Bash snippets
Do relative import of a bash library⚑
If you want to import a file lib.sh that lives in the same directory as the file that is importing it you can use the next snippet:
# shellcheck source=lib.sh
source "$(dirname "$(realpath "$0")")/lib.sh"
If you use source ./lib.sh you will get an import error if you run the script on any other place that is not the directory where lib.sh lives.
Check the battery status⚑
This article gives many ways to check the status of a battery, for my purposes the next one is enough
cat /sys/class/power_supply/BAT0/capacity
Check if file is being sourced⚑
Assuming that you are running bash, put the following code near the start of the script that you want to be sourced but not executed:
if [ "${BASH_SOURCE[0]}" -ef "$0" ]
then
echo "Hey, you should source this script, not execute it!"
exit 1
fi
Under bash, ${BASH_SOURCE[0]} will contain the name of the current file that the shell is reading regardless of whether it is being sourced or executed.
By contrast, $0 is the name of the current file being executed.
-ef tests if these two files are the same file. If they are, we alert the user and exit.
Neither -ef nor BASH_SOURCE are POSIX. While -ef is supported by ksh, yash, zsh and Dash, BASH_SOURCE requires bash. In zsh, however, ${BASH_SOURCE[0]} could be replaced by ${(%):-%N}.
Parsing bash arguments⚑
Long story short, it's nasty, think of using a python script with typer instead.
There are some possibilities to do this:
Compare two semantic versions⚑
This article gives a lot of ways to do it. For my case the simplest is to use dpkg to compare two strings in dot-separated version format in bash.
Usage: dpkg --compare-versions <condition>
If the condition is true, the status code returned by dpkg will be zero (indicating success). So, we can use this command in an if statement to compare two version numbers:
$ if $(dpkg --compare-versions "2.11" "lt" "3"); then echo true; else echo false; fi
true
Exclude list of extensions from find command ⚑
find . -not \( -name '*.sh' -o -name '*.log' \)
Self delete shell script ⚑
Add at the end of the script
rm -- "$0"
$0 is a magic variable for the full path of the executed script.
Add a user to the sudoers through command line ⚑
Add the user to the sudo group:
sudo usermod -a -G sudo <username>
The change will take effect the next time the user logs in.
This works because /etc/sudoers is pre-configured to grant permissions to all members of this group (You should not have to make any changes to this):
# Allow members of group sudo to execute any command
%sudo ALL=(ALL:ALL) ALL
Error management done well in bash⚑
If you wish to capture error management in bash you can use the next format
if ( ! echo "$EMAIL" >> "$USER_TOTP_FILE" ) then
echo "** Error: could not associate email for user $USERNAME"
exit 1
fi
Show the progresion of a long running task with dots⚑
echo -n "Process X is running."
# Process is running
sleep 1
echo -n "."
sleep 1
echo -n "."
# Process ended
echo ""
Loop through a list of files found by find⚑
For simple loops use the find -exec syntax:
# execute `process` once for each file
find . -name '*.txt' -exec process {} \;
For more complex loops use a while read construct:
find . -name "*.txt" -print0 | while read -r -d $'\0' file; do
…code using "$file"
done
The loop will execute while the find command is executing. Plus, this command will work even if a file name is returned with whitespace in it. And, you won't overflow your command line buffer.
The -print0 will use the NULL as a file separator instead of a newline and the -d $'\0' will use NULL as the separator while reading.
How not to do it⚑
If you try to run the next snippet:
# Don't do this
for file in $(find . -name "*.txt")
do
…code using "$file"
done
You'll get the next shellcheck warning:
SC2044: For loops over find output are fragile. Use find -exec or a while read loop.
You should not do this because:
Three reasons:
- For the for loop to even start, the find must run to completion.
- If a file name has any whitespace (including space, tab or newline) in it, it will be treated as two separate names.
- Although now unlikely, you can overrun your command line buffer. Imagine if your command line buffer holds 32KB, and your for loop returns 40KB of text. That last 8KB will be dropped right off your for loop and you'll never know it.
Remove the lock screen in ubuntu⚑
Create the /usr/share/glib-2.0/schemas/90_ubuntu-settings.gschema.override file with the next content:
[org.gnome.desktop.screensaver]
lock-enabled = false
[org.gnome.settings-daemon.plugins.power]
idle-dim = false
Then reload the schemas with:
sudo glib-compile-schemas /usr/share/glib-2.0/schemas/
How to deal with HostContextSwitching alertmanager alert⚑
A context switch is described as the kernel suspending execution of one process on the CPU and resuming execution of some other process that had previously been suspended. A context switch is required for every interrupt and every task that the scheduler picks.
Context switching can be due to multitasking, Interrupt handling , user & kernel mode switching. The interrupt rate will naturally go high, if there is higher network traffic, or higher disk traffic. Also it is dependent on the application which every now and then invoking system calls.
If the cores/CPU's are not sufficient to handle load of threads created by application will also result in context switching.
It is not a cause of concern until performance breaks down. This is expected that CPU will do context switching. One shouldn't verify these data at first place since there are many statistical data which should be analyzed prior to looking into kernel activities. Verify the CPU, memory and network usage during this time.
You can see which process is causing issue with the next command:
# pidstat -w 3 10 > /tmp/pidstat.out
10:15:24 AM UID PID cswch/s nvcswch/s Command
10:15:27 AM 0 1 162656.7 16656.7 systemd
10:15:27 AM 0 9 165451.04 15451.04 ksoftirqd/0
10:15:27 AM 0 10 158628.87 15828.87 rcu_sched
10:15:27 AM 0 11 156147.47 15647.47 migration/0
10:15:27 AM 0 17 150135.71 15035.71 ksoftirqd/1
10:15:27 AM 0 23 129769.61 12979.61 ksoftirqd/2
10:15:27 AM 0 29 2238.38 238.38 ksoftirqd/3
10:15:27 AM 0 43 1753 753 khugepaged
10:15:27 AM 0 443 1659 165 usb-storage
10:15:27 AM 0 456 1956.12 156.12 i915/signal:0
10:15:27 AM 0 465 29550 29550 kworker/3:1H-xfs-log/dm-3
10:15:27 AM 0 490 164700 14700 kworker/0:1H-kblockd
10:15:27 AM 0 506 163741.24 16741.24 kworker/1:1H-xfs-log/dm-3
10:15:27 AM 0 594 154742 154742 dmcrypt_write/2
10:15:27 AM 0 629 162021.65 16021.65 kworker/2:1H-kblockd
10:15:27 AM 0 715 147852.48 14852.48 xfsaild/dm-1
10:15:27 AM 0 886 150706.86 15706.86 irq/131-iwlwifi
10:15:27 AM 0 966 135597.92 13597.92 xfsaild/dm-3
10:15:27 AM 81 1037 2325.25 225.25 dbus-daemon
10:15:27 AM 998 1052 118755.1 11755.1 polkitd
10:15:27 AM 70 1056 158248.51 15848.51 avahi-daemon
10:15:27 AM 0 1061 133512.12 455.12 rngd
10:15:27 AM 0 1110 156230 16230 cupsd
10:15:27 AM 0 1192 152298.02 1598.02 sssd_nss
10:15:27 AM 0 1247 166132.99 16632.99 systemd-logind
10:15:27 AM 0 1265 165311.34 16511.34 cups-browsed
10:15:27 AM 0 1408 10556.57 1556.57 wpa_supplicant
10:15:27 AM 0 1687 3835 3835 splunkd
10:15:27 AM 42 1773 3728 3728 Xorg
10:15:27 AM 42 1996 3266.67 266.67 gsd-color
10:15:27 AM 0 3166 32036.36 3036.36 sssd_kcm
10:15:27 AM 119349 3194 151763.64 11763.64 dbus-daemon
10:15:27 AM 119349 3199 158306 18306 Xorg
10:15:27 AM 119349 3242 15.28 5.8 gnome-shell
# pidstat -wt 3 10 > /tmp/pidstat-t.out
Linux 4.18.0-80.11.2.el8_0.x86_64 (hostname) 09/08/2020 _x86_64_ (4 CPU)
10:15:15 AM UID TGID TID cswch/s nvcswch/s Command
10:15:19 AM 0 1 - 152656.7 16656.7 systemd
10:15:19 AM 0 - 1 152656.7 16656.7 |__systemd
10:15:19 AM 0 9 - 165451.04 15451.04 ksoftirqd/0
10:15:19 AM 0 - 9 165451.04 15451.04 |__ksoftirqd/0
10:15:19 AM 0 10 - 158628.87 15828.87 rcu_sched
10:15:19 AM 0 - 10 158628.87 15828.87 |__rcu_sched
10:15:19 AM 0 23 - 129769.61 12979.61 ksoftirqd/2
10:15:19 AM 0 - 23 129769.61 12979.33 |__ksoftirqd/2
10:15:19 AM 0 29 - 32424.5 2445 ksoftirqd/3
10:15:19 AM 0 - 29 32424.5 2445 |__ksoftirqd/3
10:15:19 AM 0 43 - 334 34 khugepaged
10:15:19 AM 0 - 43 334 34 |__khugepaged
10:15:19 AM 0 443 - 11465 566 usb-storage
10:15:19 AM 0 - 443 6433 93 |__usb-storage
10:15:19 AM 0 456 - 15.41 0.00 i915/signal:0
10:15:19 AM 0 - 456 15.41 0.00 |__i915/signal:0
10:15:19 AM 0 715 - 19.34 0.00 xfsaild/dm-1
10:15:19 AM 0 - 715 19.34 0.00 |__xfsaild/dm-1
10:15:19 AM 0 886 - 23.28 0.00 irq/131-iwlwifi
10:15:19 AM 0 - 886 23.28 0.00 |__irq/131-iwlwifi
10:15:19 AM 0 966 - 19.67 0.00 xfsaild/dm-3
10:15:19 AM 0 - 966 19.67 0.00 |__xfsaild/dm-3
10:15:19 AM 81 1037 - 6.89 0.33 dbus-daemon
10:15:19 AM 81 - 1037 6.89 0.33 |__dbus-daemon
10:15:19 AM 0 1038 - 11567.31 4436 NetworkManager
10:15:19 AM 0 - 1038 1.31 0.00 |__NetworkManager
10:15:19 AM 0 - 1088 0.33 0.00 |__gmain
10:15:19 AM 0 - 1094 1340.66 0.00 |__gdbus
10:15:19 AM 998 1052 - 118755.1 11755.1 polkitd
10:15:19 AM 998 - 1052 32420.66 25545 |__polkitd
10:15:19 AM 998 - 1132 0.66 0.00 |__gdbus
Then with help of PID which is causing issue, one can get all system calls details: Raw
# strace -c -f -p <pid of process/thread>
Let this command run for a few minutes while the load/context switch rates are high. It is safe to run this on a production system so you could run it on a good system as well to provide a comparative baseline. Through strace, one can debug & troubleshoot the issue, by looking at system calls the process has made.
Redirect stderr of all subsequent commands of a script to a file⚑
{
somecommand
somecommand2
somecommand3
} 2>&1 | tee -a $DEBUGLOG
Get the root path of a git repository⚑
git rev-parse --show-toplevel
Get epoch gmt time⚑
date -u '+%s'
Check the length of an array with jq⚑
echo '[{"username":"user1"},{"username":"user2"}]' | jq '. | length'
Exit the script if there is an error⚑
set -eu
Prompt the user for data⚑
read -p "Ask whatever" choice
Parse csv with bash⚑
Do the remainder or modulus of a number⚑
expr 5 % 3
Update a json file with jq⚑
Save the next snippet to a file, for example jqr and add it to your $PATH.
#!/bin/zsh
query="$1"
file=$2
temp_file="$(mktemp)"
# Update the content
jq "$query" $file > "$temp_file"
# Check if the file has changed
cmp -s "$file" "$temp_file"
if [[ $? -eq 0 ]] ; then
/bin/rm "$temp_file"
else
/bin/mv "$temp_file" "$file"
fi
Imagine you have the next json file:
{
"property": true,
"other_property": "value"
}
Then you can run:
jqr '.property = false' status.json
And then you'll have:
{
"property": false,
"other_property": "value"
}
