Command line

1.1. Command line#

last update: Feb 07, 2024

Here, we learn command line. The command line is a text interface to computers. You input text commands to your computer to do some operations, for example making or removing files, and changing files permissions, etc.

In Mac, you can use the command line in terminal.app

1.1.1. Basic#

To begin with, we check shortcuts.

Command	Description
Ctrl + C	Interrupt the current process
Ctrl + Z	Suspend the current process
Ctrl + S	Stop command output to the screen
Ctrl + Q	resume output to the screen paused by Ctrl+S
Ctrl + L	Clear the screen
Ctrl + D	Exit the shell
Ctrl + A	Move the cursor to the start of a line
Ctrl + E	Move the cursor to the end of the line
Ctrl + W	Cut the word before the cursor
Ctrl + U	Cut the part of the line before the cursor
Ctrl + K	Cut the part of the line after the cursor
Ctrl + Y	Paste the last thing you cut

Special symbols#

There are 4 special symbols and each of them represents a location.

Symbol	Meaning
`/`	root directory
`.`	current directory
`..`	parent directory
`~`	home directory

basics commands#

pwd: show the current directory

mkdir: make directory

mkdir ~/tmp

cd: change directory

cd ~/tmp

touch: make files

touch file

ls: list files or directories

ls
# file

mkdir -p; make parent directories as needed

mkdir -p dir/subdir/ssubdir
ls
# file dir

rm: remove files
rm -r: remove directories and their contents

rm file
rm -r dir

cf.

rm -v file # show filename when removed
rm -i file # comfirm whether to rmove
rm -f file # force remove

rmdir: remove empty directory

mkdir dir
ls
# dir
rmdir dir
ls
# (nothing)

cf.

mkdir -p dir/subdir/ssubdir

rmdir -p dir/subdir/ssubdir

echo:

print strings
```
echo $PATH
```
make files with contents
- Make new file or overwrite a existing file : >
```
echo [Strings] > [filename]
```
- Append : >>
```
echo [Strings] >> [filename]
```

cat: print file contents

echo hello > file1
cat file1
# hello

cat > file2
asdf  (input something)
Ctrl + D (end input)
cat file2
# asdf

cp: copy file(s) or directory(ies)

cp file1 file3
cat file3
# hello

mkdir dir1
touch dir1/file4
cp -r dir1 dir2
ls dir2
# file4

Note

When doing cp file1 file2 and the file2 already exists, you will be asked whether to overwrite the contents of file2.

If the file2 does not exist, the contents of file1 will be copied to file2.

When doing cp -r dir1 dir2 and the dir2 already exists, new dir1 will be created in dir2.

If the dir2 does not exist, the contents of dir1 will be copied to dir2. So the contents of these directories are the same.

mv: move file(s) or directory(ies). If file5 does not exist, the following operation is rename

mv file1 file5

file5 will be moved to dir3.

mkdir dir3
mv file5 dir3

If dir4 does not exist, the following operation is rename. Otherwise dir1 will be moved to dir4.

mv dir1 dir4

tree: print contents of the current directory in a tree-like format. You can use tree -d to print directory contents. You can use tree -L to print directory contents up to a certain level.

You have to install tree command in bash first.

brew install tree

tree
test
├── dir2
│   └── file4
├── dir3
│   └── file5
├── dir4
│   └── file4
├── file2
└── file3

# print directory contents up to level 2
tree -L 2

chomd: change file modes(permissions)

chmod 755 file

You can specify the mode with octal numbers.

When you specify the mode with octal number;
4 stands for “read”
2 stands for “write”
1 stands for “execute”
0 stands for “no permission”

If you permit all operations(4+2+1) to everyone, you can use 777 for example. If you permit all operations(4+2+1) to owner and read and execute(4+1) to group and others, you can use 755 for example.

You can specify the mode with symbol like u+x for user, g+x for group, o+x for others.

target	meaning	operation	meaning	mode	meaning
u	User	=	set the modes	r	read
g	Group	+	add the mode	w	write
o	Others	-	remove the mode	x	execute
a	All

For example,

chmod u=rwx,g=rx,o=r file

chmod u+x file

You can use chmod -R to change permissions recursively.

Shell globbing#

globbing patterns specify sets of filenames with wildcard characters. globbing patterns is a powerful way to find files that match a pattern. For example, *.txt means all files with the extension .txt in the current directory.

There are several common wildcard characters:

* : matches zero or more characters
? : matches any single character
[abc] : matches any character in set
[a-z] : matches any character in range
[!abc] : matches any character not in set
[!a-z] : matches any character not in range

extended pattern matching

*(pattern) more than 0 times
?(pattern) 0,1 times
@(pattern) only 1 times
+(pattern) more than 1 times
!(pattern) not in the pattern

ls
# file1 file2 file3 file4 file11 file123
ls !(file1)
# file2 file3 file4 file11 file123
ls !(file1|file2)
# file3  file4 file11 file123
ls file[0-9]
# file1 file2 file3 file4
ls file[0-9]+([0-9])
# file11 file123

find: find files and directories

# search the current directory for filename(not recursively)
find filename

# search target directory recursively for lecture0~lecture9 directories (cf. -type f)
find target -name lecture[0-9] -type d

# search the current directory recursively for files accessed within 1 day
find . -atime -1

# search the current directory recursively for files modified within 1 day
find . -mtime -1

# search the current directory recursively for files with size between 30kB and 1MB
find . -size +30k -size -1M -name '*.py'

# search target directory recursively for files or directories that match condition1 and condition2 (cf. -or, -not)
find target condtion1 -and condition2

# search the current directory recursively for .txt files and remove them all.
find . -name '*.txt' -exec rm {} +

grep: search files for patterns

# Search any line that contains `word` in filename
grep [-Options] 'word' filename

grep 'word' file1 file2 file3
grep 'string1 string2'  filename

Options	Description
-i	Ignore case distinctions
-w	The expression is searched for as a word
-v	Select non-matching lines
-n	Print line number with output lines
-h	Suppress the Unix file name prefix on output
-H	Print file name with output lines
-r	Search directories recursivly
-R	Just like -r but follow all symlinks
-l	Print only names of FILEs with matches
-L	Print only names of FILEs without matches
-c	Print only a count of selected lines per FILE
–color	Display matched pattern in colors
-m NUMBER	Stop grep command NUMBER selected lines
-o	Display only matched parts of lines

Brace Expansion#

You can generate arbitrary strings using brace expansion.

echo a{d,c,b}e
ade ace abe

echo {1,2}{3,4}
13 14 23 24

Some examples:

# move all files and directories to `dir`
mv * dir

mv ./path/{foo,bar,baz}.txt dir
# = mv ./path/foo.txt ./path/bar.txt ./path/baz.txt `dir`

# move all pyhon and shell files to `dir`
mv *{.py,.sh} dir

Pipeline and redirect#

`|`: pipe
`>`: redirect

In Unix-like systems, there are three types of streams, which connect computer programs and its environment; standard input (stdin), standard output (stdout), and standard error (stderr). The input stream is the stream that receives data from the user. The output stream is the stream that sends data to the user. The error stream is the stream that sends error messages to the user. A pipe and a redirect are techniques that connects two programs by sending output from one command to the input of the other command.

pipeline

| stdout to stdin. Second command will take the stdout of the previous command as stdin.

history | head

redirect

> stdout to file

history > hist.txt
cat hist.txt | head

>> add contents

echo asdf > test.txt
cat test.txt
# asdf

echo fdsa >> test.txt
cat test.txt
# asdf
# fdsa

To remove stderr, add 2>/dev/null to the end of the command.

Combine commands#

command2 after commmand1
command2 is executed even when command1 puts stderr
```
command1 ; command2
```
command2 and commmand1 at the same time
```
command1 & command2
```
command2 after commmand1 succeeded
```
command1 && command2
```
command2 after commmand1 failed
```
command1 || command2
```

command using the other stdout $()

date +%Y%m%d
touch file_$(date +%Y%m%d)

# nested command
dirname $(which cat)
ls $(dirname $(which cat))

Symbolic links#

ln -s: make a symbolic link.

Symbolic links is the file that refers to another file. By creating symbolic links, you don’t have to copy the same file to multiple locations. Also, if you change the contents of the file, the changes will be reflected in all locations accessed through the symbolic link. However, if the original file is deleted, the symbolic link will be broken.

ln -s original_file link_file
ls -l
# lrwxr-xr-x  1 user  group  12 Aug  4 15:18 link_file -> original_file

Note

Aliases are similar to symlinks, but they are valid after you move the original files unlike symlinks. Although it seems aliases are more useful, aliases are not compatible with unix system. You cannot use like cd alias but cd symlink.

Note

ln options:
-s, –symbolic make symbolic links instead of hard links
-f, –force make links even if the file exists

Command line

Contents

1.1. Command line#

1.1.1. Basic#

Special symbols#

basics commands#

Shell globbing#

Brace Expansion#

Pipeline and redirect#

Combine commands#

Symbolic links#

1.1.2. Reference#