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Linux Notes

The normal settings for /tmp are 1777, which ls shows as drwxrwxrwt. That is: wide open, except that only the owner of a file can remove it (that’s what this extra t bit means for a directory).

Nice commands

find duplicate files, generate rm command to remove old same size file to make it remove duplicates, remove echo from xargs.

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for FILE in *; do stat -c"%s/%n" "$FILE"; done | awk -F/ '{if ($1 in a)print $2; else a[$1]=1}' | xargs echo rm

shell programing tips

enable xtrace for shell script

  • The set -x option instructs bash to print commands and their arguments as they are executed.
  • The set -x option instructs bash to print shell input lines as they are read.

Unofficial Bash Strict Mode

  • The set -e option instructs bash to immediately exit if any command has a non-zero exit status.
  • The set -u option instructs bash to immediately exit if a reference to any variable you haven’t previously defined - with the exceptions of $* and $@.
  • The set -o pipefail option instructs bash to prevents errors in a pipeline from being masked.
  • Setting IFS to $’\n\t’ means that word splitting will happen only on newlines and tab characters.
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#!/usr/bin/env bash
# Uncomment following line for Debugging
# set -xv

set -euo pipefail
IFS=$'\n\t'
export LC_ALL=C

awk example

split and print

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awk '{FS=","} {print $6}'

expect example

handle first time ssh login

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expect <<EOF
set timeout 10

spawn ssh root@$TARGET_IP
expect {
    "Password" {send "$PWD\r";}
    "yes/no" {send "yes\r";exp_continue}
}
expect "#*"

send "mv $BIN $BIN.bak;\r"
send "exit\r"
expect eof

EOF

coredump file check simple example

to check where coredump file will be generate

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cat /proc/sys/kernel/core_pattern

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readelf -h core.21437 
objdump86 -x core.21437 
objdump -x core.21437 
gdb test core.21437

grep usage

To search the string in a single file,

grep file1.txt -e ‘techieshouts’

To search the full word and not the substring,

grep -w file1.txt -e ‘techieshouts’

To search the full word by ignoring case sensitiveness,

grep -wi file1.txt -e ‘techieshouts’

To search all the files recursively in the directory for the same word,

grep -wir /directory -e ‘techieshouts’

To search and get the line number of the word in all the files

grep -wirn /directory -e ‘techieshouts’

To search and get only the file names that contain the word

grep -wirl /directory -e ‘techieshouts’

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