Why is $? creating a file? - bash

In the following script I am testing how to count how many times a command runs.
#!/bin/bash
echo "Hello"
scount=$?
if [ $scount -eq 0 ]; then
count=$(cat ${scount})
else
count=0
fi
((count++))
echo ${count} > ${scount}
echo "Scount: $count"
This is the output I receive. I am confused as to why I am getting the cat: 0: No such file or directory message.
Hello
cat: 0: No such file or directory
Scount: 1

As said on Unix manual for cat
cat - concatenate files and print on the standard output
And as said on $? here
$? is the exit status of the last executed command.
So:
cat command wants a file to read and print in standard output
$? is a number and its value is 0, cause the last command executed in that point of the script was echo "Hello" (well executed = status 0)
You're trying to do something like cat 0 so with this:
count=$(cat ${scount})

echo "Hello"
scount=$?
if [ $scount -eq 0 ]; then
count=$(cat ${scount})
else
count=0
fi
I think you were wanting to assign 0 to count and used cat instead of echo
count=$(echo "$scount")
But that's a useless use of echo
count=$scount
But now you're assigning 0 to count in both branches of the if statement, so you can remove the whole if:
echo "Hello"
scount=$?
count=0

Related

shell: failed to save error stream code to file

I am trying to detect whenever the following script (random_fail.sh) fails --which happens rarely-- by running it inside a while loop in the second script (catch_error.sh):
#!/usr/bin/env bash
# random_fail.sh
n=$(( RANDOM % 100 ))
if [[ n -eq 42 ]]; then
echo "Something went wrong"
>&2 echo "The error was using magic numbers"
exit 1
fi
echo "Everything went according to plan"
#!/usr/bin/env bash
# catch_error.sh
count=0 # The number of times before failing
error=0 # assuming everything initially ran fine
while [ "$error" != 1 ]; do
# running till non-zero exit
# writing the error code from the radom_fail script into /tmp/error
bash ./random_fail.sh 1>/tmp/msg 2>/tmp/error
# reading from the file, assuming 0 written inside most of the times
error="$(cat /tmp/error)"
echo "$error"
# updating the count
count=$((count + 1))
done
echo "random_fail.sh failed!: $(cat /tmp/msg)"
echo "Error code: $(cat /tmp/error)"
echo "Ran ${count} times, before failing"
I was expecting that the catch_error.sh will read from /tmp/error and come out of the loop once a particular run of random_fail.sh exits with 1.
Instead, the catch script seems to be running forever. I think this is because the error code is not being redirected to the /tmp/error file at all.
Please help.
You aren't catching the error code in the proper/usual manner. Also, no need to prefix the execution with the "bash" command, when it already contains the shebang. Lastly, curious why you don't simply use #!/bin/bash instead of #!/usr/bin/env bash .
Your second script should be modified to look like this:
#!/usr/bin/env bash
# catch_error.sh
count=0 # The number of times before failing
error=0 # assuming everything initially ran fine
while [ "$error" != 1 ]; do
# running till non-zero exit
# writing the error code from the radom_fail script into /tmp/error
./random_fail.sh 1>/tmp/msg 2>/tmp/error
error=$?
echo "$error"
# updating the count
count=$((count + 1))
done
echo "random_fail.sh failed!: $(cat /tmp/msg)"
echo "Error code: ${error}"
echo "Ran ${count} times, before failing"
[ "$error" != 1 ] is true if random_fail.sh prints a lone digit 1 to stderr. As long as this doesn't happen, your script will loop. You could instead test whether there has been written anything to stderr. There are several possibilities to achieve this:
printf '' >/tmp/error
while [[ ! -s /tmp/error ]]
or
error=
while (( $#error == 0 ))
or
error=
while [[ -z $error ]]
/tmp/error will always be either empty or will contain the line "The error was using magic numbers". It will never contain 0 or 1. If you want to know the exit value of the script, just check it directly:
if ./random_fail.sh 1>/tmp/msg 2>/tmp/error; then error=1; else error=0; fi
Or, you can do:
./random_fail.sh 1>/tmp/msg 2>/tmp/error
error=$?
But don't do either of those. Just do:
while ./random_fail.sh; do ...; done
As long as random_fail.sh (please read https://www.talisman.org/~erlkonig/documents/commandname-extensions-considered-harmful/ and stop naming your scripts with a .sh suffix) returns 0, the loop body will be entered. When it returns non-zero, the loop terminates.

PIPESTATUS[0] in BASH script

I am implementing a scenario in Unix Bash scripts. I have two scripts ABC.bash and XYZ.bash. There is one condition in ABC.bash when requester does not enter Y or y scripts exit with message and do not work further. ABC.bash working fine when runs alone.Problem arises when I run it from another bash script i.e. XYZ.bash. It does not check for exit condition. Syntax of logic in XYZ.bash.
echo "Calling ABC.bash from XYZ.bash"
ABC.bash $a $b | tee -a $LOGFILE; sleep 2
if [ ${PIPESTATUS[0]} = 0 ]
then
echo "Do some work"
else
echo "Check ABC.bash input"
exit 1
fi
But when ABC.bash $a $b exit with status 2 flow still goes to IF block rather than ELSE.In log I can see message as DEBUGMODE set to 0. I need this DEBUGMODE setting as it is required but want to exit if ABC.bash exit. Ideally it should go to ELSE part as ABC.bash exit with wrong user input.
Additionally I have set up DEBUGMODE option in XYZ.bash script. Like-
if [[ -z "$1" ]]
then
echo " ">> No input so default to 0"
DEBUGMODE=0
else
echo "DEBUGMODE set to $1"
DEBUGMODE=$1
fi
enter code here
The problem is that PIPESTATUS is a volatile variable. That is it will be reset as soon as any other command is executed. You need to remove the call to sleep 2 if you want to inspect the PIPESTATUS.
In your example, PIPESTATUS reflects the status of sleep 2. So replace
ABC.bash $a $b | tee -a $LOGFILE; sleep 2
if [ ${PIPESTATUS[0]} = 0 ]
by
ABC.bash $a $b | tee -a $LOGFILE; pstat=(${PIPESTATUS[#]}); sleep 2
if [ ${pstat[0]} = 0 ]
to save the status.
As best practice, unless the your code has full control over the any variable content, better to quote the variable. This will not prevent logical error (e.g, the extra sleep that modified the PIPESTATUS), but it will avoid accidental injection of code into the script (or unexpected syntax errors)
if [ "${PIPESTATUS[0]}" = 0 ] ; then

Unix shell script: exit with returning value

I have the following unix shell script, in which i have two integer
variables namely a and b.
If a is greater then or equal to b then shell script should exit with returning 0.
Else it should exit with returning 1.
My try:
Script: ConditionTest.sh
#!/bin/sh
a=10
b=20
if [ $a -ge $b ]
then
exit 0
else
exit 1
fi
....
....
....
Running Script:
$ ./ConditionTest.sh
$
Note: I am not getting any return value after executing the file.
The shell puts the exit status of the last command in the variable ?.
You could simply inspect it:
mycommand
echo $?
... or you could use it to do something else depending on its value:
mycommand && echo "ok" || echo "failed"
or alternatively, and slightly more readable:
if mycommand; then
# exit with 0
echo "ok"
else
# exit with non-zero
echo "failed"
if
Your script looks fine; you did everything right.
#!/bin/sh
a=10
b=20
if [ $a -ge $b ]
then
exit 0
else
exit 1
fi
So here's where we run it and check the return value:
$ sh test.sh
$ echo $?
1
$
10 is not greater than or equal to 20.
Another way to test it would be like this:
$ sh test.sh && echo "succeeded" || echo "failed"
failed
As noted in the comments, you should also quote your variables, always:
if [ $a -ge $b ]
Should be:
if [ "$a" -ge "$b" ]
To add to the previous answers, the key idea you should understand is that every program provides a number when exiting. That number is used as a way to report if the command has completed its operation successfully, and if not, what type of error has occurred.
Like mentioned, the exit code of the last command executed can be accessed with $?.
The reason nothing was printed by your script, is that your script returned 1, but the exit code of a command is not printed. (This is analogous to calling a function, you get a return value from the function but it's not printed)

Cannot compare received user input via read

I have the following shell script:
#!/bin/bash
if [ "`read -n 1`" == "c" ] ; then
printf "\nfoo\n"
exit 0
fi
printf "\nbar\n"
exit 0
However, regardless of the input, I always get bar as the output:
$ ./test.sh
c
bar
$ ./test.sh
d
bar
Why is this occuring and what do I need to change in the shell script?
You need to read it into a variable first, otherwise you're just comparing the output value of read (which is empty value).
Following should work:
#!/bin/bash
read -n 1 ch
if [ "$ch" == "c" ] ; then
printf "\nfoo\n"
exit 0
fi
printf "\nbar\n"
exit 0

Check the output of a command in shell script

I'm writing a very simple shell scripts that would looked at the log of all failed tests, and print out all the name of all files in the current directory that are in the log
1 #! /bin/sh
2 for file in *
3 do
4 echo "checking: $file"
5 if [$(grep $file failed.txt -c) -ne 0]
6 then
7 echo "$file FAILED"
8 fi
9 done
When I execute it, I get this error:
line 6: [0: command not found
Does anyone have any idea why?
Thanks!!
[ is actually a command in linux (like bash or cat or grep).
$(grep $file failed.txt -c) is a command substitution which in your case evaluated to 0. Thus the line now reads [0 -ne 0], which is interpreted as run a program called [0 with arguments -ne 0].
What you should write instead is [ $(grep $file failed.txt -c) -ne 0 ]. Shell scripts require that there be spaces between the opening and closing square braces. Otherwise you change the command that is executed (the closing ] indicates that there are no more arguments to be read.
So now the command evaluates to [ 0 -ne 0 ]. You can try executing this in your shell to see what happens. [ exits with a value of 0 if the expression is true and 1 if it is false. You can see the exit value by echoing $? (the exit value of the last command to be run).
Instead of testing the count, you can test the return code of grep:
if grep -q $file failed.txt &>/dev/null
The script can be
#!/bin/sh
for file in *; do
echo "checking: $file"
grep failed.txt $file && echo "$file FAILED"
done
or, as an one-liner in user shell command history:
for file in *; do { echo "checking: $file" && grep failed.txt $file && echo "$file FAILED"; done
in man grep
EXIT STATUS
The exit status is 0 if selected lines are found, and 1 if not found. If an error occurred the exit status is 2. (Note: POSIX error handling code should check for '2' or greater.)

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