Introduction
One of my previous articles was more popular than I expected, so I want to write another article to introduce some lesser-known bash features
As I said before, because I think bash is a technology that needs to be used frequently (and need to be understood), I wrote a book while studying bash. Although many people are not familiar with bash, but I think they also think it is very important enough to be gratifying.
1) ^ x ^ y ^
A little trick I always use.
Have you never entered a similar command?
$ grp somestring somefile-bash: grp: command not found
Hey, this command is wrong, so you have to type "↑", then type "â†" until "p", and then type "e" to execute.
Or enter it like this:
$ ^ rp ^ rep ^ grep 'somestring' somefile $
One detail you may need to pay attention to is:
$ grp rp somefile $ ^ rp ^ rep ^ $ grep rp somefile
If you want to search for "rep", then you have to delve into the man page and learn to use this more powerful command:
$ grp rp somefile $ !!: gs / rp / repgrep rep somefile $
I will not explain this usage here. . .
2) pushd / popd
This is very useful in scripts, especially in loops
As shown below, suppose you are writing a for loop that enters and exits the folder:
for d1 in $ (ls -d * /) do # Store original working directory. original_wd = "$ (pwd)" cd "$ d1" for d2 in $ (ls -d * /) do pushd "$ d2" # Do something popd done # Return to original working directory cd "$ {original_wd}" done
You can use the pushd stack to rewrite the above code like this:
for d1 in $ (ls -d *) do pushd "$ d1" for d2 in $ (ls -d * /) do pushd "$ d2" # Do something popd done popddone
It can track and record the directory you switch and push or pop the stack
Note that when an error occurs when using pushd, the stack record may be lost and popd multiple times. So you may want to use set -e in the script (see previous article)
Of course you can also use cd-, but it will not use the stack-just return to the previous directory
cd ~ cd / tmpcd blahcd-# Back to / tmpcd-# Back to 'blah'cd-# Back to / tmpcd-# Back to' blah '...
3) shopt vs set
These two commands bothered me for a while.
What is the difference between the two?
Set has been introduced in the previous article, and the shopt looks similar. Just typing shopt will display a series of options:
$ shoptcdable_vars offcdspell oncheckhash offcheckwinsize oncmdhist oncompat31 offdotglob off
I found some answers here.
Fundamentally, there seems to be a series of bash (and other shells) built on top of sh, and adding the shopt command provides a way to set additional shell options
But I'm not sure ... if you know why, please tell me.
4) Here Docs and Here Strings
"Here Docs" are files created with some statements in the shell.
The "trick" is simple. Define a word for ending, then all input lines that appear before this word alone will constitute a file.
like this:
$ cat> afile here is a doc> it has three lines> SOMEENDSTRING alone on a line will save the doc> SOMEENDSTRING $ cat afilehere is a docit has three linesSOMEENDSTRING alone on a line will save the doc $
note:
· If the ending word does not appear "alone" on a line, it can constitute a file
· SOMEENDSTRING is usually END, but this is just a habit
Lesser known is "here string":
$ cat> asd
5) Operation of string variables
In the past, you might have written code as shown below, using tools such as sed to manipulate strings:
$ VAR = 'HEADERMy voice is my passwordFOOTER' $ PASS = "$ (echo $ VAR | sed 's / ^ HEADER (. *) FOOTER / 1 /')" $ echo $ PASS
But you may not know that bash itself is possible.
This means you can save a lot of sed and awk.
One way to rewrite the above code is as follows:
$ VAR = 'HEADERMy voice is my passwordFOOTER' $ PASS = "$ {VAR # HEADER}" $ PASS = "$ {PASS% FOOTER}" $ echo $ PASS
· # Means "match and delete the given pattern string from the beginning of the string"
·% Means "match and delete the given pattern string from the end of the string"
On my computer, the latter method is twice as fast as the previous one. And (to my surprise), his speed is roughly equivalent to the speed of python scripts with similar functions
If you want to use wildcard (see above) mode string and adopt greedy mode, you need to double write:
$ VAR = 'HEADERMy voice is my passwordFOOTER' $ echo $ {VAR ## HEADER *} $ echo $ {VAR %% * FOOTER}
6) The default value of the variable
These are very useful for writing scripts.
If you have a variable that has no value assigned, you can "assign default value" to it like this
Create a default.sh file and write the following:
#! / bin / bashFIRST_ARG = "$ {1: -no_first_arg}" SECOND_ARG = "$ {2: -no_second_arg}" THIRD_ARG = "$ {3: -no_third_arg}" echo $ {FIRST_ARG} echo $ {SECOND_ARG} echo $ {THIRD_ARG}
Now execute chmod + x default.sh and run the script with ./default.sh first second:
Observe how the default value of the third parameter is assigned instead of the first two.
You can also directly use $ {VAR: = defaultval} (equal sign, not dash), but note that this does not apply to position variables in scripts or functions. Try to modify the above script to see how it failed.
7) Traps
When a signal is sent to the script, the built-in trap can be used to "capture"
Here is an example I used in my chepci script:
function cleanup () {rm -rf "$ {BUILD_DIR}" rm -f "$ {LOCK_FILE}" # get rid of / tmp detritus, leaving anything accessed 2 days ago + find "$ {BUILD_DIR_BASE}" / * -type d- atime +1 | rm -rf echo "cleanup done"} trap cleanup TERM INT QUIT
Any CTRL-C, CTRL- or termination of the program that uses the TERM signal will first call cleanup
note:
· The logic of trap can be very tricky (for example, to deal with signal race conditions)
· KILL signal cannot be captured in this way
But in most cases, I will use it in a similar 'cleanup' to achieve the purpose of the function.
8) Shell variable
It is well worth knowing the standard shell variables available. These are my favorite.
RANDOM
Don't rely on this to encrypt the stack, but you can generate random numbers, for example when creating temporary files in scripts:
$ echo $ {RANDOM} 16313 $ # Not enough digits? $ echo $ {RANDOM} $ {RANDOM} 113610703 $ NEWFILE = / tmp / newfile _ $ {RANDOM} $ touch $ NEWFILE
REPLY
No need to give read a variable name
$ readmy input $ echo $ {REPLY}
LINENO and SECONDS
Easy to debug
$ echo $ {LINENO} 115 $ echo $ {SECONDS}; sleep 1; echo $ {SECONDS}; echo $ LINENO174380174381116
Note that even if you use; to separate commands, the above code should be divided into two lines
TMOUT
Can be used for timeout reading, it is really good in some scripts
#! / bin / bashTMOUT = 5echo You have 5 seconds to respond ... readecho $ {REPLY: -noreply}
9) Extglobs
If you are really addicted to bash and cannot extricate yourself, then you may want to enhance your wildcarding capabilities. You can set the extglob option in the shell. This is the setting method:
shopt -s extglobA = "12345678901234567890" B = "$ {A}"
Now let's see if you can point out the functions of the following statements:
echo "B | $ {B} |" echo "B # + () | $ {B # + ()} |" echo "B #? () | $ {B #? ()} |" echo "B # * () | $ {B # * ()} | "echo" B ## + () | $ {B ## + ()} | "echo" B ## * () | $ {B ## * ()} | "echo" B ## ? () | $ {B ##? ()} | "
Although it may be useful, it is difficult to imagine a situation where you must use this method. Usually you will use some tools that are more suitable for the corresponding task (like sed) or just give up bash and use some "appropriate" programming language like python.
10) Associative array
When it comes to porting to other languages, an important rule is that if I need to use arrays, then I will give up bash and use python (for this I even created a Docker Container to run a dedicated tool)
I didn't know it until I read it, there is an associative array in bash
The following is a demonstration:
$ declare -A MYAA = ([one] = 1 [two] = 2 [three] = 3) $ MYAA [one] = "1" $ MYAA [two] = "2" $ echo $ MYAA $ echo $ {MYAA [one]} $ MYAA [one] = "1" $ WANT = two $ echo $ {MYAA [$ WANT]}
Note only for bash 4.x + version
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