3.9 KiB
STIN, STOUT, STERR
Input is 0, output is 1, error is 2.
Pipe standard output to log.txt while also outputting it.
cat file.txt |& tee -a log.txt
Copy file and if that's successful, delete it where it stands.
scp archive.tar.gz pi@192.168.0.31:/home/pi && rm archive.tar.gz
A double pipe will try one, and do the other if that fails.
cp -r ~/Archive ~/Backup || tar czf Archive.tar.gz *
REGEX
Regular expression characters include:
\ ^ $ . | ? * + () [] {}
As a result, grep cannot read these characters as literal characters unless they are escaped. E.g.
grep wtf\? log.txt
... will search the string 'wtf?' in the file log.txt. Another version is egrep (now used with 'grep -e') which uses more characters as special characters, or fgrep, which treats all characters as literal strings.
Environmental Variables
PWD, USER, PATH
To display all environmental (but not local) variables, use
env
Set a variable with
colour=red
Display your variable with
echo $colour
Export this to the entire system using:
export colour=blue
Search for Programs
Search for commands relevant to cat.
apropos cat
Show files used in the cat program.
whereis cat
Show which file is the actual code which runs when you type cat:
which cat
Working with Text
Convert every tab to ten spaces.
expand -t 10 file.txt
Or the reverse, with 3 spaces converting to a tab.
unexpand -t 3 file.txt
Format a file by cutting text after 60 characters.
fmt -w 60 file.txt
Indent all but the first line of a paragraph.
fmt -t file.txt
Look at the new lines of a file only:
tail -f /var/log/syslog
The sort function arranges lines alphabetically. Use -r to reverse and -n to sort by number.
Sed
Change all examples of hey to hoi in greetings and show that output (does not change file).
sed 's/hey/hoi/g greetings.txt'
Change each example of 'cat' to 'dog' in the file 'animals.md'.
sed 's/cat/dog/g' animals.md
Measurement
Measure how long a script takes for super-autism powers.
time [bash script]
Aliases
Make one word equal a longer command with alias.
E.g.:
alias wot='sudo systemd-analyze blame && free -h '
Now when you type 'go', it will perform that entire operation.
Functions
Make a function which checks if something is a file, and if so, shows it on screen with cat:
function show(){ file "$1" | grep text &>/dev/null && cat "$1" }
That $1 refers to the first argument typed after the command.
If you want to run this on a file called list, then use:
show list
...and the list will output, only if it is text.
In total, this functions the same as typing:
file list | grep text &>/dev/null && list
Remove a function with:
unset my_function
Paths
Every shell has various paths from where it can execute binary files. Find out your current one with:
echo $PATH
To add a directory to a path, e.g. /usr/share/bin, you can declare it in addition to the old path with:
PATH=$PATH:/usr/share/bin
And then check it by echoing the path again.
Before this, probably best to check the path exists with:
if [ -e /usr/share/bin ]; then
echo yes
fi
Pipes, Pedantry and Brackets
Things that statements can do which [ ] statements cannot:
- Intuitive and easy 'and' statements.
- Intuitive and easy 'or' statements.
- Simple expression comparisons
- Simple expression comparisons with clumsy strings
- Vague comparisons
- [[
answer =~ ^y(es)?]]
- [[
exec
exec will start a process running as just that process. In a bash script, the line:
unison rat
... will startup unison as a sub-process of bash. But:
exec unison rat
... starts unison as its own process.
Brace expansion
mv picture{,-1}.jpg
This expands to
mv picture.jpg picture-1.jpg
for Statements
for f in *tiff;do convert "$f" "${f/.tiff/.png}" done