Bash tips part 1

Last Updated or created 2024-02-13

Best Practices

#!/bin/bash     # Set shebang .. interpreter (sh/python/bash)
#set -x            # debug flag, shows all output and variables
set -e             # exit when an error occurs, Dont use this when sourcing a scriptor using bash ./scriptnaam gebruikt. 
set -u             # exit when a variable isn't defined
set -o pipefail # exit when a pipe command fails
# Add comments to your scripts!

grep string /nonexistenddir/file | sort # Does NOT give an error, sort works!
So echo $? gives 0
When set -o pipefail is set, above example will print 1 or 2

Bash options example


usage ()
    echo >&2 "usage: $0 <list-of-options>"

main ()
    [ $# -lt 1 ] && usage
    INSTALL_DIR=`dirname $0`
    for i in $@; do
        echo "$INSTALL_DIR $i"

main $@

Execute output from a script.

Sometimes i write scripts which print the command I want to have executed. Most of the times to check the generated commandline.
simple example

ls | grep 2000 | while read ; do echo rm -f "$REPLY" ;done

Above only prints the lines
rm -f <filename with 2000>
rm -f <other filename with 2000>

Instead of removing the echo command, you can add a | bash , to have it executed.

ls | grep 2000 | while read ; do echo rm -f "$REPLY" ;done | bash

Or even shorter ( Use !! for previous command )

!! | bash 


Search with CTRL-R in your command history, and use !<number> to execute this command again.

touch file1 file2 file3 file4
chmod 664 !* # will chmod only the files

When you used typed
systemctl stop httpd
and want to start again use replace
This will take previous command containing stop and places stop with start and executes this.

Toggle history on/off with “set -o history”, depending on your setup you can use “<space>command to be executed”

History log original username per date when sudo is being used
(creates .history.20230103.fash in /root/ )

HISTFILE=~/.history.$(date +%y%b%d).$(who am i | awk '{ print $1 }')

Skip first 2 lines and join 2 lines

#skip 2 lines
tail -n +3

#join 2 lines
sed 'N;s/\n/,/'

#3rd line from a file
sed "3q;d" /etc/hosts

#join on line line using comma's
paste -sd "," -

Find Tricks

#Remove empty directories
find ??? -empty -type d -delete

#Find multiple extentions
find archieven/ \( -name "*.png" -o -name "*.xml" \)
-print0 to handle filenames with spaces

Remove huge directory structures FAST

mkdir /tmp/empty ; rsync -a --delete /tmp/empty /path/blah

Check program installed and in path

which zenity >/dev/null 2>/tmp/err || ( echo "zenity not found, please install" ; exit 1 )

Change directory to location script for relative path usage

cd $(dirname $0)  
ls relativesubdir/

IFS (Internal Field Separator)

IFS=$' '     # internal field separator, strings split here
for f in $(cat /etc/hosts) ; do echo $f ;done
#        localhost
IFS=$'\n\t'     # internal field separator, split end of line
for f in $(cat /etc/hosts) ; do echo $f ;done
# Outputs localhost 

Difference for and while read example

echo "This will print every word" > text  # Places text in file
echo "second line" >> text                # Append text
for f in $(cat text) ; do echo $f ;done   # for example

cat text | while read f ; do echo $f ; done # Read example
This will print every word
second line

Correct way to make a tempfile

tempfile=$(mktemp -d -t /tmp/log.$$)    # Temp file, with unique name $$ is the process number

Usage in script :
ls > $tempfile

rm -f "/tmp/$tempfile"    # Remove

Direcory empty test

[ "$(ls -A /tmp)" ] && echo "Not empty" || echo "Empty" # test directory Empty/filled
test -d /tmp/1 && rmdir /tmp/1           # Removes a directory when it exists, will give an error when NOT empty

Using Expand

On directories archive30_tmp till archive35_tmp setting recursive chmod 2775 

find achief{30..35}_tmp -type d -exec chmod 2775 {} \;
mkdir tmp{1..3} # will create tmp1 tmp2 tmp3

echo pr{ut,utser}s # Outputs "pruts prutsers"

Test root user

if [ $USER == "root" ] ; then … ; fi # execute only when root
if $USER is empty, this will give an error.

if [ w$USER == "wroot" ] works 
qouting $USER also, but qouting a number using less/greater test could be problematic

Mount test

grep -qs /media /proc/mounts && echo "/media is mounted"

Date tricks

datum=$(date +%Y%m%d)                    # datum is yyyymmdd 20230103
today=$(date +%F)
tomorrow=$(date --date="next day" +%F)
p3=$(date --date="$p2" +%F)
#now=$(date +%s --date="1 days ago")
now=$(date -d $(date --date="1 days ago" +%F) +%s)
p3epoch=$(date --date="$p3" +%s)
dater=$(date -d "$p3 1 year" +%F)

date -d @<UNIX timestamp> # Timestamp to date

Size test in directory

if [ "$(df /tmp |grep -v Available | awk {' print $4 }')" -lt 1000000 ] ; then echo "not enough free in /tmp" ;fi  

Parallel Tasks

4 parallel jobs  
find jpg -type f -name \*.jpg -print0 | xargs -0 -n1 -P4 ./

Cluster ssh trick

Using Clusterssh at work
## clusterssh trick
Only needed to do stuff on server having a certain directory

    clusterssh storageservers # ssh to 24 storage servers
    sudo su -
    cd /bricks/*/backup2 # change directory to this if exists
    cd /alternatedir/brick0*/backup2 # change to this alternate directory if it exists 
# directories not found? then you are still in /root
    pwd | grep testdir || logout # no testdir in found subdirs? (there are non in /root so you will be logged out)
    id | grep myuser && logout # if mortal user? then logout again .. you will be disconnected from servers not containing the backup2 directories
    Do your work as root 

Forgot to sudo?

systemctl restart httpd (wont work as user)
sudo !!
(this will do  "sudo systemctl restart httpd")

Top 5 homedir users

du -hsx /home/* | sort -rh | head -5

Removing a huge file takes a long time .. lets truncate it

: > hugefile
rm hugefile

Check memory banks using dmidecode

sudo dmidecode| grep  -i -B1 "Form Factor" (B1 means BEFORE 1 line, A is AFTER)
        Size: 8192 MB
        Form Factor: DIMM
        Size: 8192 MB
        Form Factor: DIMM
        Size: No Module Installed
        Form Factor: DIMM
sudo dmidecode -t memory | grep -i size
        Size: 4096 MB

sudo lshw -short -C memory
H/W path            Device      Class          Description
/0/0                            memory         96KiB BIOS
/0/1000                         memory         4GiB System Memory
/0/1000/0                       memory         4GiB DIMM RAM

Script log replay

    script --timing=/tmp/time.txt /tmp/script.log
start your script
    scriptreplay -t /tmp/time.txt /tmp/script.log

Deleted file takes up disk space (not being released)


httpd 12209 root 6w REG 253,3 5233639424 2097234 /var/log/httpd/access_log-20200711 (deleted)

cd /proc/12209/fd

find /proc/12209/fd -ls | grep '(deleted)'

121613145 0 l-wx------ 1 root root 64 Jul 10 12:04 /proc/12209/fd/6 -> /var/log/httpd/access_log-20200711\ (deleted)

: > "/proc/12209/fd/6"                 # Truncate

Keyword in line grep

cat file | grep -o 'skip_reason.*' # till end
cat file | grep -o 'skip_reason.*tillhere' 

Remove space filename

mv *\ * a 
rm *\ * 

Upper to lower case

 tr '[:upper:]' '[:lower:]'

Whole directories to lowercase

#print usage 
if [ -z $1 ];then
        echo "Usage :$(basename $0) parent-directory"
        exit 1

#process all subdirectories and files in parent directory
all="$(find $1 -depth)"

for name in ${all}; do
        #set new name in lower case for files and directories
        new_name="$(dirname "${name}")/$(basename "${name}" | tr '[A-Z]' '[a-z]')"

        #check if new name already exists
        if [ "${name}" != "${new_name}" ]; then
                [ ! -e "${new_name}" ] && mv -T "${name}" "${new_name}"; echo "${name} was renamed to ${new_name}" || echo "${name} wasn't renamed!"

#list directories and file new names in lowercase
echo "Directories and files with new names in lowercase letters"
find $(echo $1 | tr 'A-Z' 'a-z') -depth

exit 0

Sort by lenght

awk '{ print length(), $0 | "sort -n" }' 

Order / Delimiter AWK

echo | awk -F  "." '/1/ { print $3,$2,$1 }'

From – Till using AWK

cat file | awk '/Title1/,/-----/' 

Replace Tabs

sed -e 's/\t/ /g'

Sed Case Insensitive

sed s/a/b/gI 

And using lynx std in

cat index.html | lynx --dump -stdin

Random sleep

sleep .$[ ( $RANDOM % 4 ) + 1 ]s
#remove . for whole seconds instead of 10th of seconds

Copy pasting a lot from terminal , use xclip to copy to clipboard

xclip -sel c < bigsourcefile.c

Cache dir in memory for high IO processes

[ ! -d /ramcache ] && mkdir -p /ramcache
mount -t tmpfs -o size=2G,nr_inodes=5k,mode=777 tmpfs /ramcache

Have a dual boot machine? And want to remotely start the other OS?

WINDOWS_TITLE=`grep -i "^menuentry 'Windows" /boot/grub/grub.cfg|head -n 1|cut -d"'" -f2`
sudo grub-reboot "$WINDOWS_TITLE"
sudo reboot

Move files in subdirectory per create hour

for f in $(seq -w 0 23) ; do find . -type f -print0 | xargs -0 -r stat -c "%y %N" | grep -E " $f:" | cut -f2 -d\' | while read ; do mkdir -p $f ; mv "$REPLY" $f/ ; done ;d
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