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What this section covers
- Standard Unix streams, redirection and piping
- How and why to direct script diagnostic messages to standard error, but function results to standard output
- Argument defaulting
- Automatically creating a log file for a script
- Capturing standard output in a variable using backtick quoting
Standard streams
As described Intro Unix: Standard streams and redirection each with a well-defined stream number:
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- redirect standard output to a file, overwriting any exiting contents:
echo "Output text" > out.txt
echo "Some output" 1> out.txt - redirect standard output to a file, appending to any exiting contents:
echo "Some text" >> out.txt
echo "More text" 1>> out.txt - redirect standard error output to a file, overwriting any exiting contents:
ls xxxx 2> err.txt - redirect standard error to standard output, and redirect standard output to a file:
ls xxxx > ls.log 2>&1 - redirect standard output to standard error:
echo "Output that will be redirected to standard error" 1>&2
echo "Output that will be redirected standard error" 2> err.txt 1>&2
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For example, here's how to call the step_01.sh script so that its standard output goes to both the terminal Terminal and to a file.
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~/workshop/step_01.sh helloWorld My name is Anna | tee step_01.log |
See Standard streams and piping in the Intro Unix wiki.
When standard output and standard error streams are used
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Here's a step_02.sh script that builds on our step_01.sh work. It is located at ~/workshop/step_02.sh. Make sure it is executable (chmod +x ~/workshop/step_02.txt).
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#!/bin/bash
# Script version global variable. Edit this whenever changes are made.
__ADVANCED_BASH_VERSION__="step_02"
# =======================================================================
# Helper functions
# =======================================================================
# Echo's its arguments to std error
echo_se() { echo "$@" 1>&2; }
# Sets up auto-logging to a log file in the current directory
# using the specified logFileTag (arg 1) in the log file name.
auto_log() {
local logFileTag="$1"
if [[ "$logFileTag" != "" ]]; then
local logFilePath="./autoLog_${logFileTag}.log"
echo_se ".. logging to $logFilePath"
exec 1> >(tee "$logFilePath") 2>&1
else
echo_se "** ERROR in auto_log: no logFile argument provided"
exit 255
fi
}
# =======================================================================
# Command processing functions
# =======================================================================
# function that says "Hello World!" and displays user-specified text.
function helloWorld() {
local txt1=$1
local txt2=$2
shift; shift
local rest=$@
echo "Hello World!"
echo " text 1: '$txt1'"
echo " text 2: '$txt2'"
echo " rest: '$rest'"
}
# function that displays its 1st argument on standard output and
# its 2nd argument on standard error
function stdStreams() {
local outTxt=${1:-"text for standard output"}
local errTxt=${2:-"text for standard error"}
echo "to standard output: '$outTxt'"
echo_se "to standard error: '$errTxt'"
}
# function that illustrates auto-logging and capturing function output
# arg 1 - (required) tag to identify the logfile
# arg 2 - (optional) text for standard output
# arg 3 - (optional) text for standard error
function testAutolog() {
local logFileTag="$1"
local outTxt=${2:-"text for standard output"}
local errTxt=${3:-"text for standard error"}
auto_log "$logFileTag"
echo -e "\n1) Call stdStreams with output and error text:"
stdStreams "$outTxt" "$errTxt"
echo -e "\n2) Capture echo output in a variable and display it:"
local output=`echo $outTxt`
echo -e " echo output was:\n$output"
echo -e "\n3) Call echo_se with error text:"
echo_se "$errTxt"
echo -e "\n4)Capture echo_se function output in a variable and display it:"
output=`echo_se "$errTxt"`
echo -e "echo_se output was: '$output'"
}
# =======================================================================
# Main script command-line processing
# =======================================================================
function usage() {
echo "
advanced_bash.sh, version $__ADVANCED_BASH_VERSION__
Usage: advanced_bash.sh <command> [arg1 arg2...]
Commands:
helloWorld [text to display]
stdStreams [text for stdout] [text for stderr]
testAutolog <logFileTag> [text for stdout] [text for stderr]
"
exit 255
}
CMD=$1 # initially $1 will be the command
shift # after "shift", $1 will be the 2nd command-line argument; $2 the 3rd, etc.
# and $@ will be arguments 2, 3, etc.
case "$CMD" in
helloWorld) helloWorld "$@"
;;
stdStreams) stdStreams "$1" "$2"
;;
testAutolog) testAutolog "$1" "$2" "$3"
;;
*) usage
;;
esac |
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Here's how to call the stdStreams command specifying the text "hello world!" for standard output and "goodbye world!" for standard error.
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~/workshop/step_02.sh stdStreams '"hello world!' "'goodbye world!"' |
But how can you tell which stream is which? They're both written to the terminal Terminal by default!
exercise 1
Call the stdStreams command in some manner that can distinguish between standard output and standard error.
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