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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:
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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
).
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#!/bin/bash
# Script version global variable. Edit this whenever changes are made.
__ADVANCED_BASH_VERSION__="step_02"
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Finally redirect both standard output and standard error to a single file:
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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
).
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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" "$3" ;; *) usage ;; esac |
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# function that displays its 1st argument on standard output and # its 2nd argument on standard error function stdStreams() { local outTxt=${1:-"text for standard output"} local errTxt=${2:-"text for standard error"} echo "to standard output: '$outTxt'" echo_se "to standard error: '$errTxt'" } |
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argument defaulting
Often you will want to provide a default for arguments not explicitly provided by the user. This can be done using this rather odd syntax, with ":-" characters separating the positional argument number from the desired default. The entire defaulting construct is enclosed in "${ }".
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textArg=${1:-"text default for 1st argument"} integerArg=${2:-54321} |
simpler usage function
We've replaced the also added command-line processing and usage support for the new functions.
In the usage function, we've replaced the multiple echo lines in usage 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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Only the standard output text is written:
Remember the pipe ("|") conntects connects one program's standard output (here from the step_02.sh script) to the next program's standard input (here to the tee program). Standard error is ignored by the pipe (unless redirected). |
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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.
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# 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 } echo_se "** ERROR in 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
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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. "\nt" will be interpreted as a newline, tab character and "\tn" will be interpreted as a tab characternewline
- 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.
- 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.
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# 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?
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What log file is produced, what ? What are its contents, and why?
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