dotfiles/notes/shell/unix_bash.md

Table of Contents

1. Bash
   • Process priority
   • Job Control in Bash
   • Vim editing for Bash
   • Stop yourself from deleting files
   • Reusing commands
   • Automated interactive input
   • Bash scripting
   • Other Bash uses
2. Ed for terminal file editing
3. Awk the programming language

• Built in variables
• Regular expression
• Recipes
• Advanced examples

Bash

Process priority

Since the processor must decide priority, it uses a scale from -20 through 20 to rank what has priority, with -20 being the highest priority. By default, processes launched by the user start with a niceness of 0

# nice --10 ./start_qemu.sh
# nice -n -10 ./start_qemu.sh

Starts the process, in this case start_qemu.sh, with a given niceness level, in this case -10. The -n option increments from the base value, usually 0

# renice -2 1244
# renice -n -2 1244

Changes the niceness of a running process via process id, in this case 1244. -n increments niceness from the current value, in this case by -2

$ ps -fl -C 1244

Check the niceness of process with id 1244

Job Control in Bash

You should not use jobs if you can help it! Especially when it comes to input/output jobs are terribly unclear. Only use them for tasks that will never ask for input or give an output. Use a multiplexer (tmux) instead

$ python -m SimpleHTTPServer 3000 &

Starts a process, in this case the python server, in the background. Just append a & at the end

Oh no, I didn't launch the process in the background ^z If you want to move suspend the current process, ^z will suspend and stop it

$ bg %2
$ bg 2
$ %2 &

Continues the stopped job in the background. In this case, we use job identifier 1, which will start that specific job. You can omit the % with bg

$ fg %2

Bring a job back to the foreground. Uses identifiers much like bg

$ jobs -l

List all background jobs running. -l shows their process id

$ kill %2

Kills a background job, via identifiers. The % is not optional here!

$ wait %2

Waits for job identifier 2 to finish. Passing no arguments waits for all background jobs

$ wlsunset -t 4000 -T 65000 -g 0.9 &
$ disown %1

Launches wlsunset in the background. Then, assuming it's job 1 under jobs, disowns the process. The shell can now close with the process still running

$ wlsunset -t 4000 -T 65000 -g 0.9 & disown

Same as above. More compact

$ { wlsunset -t 4000 -T 65000 -g 0.9 & } &
$ ( wlsunset -t 4000 -T 65000 -g 0.9 & ) &

Same as above, background and disowns the subshell. {} execute the subshell in the current shell's context, while () start a fresh shell. {} are also used in brace expansion, so they need to be delimited by spaces

Vim editing for Bash

$ set -o vi

Makes bash use vi key binds. Note, this uses vi not vim bindings. Generally this isn't recommended, even for complete vim users

$ export EDITOR='vim'

Bash will run the specified command when looking for an editor

<C-x><C-r>

Open current shell line in editor. Really powerful for long lines

See the (Ex section)[#Batch-editing-with-ex] for batch editing with vim in ex mode

Stop yourself from deleting files

$ chmod a-w safe_directory

Prevents writing to the directory. Writing, in the sense of editing files is still controlled by file permissions, you just can't remove or make new ones

# chown other_user safe_directory; chmod 755 safe_directory

Makes another user own the directory. All others uses cannot make/remove files. Same as above, if ownership is given to root, though a bit more explicit

# touch safe_directory/file
# rm safe_directory/file

You can still create and delete files in this directory with root

Reusing commands

See HISTORY EXPANSION in man bash for more information

$ sudo !!
$ sudo !-1

Recalls the last command and runs it as root. Notice the -1 can be any number to recall older commands

$ !man

Rerun the last command starting with the string man

$ !?diff?:p

Print the last command that contained the string diff

$ mv !:2 ../!:2

Substitutes !:2 with the second argument of the previous command. The command itself is !:0, so it's the second argument. Here it moves a directory up

$ mv !^ !^'_name'
$ mv !$ !$'_name'

$ and ^ are aliases for the first and last arguments. Here it changes a file or directory name by adding _name at the end

$ vim -p !touch:*
$ vim -p !touch:2-4
$ vim -p !:2*

Expands argument ranges from the last touch command. *, 1-* and 1* are synonymous. If no string is specified, like !:*, the last command is used

$ vim -o !touch:1*

Open a new window for every argument in the most recent command starting with touch

$ ^touch^vim -o^
$ !!:s/touch/vim -o
$ !!:&

Replay the last command replacing the first instance of touch with vim -o. The third line replaces & with the last substitution

$ vim -o !touch:*:gs/html/md

Recall all the arguments of the last touch command. Replace all instances of html with md then pass those to vim -o

$ cat !!:$:s/machine/human

Substitute in the last argument from the previous command with human

<C-r>
<C-s>

Search backward or forward for input string. Use multiple times to scroll

awk '{a[NR] = $0} END { for (i = NR; i > 0; i--) print a[i] }' # rev lines
<C-r># rev lines

Replays the tagged command. Use trailing comments to act as a tag

diff ~/Downloads/{before,after}.txt

Checks if before.txt and after.txt are different. Braces are duplicated

$ !if:gs/$c/$a

Replace all the variables $c with $a in the previous command starting with if

So the general structure of recall is

!<cmd-identifier>[:<arg-range>][:<operator>]

Automated interactive input

$ yes | rm -ir /deep_dir

Bypasses interactive y/N prompt given by the rm command for every thing in that directory. yes prints an infinite number of 'y\n' to any program

$ printf 'y\nn\ny\n' | rm -ir /small_dir

Will remove the first and third file automatically, then ask you for further input

$ rm -ir /deep_dir < pre_made_input.txt

Enters the file contexts to the interactive prompt. Line breaks are like '\n'. Very helpful if you're using this order of input over and over

Bash scripting

Bash is an awful scripting language in every sense except portability. Always install and use shellcheck after writing a script

bash_script.sh
#!/bin/bash
#!/usr/bin/env bash
#!/usr/bin/env -S awk -f ${SOMEFILE}

Use shebang at the top to declare an interpreter. Using env is considered more portable, though the -S option is required for anything longer than one word

$ echo ${myvar:-not here}
$ echo ${myvar:-"not here"}

Expands to $myvar if it's set, otherwise expands to string "not here"

$ for i in $(seq 0 9); do python -c "print(ord('$1'))" & done

Asynchronously print the ASCII codes for range [0, 9]

Other Bash uses

$ compgen -c git

See the possible completion for the given word. This is what tab uses

$ time bash -c "make && ./a.out; rm ./a.out"

Times multiple separate bash commands

Ed for terminal file editing

Ed is the original text editor for Unix systems, which still is useful for batch editing scripts. Ranges are very similar to vim's

$ ed -p '> :' file1

Open file1 with ed. Useful for writing a script

:n      Print the current line enumerated
:100    Moves to line 100 and prints it
:ka     Set a mark `a`
:'a     Move to mark `a`
:i      Open line above current line in insert mode. Like vim's `o`
:a      Open line below current line in insert mode. Like vim's `O`
.       Exit insert mode. Should be the only character on the line

Various basic commands. Most commands accept ranges, like vim's. Unlike vim, all commands operate on entire lines at once

:g/Style/p\
a\
Style line is above ^^^^\
.\
n

For all lines matching the regex, run the command sequence. \ separate lines

See vim_batch_editing.md for practical application of this

Awk the programming language

AWK is an old, though surprisingly useable stream-editing language. It's a POSIX compliant tool to bundle grep, sed, printf, cut, xargs, and more all into one scripting language. Most its use comes in bash scripts or one-liners to quickly extract information from a text stream. The man pages are really good!

Much of the standard control flow is the same as C. Variables don't need to be initialized before use, ex: i+=1. Many functions will perform on $0 if no argument is given

#!/usr/bin/env -S awk -f
#!/usr/bin/env -S awk -F: -f

Use the string option on env to be able to specify the necessary -f option

substr(str, start_pos, chars_cut)   // Cut out part of a string
length([str])               // Length of str, or $0 if no argument is given
index(str, search_str)      // Find starting index of match in str
match(str, /reg/)           // Return index. Set RSTART and RLENGTH
split(str, array, fs)       // Split str into array elements on FieldSeperator
sub(/reg/, new_str, str)    // Subs new_str for first /reg/ match in str
sub("match", new_str, str)  // Subs new_str for first string "match" in str
gsub(/reg/, new_str, str)   // Above just global

The core awk functions with odd argument order

printf, sprintf, system, tolower, toupper, exp, log...

Core awk functions without odd argument order

Built in variables

NR  Current line's number. Literally "number of reads". Starts at 1
NF  Number of fields in this line. `$NR` expands to the last field's value
RSTART  Field number of last `match()`
RLENGTH  How many characters long the last `match()` was
$0  The entire line of text
$1  Where `1` is the `1th` field in the line

Regular expression

Regular expressions are supported. They're a separate type from strings. Unlike most modern regex interpreters, escape sequences such as \s and \d aren't supported. Instead use POSIX compliant versions [:space:] and [:digit:]

Inverting a regex is done by defining a ! in front. For example $0 !~ /hello/ $0 ~ !/hello/ $0 ~ /[^(hello)]/

If you want to store a regex in a variable for later use, DO NOT store an actual regex type. Store the regex as a string instead, effectively replacing the encapsulating / with "

my_regex = "hello[:space:]+you"
$0 ~ my_regex

Notably, using a regex instead of a string in this situation results in odd silent errors that mostly work. These can be hard to debug, so watch out

Recipes

$ cat file.txt | awk 'tolower($0) ~ /word/ { print $0 }'
$ cat file.txt | awk 'tolower($0) ~ /word/'
$ cat file.txt | grep -i 'word'

Searches through given piped text for 'word' in any casing

$ cat file.txt | awk -F ',' '
        { commas += NF - 1 }
    END { print "Commas counted:", commas }'

Counts the number of commas in a file.

$ awk -F '\t' '{ print $0 }' demo.txt
$ awk -F '\t' demo.txt

Uses tabs are a files separator between "lines"

$ current_window_id=$(chrome-cli list windows | awk -F ' ' \
        'NR == 1 { gsub(/[\[\]]/, "", $1); print $1 }')

Parses the window id from [999] Some Title to 999

$ ls -la | awk 'match($NF, /[0-9]+/) { print substr($NF, RSTART, RLENGTH) }' >
coffees.nh

Checks if the last argument is a string of consecutive numbers. Notice how $NF uses that NF is the last number to index the last field like $3

Advanced examples

END {
    arr["hello"] = 2; arr["other"]++; arr["some"] += 10;

    printf "Hello: %d, Other: %d, Some: %d, None: %d\n",
        arr["hello"], arr["other"], arr["some"], arr["none"];
}
>>> Output: Hello: 2, Other: 1, Some: 10, None: 0

Arrays can be indexed by strings! This allows for counting words or lines

$ awk '!($0 in uniq) { uniq[$0]; print }' duplicates.txt > uniques.txt

Removes duplicate lines. This uses the relational operator (expr, expr... arr) to check if an element with the string's index has been initialized in the array

$ awk '!unique[$0]++' duplicates.txt > uniques.txt

Removes duplicate lines. This exploits awk's automatic initialization of variables with falsey values, 0 here, then turns that index truthy with ++