How to set font color for STDOUT and STDERR
I want to differentiate the STDOUT and STDERR messages in my terminal. If a script or command is printing a message in terminal I want to differentiate by colors; is it possible?
(E.g. stderr font color is red, and stdout font color is blue.)
Example (using bold):
$date
Wed Jul 27 12:36:50 IST 2011
$datee
bash: datee: command not found
$alias ls
alias ls='ls --color=auto -F'
$aliass ls
bash: aliass: command not found
Solution 1:
Create a function in a bash shell or script:
color()(set -o pipefail;"$@" 2>&1>&3|sed $'s,.*,\e[31m&\e[m,'>&2)3>&1
Use it like this:
$ color command -program -args
It will show the command's stderr
in red.
Keep reading for an explanation of how it works. There are some interesting features demonstrated by this command.
-
color()...
— Creates a bash function called color. -
set -o pipefail
— This is a shell option that preserves the error return code of a command whose output is piped into another command. This is done in a subshell, which is created by the parentheses, so as not to change the pipefail option in the outer shell. -
"$@"
— Executes the arguments to the function as a new command."$@"
is equivalent to"$1" "$2" ...
-
2>&1
— Redirects thestderr
of the command tostdout
so that it becomessed
'sstdin
. -
>&3
— Shorthand for1>&3
, this redirectsstdout
to a new temporary file descriptor3
.3
gets routed back intostdout
later. -
sed ...
— Because of the redirects above,sed
'sstdin
is thestderr
of the executed command. Its function is to surround each line with color codes. -
$'...'
A bash construct that causes it to understand backslash-escaped characters -
.*
— Matches the entire line. -
\e[31m
— The ANSI escape sequence that causes the following characters to be red -
&
— Thesed
replace character that expands to the entire matched string (the entire line in this case). -
\e[m
— The ANSI escape sequence that resets the color. -
>&2
— Shorthand for1>&2
, this redirectssed
'sstdout
tostderr
. -
3>&1
— Redirects the temporary file descriptor3
back intostdout
.
Solution 2:
Here's a hack that I thought of and it seems to work:
Given the following aliases for readability:
alias blue='echo -en "\033[36m"'
alias red='echo -en "\033[31m"'
alias formatOutput='while read line; do blue; echo $line; red; done'
Now, you need to first set the font color in your terminal to red (as the default, which will be used for stderr).
Then, run your command and pipe the stdout through formatOutput
defined above (which simply prints each line as blue and then resets the font color to red):
shell$ red
shell$ ls / somenonexistingfile | formatOutput
The above command will print in both stderr and stdout and you'll see that the lines are coloured differently.
Hope this helps
UPDATE:
To make this reusable, I've put it all in a small script:
$ cat bin/run
#!/bin/bash
echo -en "\033[31m" ## red
eval $* | while read line; do
echo -en "\033[36m" ## blue
echo $line
echo -en "\033[31m" ## red
done
echo -en "\033[0m" ## reset color
Now you can use this with any command:
$ run yourCommand
Solution 3:
I color stderr red by linking the file descriptor to a custom function that adds color to everything that goes through it. Add to following to your .bashrc
:
export COLOR_RED="$(tput setaf 1)"
export COLOR_RESET="$(tput sgr0)"
exec 9>&2
exec 8> >(
perl -e '$|=1; while(sysread STDIN,$a,9999) {print
"$ENV{COLOR_RED}$a$ENV{COLOR_RESET}"}'
)
function undirect(){ exec 2>&9; }
function redirect(){ exec 2>&8; }
trap "redirect;" DEBUG
PROMPT_COMMAND='undirect;'
So what is happening? The debug trap is executed just before and immediately after executing a command. stderr is thus redirected before a command is executed to enable red output. PROMPT_COMMAND is evaluated before the prompt is shown and with this I restore stderr to its normal state. This is necessary because PS1 and PS2 (your prompt) are printed over stderr and I do not want a red prompt. voila, red output over stderr!