I'm trying to write a script to convert man pages to PDF files. The script I have right now is:
#! /usr/bin/env bash
[[ $# -ne 1 ]] && { echo "Usage: $(basename $0) [command]" ; exit 1 ; }
man -t ${1} | ps2pdf14 - > "${1}_man.pdf"
The problem is that if a man page does not exist, the script will still carry on executing, generating an empty PDF file. So I wonder if there's a way to determine if a man page exists?
Use Man's Exit Status from a Function
The man(1) manual page defines the following exit status codes:
EXIT STATUS
0 Successful program execution.
1 Usage, syntax or configuration file error.
2 Operational error.
3 A child process returned a non-zero exit status.
16 At least one of the pages/files/keywords didn't exist or wasn't
matched.
That means you can use the exit status of man itself to determine if a page is available via manpath. For example:
check_for_man_page () {
man "$1" > /dev/null 2>&1
}
With this function, you can use test conditionals on the exit status like so:
$ check_for_man_page "cat" && echo 'Found it!'
Found it!
$ check_for_man_page "quux" || echo 'Not found!'
Not found!
Using an If/Else Statement
Here's another way to do this, using an if/else statement to determine whether to run your original code:
if man "$1" > /dev/null 2>&1
then
man -t ${1} | ps2pdf14 - > "${1}_man.pdf"
else
echo "Missing man page: $1" >&2
fi
It's not neat, but this seems to do what you want:
if man -w ${1} >/dev/null 2>/dev/null ; then
man -t ${1} | ps2pdf14 - > "${1}_man.pdf"
fi
However, some man pages may exist in different sections of the manual -- for example man 1 printf and man 3 printf. You may wish to modify your script to take this into account.
Related
I want to solve a certain task with a bash script which I pass to a parameter, but unfortunately I don't get anywhere.
This is what it is about.
If a user exists, the exit status 0 should be returned and displayed. If this user does not exist then 1. This works fine so far. Now it is so that if the user should exist and has running processes, only the processes should be displayed. Otherwise only the exit status 0 if the user exists and has no running processes.
#!/bin/bash
user=$1
exists=$(grep -c $user /etc/passwd)
if [ "$exists" -ne 0 ]; then
echo $?
else
echo $?
fi
I added an additional elif, but that did not work. How can I customize the script to show me the running processes for an existing user if they should exist or if no running processes exist but the user exists only returns and displays the status 0 or 1?
Thanks a lot!
The id command can tell if user exists, without need to tap the passwd file, and works regardless of what resource type is providing the users database to the system.
#!/bin/sh
if id --user "$1" > /dev/null 2>&1;
then
ps --user "$1" --format pid=,comm= || :
else
false
fi
Or a one-liner:
sh -c 'id -u "$1">/dev/null 2>&1&&{ ps -u "$1" -o pid=,comm=||:;}' sh username
grep returns the exit status you are looking for, so you probably just want:
#!/bin/bash
user=$1
if grep -q "$user" /etc/passwd; then
echo 0;
exit 0;
else
echo 1;
exit 1;
fi
But that's quite redundant and could be more easily written:
grep -q "$user" /etc/passwd
rv=$?
echo "$rv"
exit "$rv"
However, the requirement to print the return status is odd, and if you simply drop that, your script can just be the grep.
Note that this completely disregards that fact that grepping /etc/password for the user name is not an adequate test to determine if the user exists, but that does not seem to be the heart of this question.
I am working on a script that take 1 string argument and a file. I want it so that if a file is put in that doesn't exist, then it will display the "filename cannot be read" message.
That part does work however it also displays a "grep: grep.txt: No such file or directory" message. Is there any way to stop the grep message from posting and ending the script if the first if statement is true?
#! /bin/sh
if [ ! -f "$2" ]
then
echo "$0" cannot be read 1>&2
fi
if [ $# -eq 2 ]
then
grep "$1" $2
else
echo there is more or less than 2 arguments 1>&2
fi
Exit the script with a non-zero exit code to indicate failure and stop it from continuing on to the grep.
if [ ! -f "$2" ]
then
echo "$0" cannot be read 1>&2
exit 1
fi
You can add /dev/null in grep command it will suppress the error part.
grep "$1" $2 2>/dev/null
The > operator redirects the output usually to a file but it can be to a device. You can also use >> to append.
2> file redirects stderr to file
/dev/null is the null device it takes any input you want and throws it away. It can be used to suppress any output.
You could redirect all errors from grep, for example:
grep "$1" $2 2>/dev/null
(the 2> means redirect standard error, as opposed to standard output with > or 1>).
That introduces a race condition, however: if the file disappears while your script as running, it might still exist when you check that it exists, but be gone by the time grep runs.
You could handle that by checking the exit status...
grep "$1" $2 2>/dev/null
if [[ $? -gt 1 ]]; then
echo "grep failed unexpectedly" >&2
fi
IMHO, in this example it would be better to just let grep print the error.
I am using following options
set -o pipefail
set -e
In bash script to stop execution on error. I have ~100 lines of script executing and I don't want to check return code of every line in the script.
But for one particular command, I want to ignore the error. How can I do that?
The solution:
particular_script || true
Example:
$ cat /tmp/1.sh
particular_script()
{
false
}
set -e
echo one
particular_script || true
echo two
particular_script
echo three
$ bash /tmp/1.sh
one
two
three will be never printed.
Also, I want to add that when pipefail is on,
it is enough for shell to think that the entire pipe has non-zero exit code
when one of commands in the pipe has non-zero exit code (with pipefail off it must the last one).
$ set -o pipefail
$ false | true ; echo $?
1
$ set +o pipefail
$ false | true ; echo $?
0
Just add || true after the command where you want to ignore the error.
Don't stop and also save exit status
Just in case if you want your script not to stop if a particular command fails and you also want to save error code of failed command:
set -e
EXIT_CODE=0
command || EXIT_CODE=$?
echo $EXIT_CODE
More concisely:
! particular_script
From the POSIX specification regarding set -e (emphasis mine):
When this option is on, if a simple command fails for any of the reasons listed in Consequences of Shell Errors or returns an exit status value >0, and is not part of the compound list following a while, until, or if keyword, and is not a part of an AND or OR list, and is not a pipeline preceded by the ! reserved word, then the shell shall immediately exit.
Instead of "returning true", you can also use the "noop" or null utility (as referred in the POSIX specs) : and just "do nothing". You'll save a few letters. :)
#!/usr/bin/env bash
set -e
man nonexistentghing || :
echo "It's ok.."
Thanks for the simple solution here from above:
<particular_script/command> || true
The following construction could be used for additional actions/troubleshooting of script steps and additional flow control options:
if <particular_script/command>
then
echo "<particular_script/command> is fine!"
else
echo "<particular_script/command> failed!"
#exit 1
fi
We can brake the further actions and exit 1 if required.
I found another way to solve this:
set +e
find "./csharp/Platform.$REPOSITORY_NAME/obj" -type f -iname "*.cs" -delete
find "./csharp/Platform.$REPOSITORY_NAME.Tests/obj" -type f -iname "*.cs" -delete
set -e
You can turn off failing on errors by set +e this will now ignore all errors after that line. Once you are done, and you want the script to fail again on any error, you can use set -e.
After applying set +e the find does not fail the whole script anymore, when files are not found. At the same time, error messages
from find are still printed, but the whole script continues to execute. So it is easy to debug if that causes the problem.
This is useful for CI & CD (for example in GitHub Actions).
If you want to prevent your script failing and collect the return code:
command () {
return 1 # or 0 for success
}
set -e
command && returncode=$? || returncode=$?
echo $returncode
returncode is collected no matter whether command succeeds or fails.
output=$(*command* 2>&1) && exit_status=$? || exit_status=$?
echo $output
echo $exit_status
Example of using this to create a log file
log_event(){
timestamp=$(date '+%D %T') #mm/dd/yy HH:MM:SS
echo -e "($timestamp) $event" >> "$log_file"
}
output=$(*command* 2>&1) && exit_status=$? || exit_status=$?
if [ "$exit_status" = 0 ]
then
event="$output"
log_event
else
event="ERROR $output"
log_event
fi
I have been using the snippet below when working with CLI tools and I want to know if some resource exist or not, but I don't care about the output.
if [ -z "$(cat no_exist 2>&1 >/dev/null)" ]; then
echo "none exist actually exist!"
fi
while || true is preferred one, but you can also do
var=$(echo $(exit 1)) # it shouldn't fail
I kind of like this solution :
: `particular_script`
The command/script between the back ticks is executed and its output is fed to the command ":" (which is the equivalent of "true")
$ false
$ echo $?
1
$ : `false`
$ echo $?
0
edit: Fixed ugly typo
I am using following options
set -o pipefail
set -e
In bash script to stop execution on error. I have ~100 lines of script executing and I don't want to check return code of every line in the script.
But for one particular command, I want to ignore the error. How can I do that?
The solution:
particular_script || true
Example:
$ cat /tmp/1.sh
particular_script()
{
false
}
set -e
echo one
particular_script || true
echo two
particular_script
echo three
$ bash /tmp/1.sh
one
two
three will be never printed.
Also, I want to add that when pipefail is on,
it is enough for shell to think that the entire pipe has non-zero exit code
when one of commands in the pipe has non-zero exit code (with pipefail off it must the last one).
$ set -o pipefail
$ false | true ; echo $?
1
$ set +o pipefail
$ false | true ; echo $?
0
Just add || true after the command where you want to ignore the error.
Don't stop and also save exit status
Just in case if you want your script not to stop if a particular command fails and you also want to save error code of failed command:
set -e
EXIT_CODE=0
command || EXIT_CODE=$?
echo $EXIT_CODE
More concisely:
! particular_script
From the POSIX specification regarding set -e (emphasis mine):
When this option is on, if a simple command fails for any of the reasons listed in Consequences of Shell Errors or returns an exit status value >0, and is not part of the compound list following a while, until, or if keyword, and is not a part of an AND or OR list, and is not a pipeline preceded by the ! reserved word, then the shell shall immediately exit.
Instead of "returning true", you can also use the "noop" or null utility (as referred in the POSIX specs) : and just "do nothing". You'll save a few letters. :)
#!/usr/bin/env bash
set -e
man nonexistentghing || :
echo "It's ok.."
Thanks for the simple solution here from above:
<particular_script/command> || true
The following construction could be used for additional actions/troubleshooting of script steps and additional flow control options:
if <particular_script/command>
then
echo "<particular_script/command> is fine!"
else
echo "<particular_script/command> failed!"
#exit 1
fi
We can brake the further actions and exit 1 if required.
I found another way to solve this:
set +e
find "./csharp/Platform.$REPOSITORY_NAME/obj" -type f -iname "*.cs" -delete
find "./csharp/Platform.$REPOSITORY_NAME.Tests/obj" -type f -iname "*.cs" -delete
set -e
You can turn off failing on errors by set +e this will now ignore all errors after that line. Once you are done, and you want the script to fail again on any error, you can use set -e.
After applying set +e the find does not fail the whole script anymore, when files are not found. At the same time, error messages
from find are still printed, but the whole script continues to execute. So it is easy to debug if that causes the problem.
This is useful for CI & CD (for example in GitHub Actions).
If you want to prevent your script failing and collect the return code:
command () {
return 1 # or 0 for success
}
set -e
command && returncode=$? || returncode=$?
echo $returncode
returncode is collected no matter whether command succeeds or fails.
output=$(*command* 2>&1) && exit_status=$? || exit_status=$?
echo $output
echo $exit_status
Example of using this to create a log file
log_event(){
timestamp=$(date '+%D %T') #mm/dd/yy HH:MM:SS
echo -e "($timestamp) $event" >> "$log_file"
}
output=$(*command* 2>&1) && exit_status=$? || exit_status=$?
if [ "$exit_status" = 0 ]
then
event="$output"
log_event
else
event="ERROR $output"
log_event
fi
I have been using the snippet below when working with CLI tools and I want to know if some resource exist or not, but I don't care about the output.
if [ -z "$(cat no_exist 2>&1 >/dev/null)" ]; then
echo "none exist actually exist!"
fi
while || true is preferred one, but you can also do
var=$(echo $(exit 1)) # it shouldn't fail
I kind of like this solution :
: `particular_script`
The command/script between the back ticks is executed and its output is fed to the command ":" (which is the equivalent of "true")
$ false
$ echo $?
1
$ : `false`
$ echo $?
0
edit: Fixed ugly typo
I have a Bash shell script that invokes a number of commands.
I would like to have the shell script automatically exit with a return value of 1 if any of the commands return a non-zero value.
Is this possible without explicitly checking the result of each command?
For example,
dosomething1
if [[ $? -ne 0 ]]; then
exit 1
fi
dosomething2
if [[ $? -ne 0 ]]; then
exit 1
fi
Add this to the beginning of the script:
set -e
This will cause the shell to exit immediately if a simple command exits with a nonzero exit value. A simple command is any command not part of an if, while, or until test, or part of an && or || list.
See the bash manual on the "set" internal command for more details.
It's really annoying to have a script stubbornly continue when something fails in the middle and breaks assumptions for the rest of the script. I personally start almost all portable shell scripts with set -e.
If I'm working with bash specifically, I'll start with
set -Eeuo pipefail
This covers more error handling in a similar fashion. I consider these as sane defaults for new bash programs. Refer to the bash manual for more information on what these options do.
To add to the accepted answer:
Bear in mind that set -e sometimes is not enough, specially if you have pipes.
For example, suppose you have this script
#!/bin/bash
set -e
./configure > configure.log
make
... which works as expected: an error in configure aborts the execution.
Tomorrow you make a seemingly trivial change:
#!/bin/bash
set -e
./configure | tee configure.log
make
... and now it does not work. This is explained here, and a workaround (Bash only) is provided:
#!/bin/bash
set -e
set -o pipefail
./configure | tee configure.log
make
The if statements in your example are unnecessary. Just do it like this:
dosomething1 || exit 1
If you take Ville Laurikari's advice and use set -e then for some commands you may need to use this:
dosomething || true
The || true will make the command pipeline have a true return value even if the command fails so the the -e option will not kill the script.
If you have cleanup you need to do on exit, you can also use 'trap' with the pseudo-signal ERR. This works the same way as trapping INT or any other signal; bash throws ERR if any command exits with a nonzero value:
# Create the trap with
# trap COMMAND SIGNAME [SIGNAME2 SIGNAME3...]
trap "rm -f /tmp/$MYTMPFILE; exit 1" ERR INT TERM
command1
command2
command3
# Partially turn off the trap.
trap - ERR
# Now a control-C will still cause cleanup, but
# a nonzero exit code won't:
ps aux | grep blahblahblah
Or, especially if you're using "set -e", you could trap EXIT; your trap will then be executed when the script exits for any reason, including a normal end, interrupts, an exit caused by the -e option, etc.
The $? variable is rarely needed. The pseudo-idiom command; if [ $? -eq 0 ]; then X; fi should always be written as if command; then X; fi.
The cases where $? is required is when it needs to be checked against multiple values:
command
case $? in
(0) X;;
(1) Y;;
(2) Z;;
esac
or when $? needs to be reused or otherwise manipulated:
if command; then
echo "command successful" >&2
else
ret=$?
echo "command failed with exit code $ret" >&2
exit $ret
fi
Run it with -e or set -e at the top.
Also look at set -u.
On error, the below script will print a RED error message and exit.
Put this at the top of your bash script:
# BASH error handling:
# exit on command failure
set -e
# keep track of the last executed command
trap 'LAST_COMMAND=$CURRENT_COMMAND; CURRENT_COMMAND=$BASH_COMMAND' DEBUG
# on error: print the failed command
trap 'ERROR_CODE=$?; FAILED_COMMAND=$LAST_COMMAND; tput setaf 1; echo "ERROR: command \"$FAILED_COMMAND\" failed with exit code $ERROR_CODE"; put sgr0;' ERR INT TERM
An expression like
dosomething1 && dosomething2 && dosomething3
will stop processing when one of the commands returns with a non-zero value. For example, the following command will never print "done":
cat nosuchfile && echo "done"
echo $?
1
#!/bin/bash -e
should suffice.
I am just throwing in another one for reference since there was an additional question to Mark Edgars input and here is an additional example and touches on the topic overall:
[[ `cmd` ]] && echo success_else_silence
Which is the same as cmd || exit errcode as someone showed.
For example, I want to make sure a partition is unmounted if mounted:
[[ `mount | grep /dev/sda1` ]] && umount /dev/sda1