to the `tee`
# command in the bg process
wait $teepid # wait for bg process to exit
fi
It works - I tested it.
References:
Force bash script to use tee without piping from the command line # superuser.com - helped a lot
I/O Redirection # tldp.org
$! - PID Variable # tldp.org
TEST Operators: Binary Comparison # tldp.org
For simple redirection of bash code block, without using a dedicated function, do:
(
echo "log this block of code"
# commands ...
# ...
# ...
) &> output.log

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