Bash Index : T - The 'T' Bash commands : description, flags and examples

cols x lines : 126 x 48

type type

type is a shell builtin

type ln

ln is /bin/ln

type -a ls

ls is aliased to `ls --color=auto'
ls is /bin/ls

type -t ln

file

'myFunction' is a function
'aFunctionThatDoesNotExist' does not exist

sed-based :

awk-based :

dirToDisplay='/path/to/directory'
cd "$dirToDisplay"; ls -R | awk -F '/' '/:$/ {	\
	if (length($NF)==2)	{ next }		\
	if (NF>0)		{ printf " " }	\
	for (i=0;i<NF-2;i++)	{ printf "|   " }	\
	if (NF>1)		{ printf "+--" }	\
	cleanDirName=gensub(/:$/, "", "g", $NF); print cleanDirName \
	}'; cd - 1>/dev/null

• 0 or EXIT : run commands when the shell exits
• DEBUG : run commands before every simple command
• RETURN : run commands each time a function or script finishes executing
• ERR : run commands every time a command exits with a non-zero status (except for if, while and until loops and AND &&, OR || and NOT ! logical operations)
  these situations and exceptions are those of the shell -e flag

finish() {
	# clean your mess before leaving
	}

trap finish EXIT

• It may be possible to do the same with sed's transliteration operator y///
• If it's about renaming files to turn their name uppercase / lowercase, rename can do it easily

Replace [newline] with [space] :

tr '\n' ' ' < inputFile

Same as above + write to outputFile :

tr '\n' ' ' < inputFile > outputFile

Delete line endings (which will turn the input into a single giant line) :

• echo -e 'foo\nbar\nbaz' | tr -d '\n'

  foobarbaz

• tr -d '\n' < inputFile

tr -d '\r' < fileToConvert > convertedFile

1. *
2. +
3. ,
4. -

This is no big deal unless you mean * and + and -. In such case, specify *+-.

D : uninterruptible sleep

• sleep state that cannot handle a signal, such as waiting for disk or network I/O
• details

I : idle

R : runnable

process in the run queue : ready to run or currently running

S : sleeping

T : stopped by job control signal

(traced)

t : stopped by debugger during trace

Z : zombie

• the process itself has actually ended, but the parent process has not reclaimed its resources (PID, ...)
• find zombies

• us : Percentage of CPU time spent in user space
• sy : Percentage of CPU time spent in system (=kernel) space
• ni : Percentage of CPU time spent in low priority (=nice) processes
• id : Percentage of CPU time spent idle
• wa : Percentage of CPU time spent waiting for I/Os (disk, network), aka IOWait
• hi : Percentage of CPU time spent in handling hardware interrupts
• si : Percentage of CPU time spent in handling software interrupts
• st : steal time : the amount of CPU 'stolen' from this virtual machine by the hypervisor for other tasks (such as running another virtual machine).

More about the IOWait value (source) :

1. top + f (toggle visible fields)
2. browse down to SWAP
3. d (toggle display)
4. While still on the SWAP line, you can enable sorting by swap usage with s (which will update the 1^st line : ...whose current sort field is SWAP)
5. Then leave with q
6. On the processes page, use < / > to sort asc / desc

How Linux manages the swap :

• top + z (use colors) + i (hide idlers)
  Processes in IOWait are those showing a D in the S (status) column.
• watch -n 1 "(ps aux | awk '\$8 ~ /D/ { print \$0 }')"

top 3.2.8 :

top 3.3.3 :

• touch myFile
• touch -am myFile

-rw------- 1 stuart developers 0 Jul  7 14:13 myFile
-rw------- 1 stuart developers 0 May 26 14:13 myFile

• time { compoundCommand; } (details) Mind the spaces surrounding the curly brackets !
• time sh -c 'compoundCommand'

• jump to solution
• workaround : /usr/bin/time -f %E true
  This can only be used to launch a binary, not a script function (details).

• time true | grep real

  real	0m0.004s
  user	0m0.003s
  sys	0m0.001s

  grep seems to do nothing . This is because time outputs to stderr. Let's redirect the output :
• time true 2>&1 | grep real

  real	0m0.001s
  user	0m0.002s
  sys	0m0.000s

  Still no luck. What's wrong this time (no pun intended ) is that we're actually redirecting the output of the "timed" command, instead of the output of time itself. Check it :
• time ls 1>/dev/null

  real	0m0.003s
  user	0m0.000s
  sys	0m0.003s

  so we need to "catch" the output of time and redirect it to stdout before feeding grep :
• { time true; } 2>&1 | grep real

  real	0m0.000s

tail -f test.log | egrep --line-buffered "error" | tee -a test.log_SHORT

this fails :

echo hello > myFile{1..5}

bash: myFile{1..5}: ambiguous redirect

this works :

for i in {1..5}; do echo hello > "myFile$i"; done

(no output)

this works even better :

echo hello | tee myFile{1..5}

hello

this writes into files in >-mode (details)

• telnet host tcpPort
• Can I probe UDP ports ?

• a welcome screen
• a prompt
• a blank screen (?)
• anything different from a Connection Refused, a Connection timed out or an error message

• on Windows : CTRL-$
• on Linux : ALT Gr-CTRL-°

then quit.

1. telnet www.google.com 80
2. GET / HTTP/1.1
3. Host: www.google.com

• Requests are case-sensitive : trying get will return an HTTP 405 error
• You have to press twice to send the request.
• If the requested page is large, you can limit the length of the output to see the HTTP headers with : telnet host tcpPort | head -30

No, you can't. Use nc -u instead.

UDP is a connectionless protocol, which means it basically just sends packets out to the specified destination. TCP is connection-oriented which means it establishes a connection to the other end using the 3-way handshake.
So it makes sense to apply the telnet paradigm to TCP : you open a connection to a specific host and port, you still remain connected (for a period of time) even if you aren't sending any data and you can send and receive data continuously without having to reconnect in-between.
UDP, on the other hand, doesn't really fit the telnet model : its more of a fire-and-forget system where you fire-off a series of packets towards the destination. You then go on with something else (or just wait doing nothing) until (or if) the remote process sends some packets back.

version 1.15 - Sergey Poznyakoff, 2004-12-20

* Compressed archives are recognised automatically, it is no longer
necessary to specify -Z, -z, or -j options to read them. Thus, you can
now run 'tar tf archive.tar.gz'.

Let's check this :

Testing with '-z'
./archive: gzip compressed data, last modified: Tue May 14 08:02:09 2019, from Unix		archive type reported by file
total 12
-rw------- 1 stuart developers 142 May 14 10:02 archive
-rw------- 1 stuart developers   6 May 14 10:02 hello					files properly un-archived
-rw------- 1 stuart developers   6 May 14 10:02 world

Testing with '-j'
./archive: bzip2 compressed data, block size = 900k						archive type reported by file
total 12
-rw------- 1 stuart developers 143 May 14 10:02 archive
-rw------- 1 stuart developers   6 May 14 10:02 hello					files properly un-archived
-rw------- 1 stuart developers   6 May 14 10:02 world

• 1^st method :
  1. tar -cf archiveName.tar file1 file2 ... file_n to build the archive (= package of several files in a single file)
  2. gzip archiveName.tar to compress the archive
• 2^nd method : tar cfz archiveName.tgz file1 file2 ... file_n
• 3^rd method : tar -cjf archiveName.bz2 file1 file2 ... file_n

• It is also possible to specify a list of exclude patterns in an external file with --exclude-from=file (details)
• As of tar version 1.23, the flag --exclude-vcs excludes ALL version control system files automatically (source)

Reproduce the problem : touch {a,b,c}.tar; tar xf *tar, outputs :

tar: This does not look like a tar archive this is normal as these are not genuine tar archives
tar: b.tar: Not found in archive
tar: c.tar: Not found in archive
tar: Exiting with failure status due to previous errors

What causes this behavior is that the command :

Solutions :