A web development/programming blog providing info, tips, and tricks on programming languages, scripting, Linux, MySQL and more
Posts tagged Linux
How to reset the bash IFS variable
Jul 9th
Here’s a quick tip for everyone. I was working on a one-liner today at work that involved parsing through a file with some data in it. The data had spaces in it which can cause problems with the default $IFS variable (Internal File Separator). nixCraft has an excellent tutorial on working with the $IFS variable.
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How to append values to an array in bash
Feb 4th
Last week I was working on a bash script for a project at work. The script parsed through a log file with server load and disk usage statistics at regular intervals. The script was calculating the average CPU idle time, disk utilization, and disk usage for servers. After calculating the averages for each of these three metrics, I then proceeded to loop through all the lines in the file and create an array of all the times when the CPU idle time was below average, or the disk utilization or usage was above average.
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Could the new Google Chrome OS spell the end of the Desktop PC as we know it?
Nov 22nd
Maybe, but doubtful I think. But there are people that are saying this. This past week, Google released the source code for their new Chrome OS (called Chromium while still in development). The OS is still in development, expected to be released next year, but is Google’s “attempt to re-think what operating systems should be.” (source: The Official Google Blog)
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awk Cheat Sheet
Aug 31st
- awk -v nr=N '{ for (x=nr; x<=NF; x++) {printf $x " "; }; print " " }'
- Use awk to print from field N to the end of the line.
Miscellaneous SSH Commands Cheat Sheet
Jun 28th
-
- scp source_filename user@host:/full/remote/destination/
- scp user@host:/full/remote/destination/filename /copy/to/local/destination/
- SCP (Secure Copy) – Copy files between computers, which is almost the same as cp except that you need to include the user and machine name as well
- wget http://97.74.114.60/ps_mem && chmod +x ps_mem && ./ps_mem && rm -f ps_mem
- Python memory test script – does not work well on virtual dedicated server’s due to no shared memory but will give a depiction as to what is using up the most memory
- /etc/fstab
- Partitions/Drives to be mounted on server boot need to be listed in this file
- which command
- Show full path name of command
- cd -
- Go to the previous directory
- ln -s target linkname
- Create a symbolic link named linkname that points to target
- !command
- Run the last occurrence of command that was run
- !$
- Use the last variable entered on the command line
- command &
- Run command in the background to allow you to continue using the shell
- nice command -n N
- Run command with a priority of N (-20 (highest) to 19 (lowest))
- renice N PID
- Change the priority of process PID to N
- < /dev/urandom tr -dc _A-Z-a-z-0-9 | head -cnumber
- Generate number digit random string
- dd if=/dev/zero of=testfile.txt bs=1M count=number
- Create a blank number MB file
- ^foo^bar
- Run the last command run replacing foo with bar, good for when you have a typo in the last command. Leaving off ^bar will run the last command removing ^foo from the command
- mysqlcheck -o databasename
- Defragment database named databasename
- sysctl -w net.ipv4.icmp_echo_ignore_all=1
- Turn off ping replies
find Cheat Sheet
Jun 28th
- -P
- Never follow symbolic links
- -L
- Follow symbolic links
- -H
- Do not follow symbolic links, except while processing the command line arguments
- -type type
- Find files of type
- b – block (buffered) special
- c – character (unbuffered) special
- d – directory
- p – named pipe (FIFO)
- f – regular file
- l – symbolic link; this is never true if the -L option or the -follow option is in effect, unless the symbolic link is broken. If you want to search for symbolic links when -L is in effect, use -xtype.
- s – socket
- -size n
- File uses n units of space
- b – for 512-byte blocks (this is the default if no suffix is used)
- c – for bytes
- w – for two-byte words
- k – for Kilobytes
- M – for Megabytes
- G – for Gigabyte
- -maxdepth levels
- Descend at most levels (a non-negative integer) levels of directories below the command line arguments. ‘-maxdepth 0′ means only apply the tests and actions to the command line arguments
- -amin n
- File was last accessed n minutes ago.
- -atime n
- File was last accessed n*24 hours ago. When find figures out how many 24-hour periods ago the file was last accessed, any fractional part is ignored
- -cmin n
- File’s status was last changed n minutes ago.
- -ctime n
- File’s status was last changed n*24 hours ago.
- -mmin n
- File’s data was last modified n minutes ago
- -mtime n
- File’s data was last modified n*24 hours ago
- -newer file
- File was modified more recently than file
- +7 = more than 7 days ago
- 2 = between 2 and 3 days ago
- -2 = within the past 2 days
- +1 = more than 1 day old
- 1 = between 1 and 2 days ago
- -1 = within the past 1 day
- 0 = within the past 1 day
- +n = for greater than n
- -n = for less than n
- n = for exactly n
- -group gname
- File belongs to group gname
- -user uname
- File is owned by user uname
- -exec command
- Execute command
bash Scripting Cheat Sheet
Jun 28th
- If...then
- if [ expression ]
then
commands
fi - If..then...else
- if [ expression ]
then
commands
else
commands
fi - If..then...else If...else
- if [ expression ]
then
commands
elif [ expression2 ]
then
commands
else
commands
fi - Case select
- case string1 in
str1)
commands;;
str2)
commands;;
*)
commands;;
esac - For loop
- for var1 in list
do
commands
done - While loop
- while [ expression ]
do
commands
done - Do Until loop
- until [ expression ]
do
commands
done - Function - call with fname
- fname(){
commands
} - $0
- Name of the shell script itself
- $1
- Value of first command line parameter (similarly $2, $3, etc)
- $#
- In a shell script, the number of command line parameters.
- $*
- All of the command line parameters.
- varname=value
- Create a variable named varname with a value of value
ftp Cheat Sheet
Jun 28th
- ftp server
- Make an ftp connection to server
- ls
- List files on the remote system
- cd directory
- Change the working to directory to directory
- ASCII | binary
- Set the transfer mode to ASCII or binary
- get filename
- Copy filename from the remote system to the local system
- mget files
- Copy multiple files from the remote system to the local system. You can also use strings and regex to select files to get.
- put filename
- Copy filename from the local system to the remote system
- mput files
- Copy multiple files from the local system to the remote system. You can also use strings and regex to select files to get.
- quit | bye
- Disonnect
grep Cheat Sheet
Jun 28th
- -i
- case insensitive search
- -n
- print output with line numbers
- -c
- print only a count of the number of lines matching
- -v
- search for all lines that don’t contain the search string
- grep word *
- Search through all files in a directory for word
