Page Comparison

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• redirect standard output to a file, overwriting any exiting contents:
  echo "Output text" > out.txt
echo "More 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:
  ls xxxx > ls.log 2>&1
• redirect standard output to standard error:
  echo "Output that will go to standard error" 1>&2
echo "Output that will go to standard error" 2> err.txt 1>&2


There's also the tee program, which takes its standard input and writes it to the specified file as well as to its standard output. 

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# list 2 files, one that exists and one that does not
ls ~/.profile xxx 

Produces this output in your terminal:

ls: cannot access 'xxx': No such file or directory
.profile

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ls ~/.profile ~/xxx 1>stdout.txt 2>stderr.txt
cat stdout.txt
cat stderr.txt

The step_02.sh Script

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 ~/step_02.txt).

#!/bin/bash

# Script version global variable. Edit this whenever changes are made.
__ADVANCED_BASH_VERSION__="step_02"

Finally redirect both standard output and standard error to a single file:

ls ~/.profile ~/xxx > ls.log 2>&1
# or
ls ~/.profile ~/xxx 2>1 1> ls.log

# But this does not redirect stdout
ls ~/.profile ~/xxx 2>&1 1> ls.log 

The step_02.sh Script

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 ~/step_02.txt).

#!/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 autoLogauto_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

...

# 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'"
}

...

argument defaulting

Often you will want to provide a default for arguments not explicitly provided by the user. This can be done using this rather odd syntax, with ":-" characters separating the positional argument number from the desired default. The entire defaulting construct is enclosed in "${ }".

textArg=${1:-"text default for 1st argument"}
integerArg=${2:-54321}

simpler usage function

We've replaced the multiple echo lines in usage also added command-line processing and usage support for the new functions.

In the usage function, we've replaced the multiple echo lines with a single echo'd string, illustrating that the invisible line feeds in the echo'd text are faithfully preserved. The function also exits with a non-0 ("failure") return code, since no valid command processing was performed.

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exercise 3

What is written to the ssout.txt file when the following is executed, and why?

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to standard output: 'hello world'

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Importantly, auto_log reports an error and terminates script execution if no tag string is specified, via exit 255 (we'll see much more on error handling shortly).

The test is made using a bash if/else/fi block, where double brackets ('"[[ ]]") enclose the test, always followed by a semicolon (";"). Here the test is for string equality (see https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html for a complete list of bash comparison operators). Note that there must always be a space after the open brackets, and one before the close brackets.

# 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 autoLog: no logFile argument provided"
    exit 255
  fi
}auto_log: no logFile argument provided"
    exit 255
  fi
}

The general form of an if/then/else/fi statement is:

if [[ some_test ]]
then
   what_to_do_when_some_test_is_true (0)
else
   what_to_do_when_some_test_is_false (not 0)
fi

As always in bash, clauses (technically commands themselves) can be put on one line if separated by a semicolon ( ; ).

if [[ some_test ]]; then echo "Test was true"; else echo "Test was false"; fi

testAutolog function and command processing

The new testAutolog command processing function has a lot going on.  It:

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• Starts automatic logging to a log file named using its 1st logFileTag argument, by calling the auto_log function.
• Uses echo -e where the -e argument to echo enables interpretation of backslash escapes.
  • e.g. "\t" as a tab character and "\n" will be interpreted as a newline
• Calls stdStreams with its 2nd and 3rd arguments.
• Calls echo capturing its output in a local variable using backtick execution syntax, then displays the captured text.
  • see Quoting in the shell
• Calls the echo_se function with some text.
• Calls the echo_se function again, capturing its output in a local variable, then displays the captured text.
  
  

# 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'"
}

...

'"
}

exercise 4

Call the testAutoLog testAutolog command with no further command line arguments. What happens, and why?

~/workshop/step_02.sh testAutoLogtestAutolog

** ERROR in autoLogauto_log: no logFile argument provided

...

What output is produced when you call the testAutoLog testAutolog command with a tag string of "test1".

~/workshop/step_02.sh testAutoLogtestAutolog test1

.. logging to ./autoLog_test1.log

1) Call stdStreams with output and error text:
to standard output: 'text for standard output'
to standard error:  'text for standard error'

2) Capture echo output in a variable and display it:
   echo output was:
text for standard output

3) Call echo_se with error text:
text for standard error

4)Capture echo_se function output in a variable and display it:
text for standard error
echo_se output was: ''

Why are both standard error and standard output text strings displayed in segment 1) when stdStreams is called?

:
text for standard error
echo_se output was: ''

Why does the $output variable contain the outTxt the output text in 2) when echo is called with output being captured ?

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What log file is produced, what ? What are its contents, and why?

1) Call stdStreams with output and error text:
to standard output: 'text for standard output'
to standard error:  'text for standard error'

2) Capture echo output in a variable and display it:
   echo output was:
text for standard output

3) Call echo_se with error text:
text for standard error

4)Capture echo_se function output in a variable and display it:
text for standard error
echo_se output was: ''

...