Shell/Git

1 Shell

A shell is a user interface for access to an operating system. Some command line shells include: Bash shell (Unix shell), Windows PowerShell, Command prompt (cmd.exe in windows).

• Shells are REPL’s that can read/execute commands from a file (shell script/bash script/script)
• Git Bash for Windows: an emulated bash shell

Common Bash Commands

Some of these commands/examples were inspired by Codecademy’s Learn the Command Line course

Command	Description
ls	List file system (default current directory)
pwd	Print working directory
cd newdirectory; cd ..	Change to newdirectory, move up one directory
mkdir dirname	Make new directory
touch newfile	Create new file in working directory
cp old.txt new.txt	Copy old into new
mv file.txt directory	Move file in current directory to new directory
mv file.txt newname.txt	Rename file
rm file, rm -r directory	Delete file or directory (including sub-directory)

Redirecting Input and Output

Command	Description
export var="myvar"	Set var to “myvar”
echo "string"; echo $var	Displays in terminal
echo "string" > file.txt	Write “string” to file
cat file.txt	Print contents of file
cat file1.txt > file2.txt	Replace file2 with file1
cat file1.txt >> file2.txt	Append file1 to file2
cat file1.txt |wc| cat > file2.txt	Compute # lines, words, characters in 1, redirect to 2
sort file1.txt | uniq > file2.txt	Sort alphabetically, save uniq to 2
Note: uniq	Filters out adjacent, duplicate lines
grep "string" file.txt	Returns lines with “string” (grep: global regular expression print)
sed 's/old/new/' file.txt	Substitute first ‘old’ with ‘new’ per line (sed: stream editor)
sed 's/old/new/g' file.txt	Substitute all ‘old’ with ‘new’ per line

Configuring the Environment

The .bash_profile and .bash_rc are configuration files for bash shell. .bash_profile is run on log-in shells (generally on start-up), while .bash_rc is run on normal shells.

Some commands that could be run the terminal or stored in .bash_profile or .bash_rc:

Command	Description
echo "Welcome"	Welcome message displayed
alias ls="ls --color"	Write “string” to file
export PS1=">> "	Prompt changed from $ to >>
export USER="Name"	Create variable
source ~/.bash_profile	Activate changes to current session

nano is a command line text editor. It can be used to edit any file, for instance nano ~/.bash_profile.

Bash Scripting

In a file (e.g. myfile, myfile.sh), a shebang is given at the beginning to tell which interpreter to use when executing the file (e.g. #!/bin/bash, #!/bin/python). To run the file: ./myfile. To run explicitly with different interpreter: python myfile.

• To give permission to execute the file: chmod +x myfile.sh

2 Git

Git is a version control system often used to coordinate work among programmers, but can be used to track changes in any set of files. Generally, users make changes to their local repository and then share these changes to a common remote repository.

Commands listed can be run in Git Bash, or a Git GUI (like SourceTree) can be used.

Some links: Glossary, Reference, Commands

• Initialize/create a new local repository, tells git to begin tracking changes
  • git init
• Check out a repository
  • git clone /path/to/repository
• Add files to staging
  • git add <filename>
• Commit staging area to checked out branch
  • git commit -m “Commit message”
• View list of commits
  • git log
• See files changed and what still needs committed
  • git status
• Send changes to remote branch on remote repo
  • git push origin <branchname>
  • git push -u origin <branchname> (if branch doesn’t exist remotely)

Branches:

active line of development

• List all branches and see current branch
  • git branch
• Create new branch (and switch to it)
  • git checkout -b <branchname>
• Switch branches
  • git checkout <branchname>

Merge:

Bring contents of one branch into current branch

• Merge a different branch into the active branch
  • git merge <branchname>
• Fetch and merge changes from remote to working repository
  • git pull
• Conflicts: a file has multiple versions (perhaps from multiple branches)

Pull requests:

In a properly set up repository, can’t directly push to remote master. Instead, work on local branch, rebase or merge (preference) to resolve conflicts, push to remote branch, raise PR for someone to review and approve.

To resolve conflicts:

1. Merging: Made changes to local branch. Merge local master into local branch, resolve conflicts.

2. Rebase: To reapply a series of changes from a branch to a different base, and reset the head of that branch to the result.
   Made changes to local branch. Rebase changes onto local master: Make changes to local branch to effectively change starting point of local branch from where it was to current master branch location, resolve conflicts.

Then, push local branch to remote branch, raise pull request from remote branch to remote master. Pull remote master into local master.